Symposium EM7 : Functional Plasmonics

The progresses of metamaterials research has brought it to the stage that increasing attention has been paid to realize large scale metamaterial devices for real applications. In particular, a fascinating application is to implement perfect lens which is enabled by negative refractive index metamaterials (NIM) with both negative effective electric permittivity and magnetic permeability.
However, the realization of such a practical device has been hindered due to the obstacle in achieving isotropic negative index metamaterials at optical frequency which are insensitive to the angle of incidence and polarization of light. The design and fabrication of such an isotropic optical metamaterial has turned out to be an extremely challenging task. In the past years, many attempt to tackle the problem were all based on the canonical designs of split ring resonator (SRR) structures. The idea is to construct SRR in all three dimensions in order for the isotropic light-matter interactions. However, these structures require sophisticated fabrications that are really difficult to achieve, especially for metamaterials at optical frequency and at a large scale. Moreover, the SRR based metamaterials structures is almost impossible to scale to optical frequency due to the saturation effect. Here we show a realistic structural design and experimentally demonstrate large scale fabrication of isotropic negative index metamaterials at optical frequency.

Optical isolation is a key requirement for chip-based information processing networks. As in electronic circuits, isolation of forward and backward propagating signals is of critical importance.¹ Typical commercially available isolators are rather bulky, hard to scale and integrate, and expensive. Design of an efficient and compact optical isolator would foster the development of integrated all-photonic networks.¹
There are three known ways to break the symmetry of light propagation in forward and backward directions¹: with the use of magneto-optical response (commonly employed in optical circulators and isolators), time-varying² and nonlinear structures³. The latter two approaches suffer from a number of fundamental constrains. Hence, isolation in time varying systems would require ultrafast modulation of material parameters² (desirably in a THz range), which is hard to achieve in practice, whereas nonlinear systems (typically third order) require high input power and have a limited operation bandwidth³.
Here we propose theoretically a conceptually different paradigm of the signal isolation based on a hybrid approach that combines the principles of isolation of both time-varying and nonlinear systems. Specifically, we show that optical signal isolation is possible in optical light guiding structures with second order optical nonlinearity. We develop an analytical model and demonstrate that in the presence of a relatively strong pump a dynamic nonreciprocal coupling between signal and idler waves is possible. In particular, we show that in the forward direction of propagation signal wave propagates without any perturbation, whereas in the backward direction it may be fully converted into an idler wave in the presence of a control pump beam. We further elaborate our model and demonstrate that in the undepleted pump approximation it is equivalent to that of ultrafast time-varying systems.
Next we apply our model to a realistic material system. To be specific, we consider optical isolation of a telecom wavelength signal (1550nm) in a 130nm gallium phosphide (GaP) waveguide. Gallium phosphide offers an exciting platforms due to its high (over 100 pm/V) second order nonlinear coefficient, high refractive index, low material losses, and its CMOS compatibility. We show that with a 10 mW pump at ~1350 nm a 1 µW signal may be completely isolated over a 10 mm propagation length. Note that the propagation length in this scheme is several orders of magnitude smaller than that reported for electro-optical time-varying systems and for Brillouin scattering based isolation in fibers. We discuss scenarios to reduce the device footprint with the help of optical ring resonators.

References:
1. Jalas, D.; et al. Nature Photon. 2013, 7, 529.
2. Yu, Z.; Fan, S. Nature Photon. 2008, 3, 91 - 94.
3. Shi, Y.; Yu, Z.; Fan, S. Nature Photon. 2015, 9, 388–392.
4. Poulton, C.G.; et al. B.J. Opt. Express 2012, 20, 21235.

Plasmonic nanoparticles are ideal candidate systems for coupling photons into phonons, allowing for control over opto-mechanical properties in nanoscale dimensions. Upon excitation of a gold nanoparticle with an ultrafast laser, a series of events occur: initial creation of a surface plasmon and dephasing via electron-electron scattering, followed by eventual transfer of energy to the lattice and impulsive excitation of acoustic modes. Although such acoustic modes have been observed on single nanoparticles using optical pump-probe experiments, the inherently subwavelength spatial scale of plasmonic structures and devices make new imaging and spectroscopic techniques critical for understanding the relationship between plasmonic nanostructures and observed acoustic modes.

We use ultrafast electron microscopy (UEM), a pump probe technique that combines the spatial resolution of TEM with the temporal resolution of ultrafast pump-probe spectroscopic methods, to study the dynamics of gold nanorods. These experiments use an unprecedented combination of spatial and temporal resolution, resolving dynamics at below 3 ps with less than 5 nm spatial resolution.

The experiment uses two optically delayed pulses to trigger dynamics on a materials system in a TEM, and then images the sample with an optically triggered electron pulse. Using pump-probe stroboscopic imaging within the UEM we are able to image the laser induced dynamics within a single gold nanorod. We spatially map through changes in diffraction contrast the cooling rate of the lattice to the environment within a single Au nanorod, showing changes of diffraction strength of up to 5% from the pre-interaction level, and cooling rates to the substrate of approximately 1 ns. In addition to the kinetic response, there are clear oscillations observable in the diffraction contrast of the nanoparticle, which we assign to thermal shock induced acoustic modes of the nanoparticle. These modes are spatially mapped within the single nanorod and are observed at 3.9, 9.4, and 27 GHz, and are assigned to a bend, symmetric stretch, and symmetric overtone stretch, respectively. These assignments are consistent with the predicted frequencies from finite element simulations.

The technique was then extended to observe dynamics on nanoparticle dimers, tetramers, and larger clusters. Notably, we observe acoustic modes that are distinct from the modes observed in single nanoparticles, including very strong localized response where two nanoparticles come into direct contact. This localized response occurs at 25.8 and 27.3 GHz.

These studies indicate the complex relationship between the structure of plasmonic assemblies and the resulting dynamics, and resolve these dynamics with high resolution spatiotemporal mapping. We believe that this technique would allow for observation of dynamics in plasmonic devices, and could be used to answer questions pertaining to thermal transport, nanomechanical behavior, and optical losses.

Optical metamaterials are often fabricated on flat substrates made from materials that are topographically and chemically compatible with conventional nanofabrication techniques, such as electron beam lithography. The ability to fabricate on unconventional substrates will enable the next generation of optical metamaterials.
We fabricate metamaterials using atomic calligraphy, a microelectromechanical systems (MEMS)-based dynamic stencil lithography technique [1]. We present a flip-chip technique that enables us to write on a variety of substrates. While the areal coverage of atomic calligraphy is approximately 100 μm², we use a stage system to extend the range to square centimeters. Because atomic calligraphy is a direct write technique, it is resist- and liftoff-free. Therefore, it can be used to fabricate on substrates with topographical features or that preclude them from use with resists or on substrates that are chemically incompatible with other lithography techniques [2]. We fabricate metamaterials on foreign substrates and characterize their infrared spectra.
This technique will enable us to fabricate metamaterials on novel substrates that lend additional degrees of freedom. For example, fabricating metasurfaces on two dimensional auxetic mechanical metamaterials will enable us to fabricate tunable metasurfaces.

Plasmon excitation enables extreme light confinement at the nanoscale, localizing energy in subwavelength volumes and thus can enable increased absorption in photovoltaic or photoconductive detectors. Nonetheless, plasmon decay also results in energy transfer to the lattice as heat which is detrimental to photovoltaic detector performance. However, heat generation in resonant subwavelength nanostructures also represents a power source for energy conversion, as we demonstrate here via design of resonant thermoelectric (TE) plasmonic absorbers for optical detection. Though TEs have been used to observe resonantly coupled surface plasmon polaritons in noble-metal thin films and microelectrodes, they have not been employed previously as resonant absorbers in functional TE nanophotonic structures.

We demonstrate nanostructures composed of TE thermocouple junctions using established TE materials – chromel/alumel and bismuth telluride/antimony telluride – but patterned so as to support guided mode resonances with sharp absorption profiles, and which thus generate large thermal gradients upon optical excitation and localized heat generation in the TE material. Unlike previous TE absorbers, our structures feature tunable narrowband absorption and measured single junction responsivities 10 times higher than the most similar (albeit broadband) graphene structures, with potential for much higher responsivities in thermopile architectures. For bismuth telluride – antimony telluride structures, we measure thermoelectric voltages up to 850 μV with incident optical power densities of 3.4 W/cm². The maximum responsivity of a single thermocouple structure was measured at 119 V/W, referenced to incident illumination power. We also find that the small heat capacity of optically resonant TE nanowires enables a fast, 3 kHz temporal response, 10-100 times faster than conventional TE detectors. We show that TE nanophotonic structures are tunable from the visible to the MIR, with small structure sizes of 50 µm x 100 µm. Our nanophotonic TE structures are suspended on thin membranes to reduce substrate heat losses and improve thermal isolation between TE structures arranged in arrays suitable for imaging or spectroscopy. Whereas photoconductive and photovoltaic detectors are typically insensitive to sub-bandgap radiation, nanophotonic TEs can be designed to be sensitive to any specific wavelength dictated by nanoscale geometry, without bandgap wavelength cutoff limitations. From the point of view of imaging and spectroscopy, they enable integration of filter and photodetector functions into a single structure.

Metal nanoparticles exhibit broad localized surface plasmon resonances that increase in width as the particle size increases. However, when these nanoparticles are organized into arrays with spacings on the order of hundreds of nanometers, narrow lattice plasmon resonances can result. The talk will describe a new way to achieve even narrower resonances via superlattice plasmons, collective excitations that are supported by hierarchical gold nanoparticle arrays, where finite arrays of particles (patches) are organized into arrays with larger periodicities. Superlattice plasmons resonances are often significantly narrower than that of single-patch lattice plasmon resonances and exhibit stronger local peak fields. We will also discuss how ultra-narrow resonances can be achieved and manipulated in emerging plasmon materials.

Planar nanostructures with subwavelength features have enabled miniaturized optical components with superior properties over traditional bulk optics. However, these structures typically suffer from strong chromatic aberration that limits their applications to narrow wavelength ranges. Here we report a platform based on gold nanoparticle (NP) lattices and an evolutionary method that can design flat lenses for a wide range of wavelengths. Our optical components can access the desired wavelengths by tuning the plasmon resonances of the lattice elements with their size and shape. The design approach allows multi-objective optimization, which enable us to design multi-resonance lattices using multiple particle shapes that can focus light to the same focal point at up to three wavelengths. We expect that the design strategy will be applicable to all wavelengths ranges by choosing the suitable materials and structures as the lattice building blocks.

In order to overcome the limitations imposed by the pre-defined dielectric function of metals we implement alloys [1]. First, we built a library of the optical response of thin-film alloys formed by the binary combination of Ag, Au, Cu and Al. For that, we combine ellipsometry and SPP measurements and calculations, and find an excellent agreement between the two methods. Surprisingly, we find that some compositions present a quality factor higher than their pure counterparts. Second, we fabricated alloyed nanoparticles and investigated their optical response by near-field scanning optical microscopy (NSOM), in conjunction with full-field simulations of light-matter interactions in the visible and NIR ranges of the spectrum [2]. Our findings pave the way for the development of optically engineered building blocks for nanophotonics based on alloys, where the chemical composition of the nanostructures can be used as an additional knob to tune their optical properties.

[1] C. Gong et al. ACS Photonics, 3, 507 (2016). Front COVER.
[2] C. Gong, M. Dias et al. Adv. Optical Materilas, DOI:10.1002/adom.201600568 (2016).

Diffraction limit basically limits the resolution of conventional optical microscopy, which means that the object smaller than the diffraction limit is difficult to be distinguished [1]. Many kinds of imaging techniques have been introduced and demonstrated to overcome this physical limitation. Near-field scanning optical method, many fluorescent microscope, like STED, STORM, NSOM are introduced and develop an effective way to overcome resolution barrier [2-4].
New superlens imaging concept without scanning and reconstruction is introduced and received with great interest. The fact that evanescent field can be amplified by a metamaterial is demonstrated [5]. Far-field super-resolution concept also accomplished by a sub-diffraction-limit imaging technique called hyperlens [6]. Hyperlens is a spherical geometry with multiple periodic metal and dielectric layers. It enables waves with large tangential wave vectors to propagate in far field. Subwavelength small object information is magnified and can be propagated to the far field, which can be applied to conventional optical microscopy for super resolution imaging.
In this study, we introduce a proper method for large scale hyperlens array fabrication using nano-imprint and provide optimal parameters from numerical simulations. The limit of existing hyperlens is hard to locate a sample exquisitely. Hyperlens array with hexagonal pattern can solve the problem. It can be a guideline for the practical use of imaging system with hyperlens. We expect this approach will have useful applications in biology, pathology and medical science and nanotechnology.

References
1. E. Abbe, Arch. Mikroskop. Anat. 9, 413 (1873)
2. E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner and R. L. Kostelak, Science 251, 1468-1470 (1991)
3. S. W. Hell, Nature Biotechnol. 21, 1347-1355 (2003)
4. Rust, J. Michael, M. Bates and X. Zhuang, Nature Methods 3.10, 793-796 (2006)
5. N. Fang, H. Lee, C. Sun and X. Zhang, Science 308, 534-537 (2005)
6. J. Rho, Z. Ye, Y. Xiong, X. Yin, Z. Lui, H. Choi, G. Bartal and X. Zhang, Nature Commun. 1, 143 (2010)

Recently, exciting new physics of plasmonics has inspired a series of key explorations to manipulate, store and control the flow of information and energy at unprecedented dimensions. For example, surface plasmons of different chirality can be excited in two dimensional materials that support transverse currents. In this talk, we propose a method to optically excite and characterize the electromagnetic response and surface electromagnetic modes in a generic gapped Dirac material under pumping with circularly polarized light. The valley imbalance due to pumping leads to a net Berry curvature, giving rise to a finite transverse conductivity. Guided by our theoretical work, we argue the appearance of nonreciprocal chiral edge modes, their hybridization and wave guiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays. We seek to show that the new materials system can allow for electrical manipulation of light not possible with known materials. We also demonstrate experimentally ultrafast quenching of 2D molecular aggregates at picosecond timescale assisted by surface plasmons. Our analysis reveals that the metal-mediated dipole-dipole interaction increases the energy dissipation rate by at least ten times faster than that predicted by conventional models. Our results can offer novel design pathways to the light-matter interaction in a variety of photon-exciton systems with applications such as high speed visible light communication.

The intrinsic losses associated with plasmonic nanoparticles at optical frequencies can be useful. The optical response of individual gold nanorods depends strongly on orientation, however in suspension the nanorods are randomly oriented giving rise to an isotropic response. Control over orientation is required to access the full range of possible material responses. Unlike individual liquid crystal molecules, the susceptibility of individual gold nanorods is sufficiently large that their interaction energy with an electric field is strong enough to overcome the disordering effects of thermal excitations offering an exciting new paradigm for liquid crystal molecules with a wide variety of potential phases, structures and applications. We carried out experiments measuring the optical absorption from gold nanorod suspensions aligned using external electric fields [1]. We show that the absorption from these suspensions depends linearly on the orientational order parameter and develop a technique to determine the imaginary parts of the longitudinal and transverse electric susceptibilities of the nanorods. We provide evidence that the critical electric field needed to orient the gold nanorods is proportional to the nanorod volume and depolarization anisotropy and demonstrate for suspensions with two different nanorod sizes that the alignment of each population can be controlled. These suspensions are expected to be significantly thinner than current liquid crystal based displays, they do not require surface alignment layers, and they are continuously color tunable and chemically stable, unlike dichroic dyes. They are also generally expected to have faster switching times compared to typical liquid crystal displays. This work was supported with funding provided from the Office of Naval Research Global under ONRG-NICOP-N62909-15-1-N016. [1] Fontana et al, Applied Physics Letters, 108, 081904 (2016)

The development of multiband plasmonic nanoantennas with a large density of spectral resonances offers exciting opportunities for the engineering of novel optical sensors and spectroscopic techniques. In our presentation we will discuss the design, fabrication and testing of novel types of Au plasmonic antennas based on the inverse Cesaro space-filling fractal curve. Differently from recently proposed fractal structures, Cesaro nanoantennas have the remarkable property that the number of their resonant bands, which extend from the visible to the long-infrared range, does not depend on the overall size of the devices. In other words, inverse-Cesaro nanoantennas give rise to a high density of spectral resonances over a compact device area with sub-wavelength footprint, and can be conveniently integrated in future plasmonic-photonic active platforms. In particular, in this talk we will present our systematic study of the scattering and near-field resonant properties of devices fabricated by electron-beam lithography (EBL) on CaF₂ substrates by resorting to FDTD simulations in combination with experimental Fourier transform infrared microscopy. Our findings demonstrate that large values of electric and magnetic near-field enhancement with multi-scale distributions of resonant modes are obtained in Cesaro-type nanoantennas across multiple bands controlled by their fractal iteration number, rendering these plasmonic systems ideally suited for multispectral chemical detection. In order to demonstrate the full potential of Cesaro fractal antennas we measured by differential reflectance spectroscopy the absorption bands of thin poly(methyl methacrylate) (PMMA) layers deposited atop the structures. Our data unambiguously demonstrate reliable and simultaneous detection of three absorption bands of PMMA films with nanoscale thickness. We believe that the engineering of Cesaro-type plasmonic nanoantennas provides a novel strategy for the realization of active devices with a large spectral density for energy harvesting and optical biosensing on a compact plasmonic chip.

Recently, CMOS-compatible materials with tailorable optical properties such as transition metal nitrides (titanium- and zirconium nitrides) and transparent conducting oxides (TCOs) (such as highly doped zinc oxide and indium tin oxide) have been proposed for photonic and plasmonic applications in the visible and telecommunication wavelength ranges. TiN and ZrN are gold-like, robust and high-temperature stable materials with their dielectric permittivities’ cross-over wavelength near 500 nm. Partnering TiN and ZrN with CMOS-compatible silicon nitride enables a fully solid-state waveguide which offers a propagation length greater than 1 cm for a ~8 μm mode size at 1.55 μm. Transition metal nitrides also enable durable metasurfaces for applications in high-intensity flat optics for advanced beam control and solar thermo photovoltaics. Utilizing highly doped zinc oxide films as a dynamic photonic material, high performance modulators can be realized. Together, these alternative materials form the base of a fully integrated nanophotonic system, capable of exceptional performance with speeds greater than 1 THz. Due to the ability of TCO nanostructures to support strong plasmonic resonance in the near infrared (NIR), metasurface devices, such as a quarter wave plate, have been demonstrated whose properties can be easily adjustable with post processing such as thermal annealing. Additionally, TCOs can be used as epsilon near zero (ENZ) materials in the NIR. TCOs are shown to be extremely flexible materials, enabling fascinating physics and unique devices for applications in the NIR regime.

Plasmonic materials offer interesting optical properties, as the nanoscale metallic features localize the electromagnetic field at the surface and greatly increase the intensity of the field. These materials can be useful for many spectroscopy applications, including surface-enhanced Raman spectroscopy (SERS) in which localized surface plasmons enhance the weak Raman scattering of molecules. Here, we demonstrate the simulated Raman correlation spectroscopy (SRCS) process in which we utilize the SERS signals of cytosine-silver composites to characterize the systems and understand the optical properties of these hybrid plasmonic systems. The technique successfully implements time-dependent density functional theory (TD-DFT) for Raman frequency calculations that are then numerically compared to experimental measurements.
We begin the SRCS process by calculating the several potential binding configurations of cytosine-silver composites and experimentally measuring the corresponding systems. For TD-DFT simulations, geometrical optimization and frequency mode calculations are performed on the cytosine-silver composites using the B3LYP method and LANL2DZ basis set. The four potential binding sites of cytosine (N1, N3, NH2, and O) are each attached to a 20 atom silver structure and the Raman frequency modes are calculated for each potential binding site. For comparison, cytosine is functionalized on plasmonic silver films and the Raman signal is experimentally measured. Analysis of the spectra show that there are many differences between the simulated Raman spectra and the experimental measurement. The discrepancies are due to multiple potential binding sites, rather than a single one.
We have developed the SRCS process to determine the preferential binding site composition. First, the Raman intensity bands for each system are categorized based on the Raman frequency mode (e.g. ring-breathing-mode). Then, the frequency mode intensities are normalized with respect to the total intensity of the prominent Raman frequency intensities. Finally, to calculate the optimal weighted binding coefficients, we maximize the correlation between the experimental normalized frequency modes and the simulated normalized frequency modes as we vary the composition of binding coefficients.
The SRCS algorithm performs approximately 175,000 iterations and reports the maximum . For the case of cytosine functionalized to silver nanoparticles, the optimal value is 0.81 with a binding site composition of 9% N1, 63% N3, 26% NH₂, and 2% O. This coefficient of determination is higher than assuming a single binding site, which has poor correlation with experimental measurements. Thus, we have demonstrated that we can achieve higher correlation between simulated and experimental measurements by optimizing the weighted coefficients of potential binding sites. The SRCS process can be applied to the other nucleic acids and molecules that have multiple potential binding sites.

Semiconductor quantum dots (QDs) have attracted immense interests due to their unique optical and electrical properties that arise from quantum confinement effects. The integration of QDs with plasmonic materials enables superior hybrids for applications in biosensing, energy and information technology. However, to realize such applications, it is crucial to achieve extremely precise patterning of QDs on the plasmonic substrates at a reasonably high throughput. Typical printing techniques such as inkjet printing, electrohydrodynamic jet printing, Langmuir-Blodgett printing, and micro-transfer printing cannot simultaneously achieve a high resolution, high patterning speed, and low post-processing times.

In this work, we develop and apply a new type of technique known as bubble printing to achieve the ultrahigh-resolution patterning of QDs over plasmonic substrates (i.e., gold nanoisland). A mesobubble (bubble with dimension <1000 nm) generated upon incidence of a laser beam collects the QDs via Maragoni convection, and immobilize them on the substrate with the van der Waals force and thermal effects. By scanning the mesobubble over the substrate, we are able to pattern QDs of arbitrary geometries with a linewidth of 600nm, a speed of up to 10^-2 m/s, and post-processing time of under 1 ms. By optimizing the incident laser power and scanning speed, control over the bubble dimensions is achieved. This manifests as the versatility in the tuning of the QD concentration and linewidth of the patterns. Moreover, we have shown that the bubble printing bypasses the inherent limitations of conventional printing techniques to create intricate patterns of multi-color QDs (Red, Green and Blue emissions). Generation of mesobubble in the nanosecond regime further corroborates our technique’s applicability. Subsequently, we have achieved patterning of QDs on plasmonic structures across multiple platforms including glass slides, flexible polymers, and three-dimensional microspheres. Fluorescence spectroscopy measurements on the patterns indicate no significant damage to the QDs. The interaction between QDs and plasmonic substrates, as revealed by fluorescence lifetime imaging microscopy, leads to 10-fold reduction in the lifetime of the QDs.

Color generation is commonly pigmentation-related and is spatially limited to tens of microns, two orders of magnitude above the diffraction limit. Colors can also be generated with interference devices such as photonic crystals and subwavelength plasmonic structures. ^[1-2] The latter are suggested as the next generation for color display, because they have the potential to reach the diffraction limit resolution using advanced fabrication techniques. Furthermore, light can be efficiently manipulated by such plasmonic structures followed by polarization for instance. Hence, one can control the simultaneous tuning of the generated color. Plasmonic nanostructures such as hole arrays, grooves, disks, and slits have been shown to generate colors efficiently, and have the potential to function as dynamic color pixels. Yet, their size is still limited to several microns. ^[3-5] Therefore, We exploit the plasmonic-hybridization of nano cavities milled in metallic films, which are excited by propagating surface plasmons, to induce coupling between them. This is where Babinet’s principle does not hold, namely, holes cannot be considered complimentary to nanoparticles. Following hybridization, new states are formed: the ‘in-phase’ and ‘out of phase’ states, in analogy to molecular orbitals. The polarization state of the incoming optical field modifies the charge distribution around the cavities, thus, one can actively achieve the whole energy landscape of the optical range.
Herein, we report on such active, sub-micron plasmonic devices. Despite their small size, we are able to generate multiple colors from these structures, depending on the polarization state of the incoming optical field. ^[6] To examine the whole structure which acts as a unified entity, we utilize both optical far field microscopy, alongside cathodoluminescene (CL) spectroscopy. The properties of these plasmonic devices are unique and related to the interactions between the neighboring cavities. We present a thorough study of the modes which give rise to the enhanced mutual coupling between these cavities. This examination is possible due to spatial mapping of the photon emission for a given energy, which can easily be obtained by CL - providing a direct way to probe the local electric field.

[1] S. P. Burgos, S. Yokogawa, H. A. Atwater, ACSnano 2013, 7, 10038.
[2] L. B. Sun, X. L. Hu, B. Zeng, L. S. Wang, S. M. Yang, R. Z. Tai, H. J. Fecht, D. X. Zhang, J. Z. Jiang, Nanotechnology 2015, 26, 305204.
[3] A. Salomon, Y. Prior, M. Fedoruk, J. Feldmann, R. Kolkowski, J. Zyss, J. Opt. 2014, 16,114012.
[4] C. Genet, T. W. Ebbesen, Nature 2007, 445, 39.
[5] S. Balci, E. Karademir, C. Kocabas, Optics Letters 2015, 40, 2.
[6] E. Segal, A. Weissman, D. Gachet, A. Salomon,Nanoscale 2016, 8, 15296.

Recently, structural color generation by micro- or nano-structures has been widely investigated as a good alternative to the conventional pigment based color generation, due to sensitive color tuning as well as low photodegradation [1]. Various applications of structural color generation such as filter-free image sensing [2], omnidirectional color reflection [3], and stereoscopic color printing [4] have been demonstrated. Plasmonic resonances of metallic nano-structures allow structural color with higher resolution, higher brightness and contrast, and more sensitive color tunability [5]. The noble metals, gold, silver and copper, are not suitable to generate plasmonic structural color over the whole visible wavelength range. On the other hand, aluminum, of which the interband transition is located in the near-infrared wavelength, is the most promising medium for structural color generation not only in the visible region but also in the ultraviolet region. Operation in the ultraviolet region provides structural color generation with further applications including color multiplexing [6], image steganography [7], and optical data storage [8].
In this research, we present ultraviolet structural color generation using aluminum nanodisk arrays on a quartz substrate. The plasmonic resonance of the aluminum nanodisks produces vivid reflective structural color based on the strong backward scattering. To characterize ultraviolet structural color generation, we measured the reflectance spectrum of the aluminum nanodisks arranged in a square lattice. As increasing the diameter of the nanodisk and the period of the square lattice from 74 to 94 nm and from 190 to 260 nm, respectively, the resonance wavelength (λ_res) in the reflectance spectrum gradually moves from ~327 to ~401 nm. Here, the filling ratio of the nanodisk in a unit cell is fixed to ~11 %. Even though the thickness of the nanodisk arrays is only ~35 nm, high reflectance values of ~35 % on average are achieved at the resonant condition. Typical full-width at half-maximum of the resonance peak is ~λ_res/5, which is narrow enough to generate vivid structural color. The numerical simulations employing the finite-difference time-domain method successfully reproduce the experimental results. We also demonstrated ultraviolet structural color pixels with different resonance wavelengths and examined their performances by measuring bright-field microscope images of the aluminum nanodisk arrays under different illumination conditions using 325/25, 340/25, 365/25, 380/10, 400/25, and 420/20 nm bandpass filters.
[1] Zhao, Y. et al. Chem. Soc. Rev. 41, 3297 (2012)
[2] Park, H. et al. Nano Lett. 14, 1804 (2014)
[3] Chung, K. et al. Adv. Mater. 24, 2375 (2012)
[4] Goh, X. M. et al. Nature Commun. 5, 5361 (2014)
[5] Kumar, K. et al. Nat. Nano. 7, 557 (2012)
[6] Huang, Y.-W. et al. Nano Lett. 15, 3122 (2015)
[7] Dean, N. Nat. Nanotech. 10, 15 (2015)
[8] Mansuripur, M. et al. Opt. Express 17, 14001 (2009)

Plasmonics is a newly emerging field that has profound impact on energy storage and conversion, sub-wavelength light manipulation, photothermal cancer therapy, and ultrasensitive biomolecular sensing. It has been recently observed that the localized surface plasmon resonance supported by metallic nanostructures plays a crucial role in driving or enhancing a series of interesting chemical or photochemical reactions on metallic nanoparticle surfaces, though the detailed mechanisms of these plasmon-mediated reactions are still poorly understood and under intense debate. Therefore, it is imperative to gain quantitative insights into the kinetics and underlying pathways of these plasmon-driven photoreactions to fully understanding the obstacles that might limit the wide applications of plasmonic nanostructures as high-performance photocatalysts. In this presentation, I will talk about our latest progress on developing quantitative understanding of the kinetics and underlying pathways of two interesting plasmonic hot electron driven reactions, oxidative coupling of 4-aminothiophenol and reductive coupling of 4-nitrothiophenol. We use single-particle surface-enhanced Raman spectroscopy (SERS) to precisely monitor, in real time, the plasmon-driven photoreaction kinetics at the molecule-nanoparticle interfaces. A unique hybrid nanostructure composed of a Fe₃O₄ (or SiO₂) bead decorated with Ag nanocubes was used as a plasmonically addressable substrate for SERS measurement. The plasmon-driven dimerization of thiophenol-derivates were chosen as a model reactions to explore the effects of plasmon excitations, molecular adsorption states, local field enhancements, and photothermal processes, on the plasmon-driven photoreactions. In addition to the energetic hot electrons/holes generated during the plasmon decay on metallic nanoparticle surfaces, the peculiar role of active oxygen species in guiding the plasmon-driven photocatalytic reactions was also proposed and discussed in detail.

Interference coatings using dielectric thin-film stacks have been used in a variety of applications, including thin-film optical filters and anti-reflection coatings. We have previously shown that strong interference effects can also be observed in a lossy ultra-thin film with thickness as small as λ/100 on an opaque substrate, resulting in perfect absorption.

The amount of absorption can be dynamically tuned by employing materials with tunable optical properties. One of the most promising materials with a dramatic change of its optical properties is vanadium dioxide (VO₂), which exhibits a reversible insulator-to-metal transition (IMT) as the temperature is increased above a critical temperature T_C ~ 68°C. Previously, sapphire in its Reststrahlen band region has been identified as a suitable substrate material to observe the perfect absorption effect; however, this configuration does not allow for wavelength agility.

Another set of candidate materials includes heavily doped semiconducting metal oxides in the spectral region of anomalous dispersion, which is coupled to their adjustable plasma frequency. Here, we demonstrate a tunable perfect absorber comprising an ultra-thin layer of VO₂ on aluminum-doped zinc oxide, a transparent conducting oxide. The perfect absorption wavelength can be tuned over a wide wavelength range across the near-infrared and mid-infrared by locally controlling the free-carrier concentration by means of ion beam Al⁺ doping. We also demonstrate how area-selective ion beam irradiation can be used to locally modify the VO₂ phase transition via the intentional creation of oxygen vacancies, decreasing the transition temperature – even to below room temperature – of the irradiated regions.

Unlike existing means of semiconductor doping and IMT modification via impurity doping during growth, ion beam irradiation can be combined with lithographic patterning to create complex optical meta-devices with designer phase transitions at any wavelength of choice. Using this approach, we demonstrate ultra-thin (thickness ~ λ/100) wavelength-customized temperature-tunable perfect absorbers and reconfigurable polarizers for the mid-infrared.

Plasmon resonances with their dramatically enhanced cross sections for light harvesting can serve as efficient generators of hot electrons and holes. Such hot carriers can be exploited in a wide range of photophysical and photochemical processes. Hot carrier generation is a quantum mechanical process in which one plasmon quantum is transferred to the conduction electrons of the nanostructure by excitation of an electron below the Fermi level into a state above the Fermi level but below the vacuum level. In my talk I will discuss the basic mechanism of plasmon-induced hot carrier formation, hot carrier relaxation, and how the hot carrier distribution in a metal can be tuned using bimetallic antenna-reactor structures. In this structure, the near-field induced around a plasmonic antenna can be used to drive hot carrier generation in a nearby catalytic transition metal. Finally I will discuss several recent applications of the antenna-reactor concept in photocatalysis.

In recent years, plasmonic nanocrystals (NCs) have attracted a great deal of attention in the field of photochemistry, where plasmon-induced generation of hot electrons is one of the main mechanisms for enhancing the photochemical activities in the visible and near infrared spectral intervals. Here we numerically study the process of plasmon-induced hot electron generation in terms of shape and size of NCs. The rate of hot electron generation in a metal NC depends directly on the strength of electromagnetic enhancement inside a NC. Mathematically, we express the rate of generation in terms of the normal-to-surface component of the electric field near the metal surface in the metal NC. NCs with multiple tips such as nanostars exhibit the largest rates of hot electron generation as compared to the other shapes including nanospheres, nanorods and nanocubes. For plasmonic nanostars, most of the hot electrons are generated in the plasmonic hot spots near the nanostar tips. In NCs with complex shapes, the total rate of generation of hot plasmonic electrons is mainly affected by the two factors: The surface area of a NC and the strength of field enhancement inside a NC. The injection rate of hot electrons from the nanostar is nearly isotropic with respect to the polarization of incident light. Overall we show that the nanostars and other NCs with complex shapes are excellent candidates for plasmon-assisted photochemistry [1].


[1] A. Sousa-Castillo, M. Comesaña-Hermo, B. Rodríguez-González, M. Pérez-Lorenzo, Z. Wang, X.-T. Kong, A. O. Govorov, and M. A. Correa-Duarte, J. Phys. Chem. C, online, DOI:10.1021/acs.jpcc.6b02370

Noble metals that generate hot carriers by plasmon decay promote efficient charge separation with visible light irradiation, which opens a new prospect in the field of photocatalysis, photovoltaics, and photodetection. While localized surface plasmon resonance (LSPR)-induced hot carrier generation is evidenced in diverse metal nanostructures, inhomogeneous metal-semiconductor mixtures hinder efficient control over photocarrier generation and therefore reproducible carrier-mediated photochemistry. Here, we generate surface plasmon polaritons (SPPs) at the interface between a noble metal film and aqueous solution allowing simultaneous optical and electrochemical interrogation of plasmon-mediated chemistry in a system whose resonance may be continuously tuned via the incident excitation angle. This is the first experimental demonstration of SPP-induced hot carrier generation for photocatalysis to the best of our knowledge. We find electrochemical photovoltage and photocurrent responses as SPP-induced hot carriers drive solution chemistry in a mixture of methanol and sodium hydroxide, and demonstrate the role of SPPs in methanol oxidation. A strong SPP angle dependence and linear power dependence in the electrochemical response validates that the methanol oxidation is a SPP-driven reaction. We will also characterize the effect of a TiO₂ layer in carrier transport, whose thickness and morphology can be tuned during atomic layer deposition. The results will provide the design criteria for a metal-semiconductor hybrid system with enhanced hot carrier generation and transport, which is important for the understanding and application of SPP-induced photocatalysis.

Light absorption in metals can lead to coupled electromagnetic and charge oscillations known as surface plasmons. These plasmons can decay into hot carriers (electrons and holes) that can be used for carrier injection into a semiconductor, enhanced chemical reactivity, sensor applications, etc. Gold and silver have been the most widely used for such applications; however, many other materials may provide equal or better performance based on the application. Here we will present our recent work on hot carrier devices using transparent conducting oxides [1] and aluminum, as well as the opportunities that exist for not traditional materials, alloys, and nanostructures [2]. Further, we will discuss fabrication and testing of both planar and nanostructured devices that are enabled by hot carrier effects to create novel sub-bandgap photodetectors and solar energy harvesters. Our planar Al devices show a responsivity of ~240 nA/W at shorter wavelengths and an open-circuit voltage that depends on the incident photon energy. Al nanowires lead to ~70% absorption of the incident light and show a promising avenue for improved device performance. The outlook for future devices based on new materials will be discussed.

[1] Tao Gong and Jeremy N. Munday, Nano Lett., 15, 147–152 (2015)
[1] Tao Gong and Jeremy N. Munday, Opt. Mat. Exp., 5, 2501-2512 (2015)

The dynamics of optically-excited electrons and holes at femtosecond time scales and nanometer length scales is critical in many applications including photovoltaics, photocatalysis and photodetectors. Decay of surface plasmons to hot carriers is a new direction that has attracted considerable fundamental and application interest, yet a theoretical understanding of ultrafast plasmon decay processes and the underlying microscopic mechanisms remain incomplete. While ultrafast experiments provide insights into the relaxation of non-equilibrium carriers at the tens and hundreds of femtoseconds time scales, they do not yet directly probe shorter times with nanometer spatial resolution. Theoretical calculations can access these scales and complement such experiments, but have so far been primarily restricted to free electron-like models.

A theoretical understanding of plasmon-driven hot carrier generation and relaxation dynamics from the femtosecond to nanosecond timescales is presented here. We combine first principles calculations of electron-electron and electron-phonon scattering rates with Boltzmann transport simulations to predict the ultrafast dynamics and transport of carriers in real materials. In particular, we calculate the distributions of hot carriers generated by plasmon decay and their transport in metallic nanostructures which guide material selection and geometry design for plasmonic energy conversion devices. We also predict the evolution of electron distributions in ultrafast experiments on noble metal nanoparticles from the femtosecond to picosecond time scales. In all these studies, we find that details in the electronic structure and electron- phonon coupling matter significantly, especially when d bands are involved.

The role of nonlinear multi-plasmon processes in measurements of plasmon decay has been suggested, yet the extent and spectral signatures of these contributions is an open question. Here we show calculations for multiplasmon and nonlinear processes in the ultrafast regime from the mid-IR to visible and in different geometries. Extending these predictions, we propose experimental signatures of the nonlinear response
that utilize high-energy `up-converted' carriers inaccessible from the linear response (conventional plasmon decay), including ultrafast measurements on metal-semiconductor interfaces and plasmon-enhanced Raman spectroscopy.

Plasmonic nanopore arrays have been fabricated to contain multiple nanoscale electrodes, which can moderate the interaction between single electron-transfer events and fluorescence emission, in zeptoliter optical confinement volumes. Furthermore, arrays of nanoscale-recessed dual ring electrodes fabricated using layer-by- layer deposition coupled with focused ion beam etching can function both as working generator-collector electrode pairs and also as zero-mode waveguide (ZMW) arrays. The dual functionality makes it possible to perform single molecule spectroelectrochemical measurements under redox cycling conditions – both when the upper electrode is potential-controlled and using self-induced redox cycling. Flavin mononucleotide, FMN, contains an isoalloxazine chromophore which is fluorescent in the oxidized state, while the reduced state, FMNH₂ , exhibits a substantially lower quantum efficiency, thus permitting the redox state of single FMN molecules to be followed by observing their fluorescence behavior. Because the ~100 zeptoliter volumes of these nanopores dictate very short residence times, evidence for single molecule redox cycling is obtained from the fluorescence dynamics. Freely diffusing species exhibit characteristic behavior in which the probability of observing single reduced molecules increases as the potential is scanned to more negative values. Conversely, single molecule cycling behavior is evidenced by the distribution of on- and off-times, which are altered relative to freely diffusing FMN/FMNH₂ . Capture efficiencies are characterized as a function of the potential applied to the upper ring electrode.

The decay of surface plasmon resonances is usually a detriment in the field of plasmonics, but the possibility to capture the energy normally lost to heat has open new opportunities in photon sensors and energy conversion devices. In particular, the large extinction cross-section at a surface plasmon resonance enables nanostructures to absorb a significant fraction of the solar spectrum in very thin films and generate energetic carriers which could be exploited to drive photochemical processes.

Previously, we analyzed theoretically the quantum decay of surface plasmon polaritons, with and without phonons, and found that the prompt distribution of generated carriers is extremely sensitive to the energy band structure of the plasmonic material. In particular, the onset of interband transitions, occurring in the VIS regime (around 2 eV) for most common plasmonic metals (Au, Cu, Ag), is expected to significantly modify the hot-carriers distribution. Based on the theoretical work, we have designed an experimental system using GaN/noble-metal nanoscale heterostructures to investigate the photocurrent generated upon plasmon decay across the extended UV-visible regime. We indeed fabricated planar Schottky diodes where the Schottky contact was patterned with e-beam lithography to concurrently serve as plasmonic resonator, either in the form of hole array or metallic strips. Using a super continuum Fianium laser as light source to cover the energy range from 3.1 eV to 1.2 eV, we measured the photocurrent as a function of photon energy. Thanks to the use of an optically transparent substrate which allows a direct experimental quantification of the absorption spectrum of our plasmonic structures, we can determine the internal quantum efficiency (IQE) spectrum of each device. By direct comparison with ab-initio calculations, we aim at assessing the contribution of direct and indirect transitions for metals with different band structures, such as Au and Al, and we identify guidelines for engineering the generation of plasmonic hot-carriers in the UV-VIS regime.

The exquisite sensitivity of localized and propagating surface plasmons to their dielectric environment, when coupled to the extraordinarily large change in dielectric function in vanadium dioxide (VO₂) as it undergoes its characteristic insulator-to-metal transition (IMT), argue compellingly for an active-plasmonics design strategy embodying this phase-changing oxide. In this talk, I will briefly review the attractive electronic and materials properties of VO₂ from recently published and still unpublished work, emphasizing the dynamics of the optically switched IMT, including the effects of dopants. The central focus will highlight several strategies for modulating the radiative and memory functions of noble-metal plasmonic structures using the metal-insulator transition in vanadium dioxide, including optical limiting, the control of near- and far-field radiation from nanoscale antennas, tunable metamaterial perfect absorbers and an optical analog to memristive switching. There are challenging materials and nanofabrication issues to be mastered in making practical plasmonic devices that will be addressed in conclusion, including the opportunities opened up by dielectric metamaterials.

Research partially supported by the Office of Science, U. S Department of Energy under grant DE-FG02-01ER45916 and by the National Science Foundation (DMR-1207507).

The development of active meta-optics has long been a goal of the community and vanadium dioxide (VO₂) is a prime candidate for an actively tunable element due to its ultra-fast insulator to metal phase transition. The use of VO₂ is particularly attractive as its phase transition occurs near room temperature and is accompanied by a dramatic change in both its electrical and optical properties. In this work we combine VO₂ with a plasmonic metasurface to realize tunable optical filters and modulators in the near-infrared regime. The metasurface is designed to concentrate light within small patches of VO₂ in order to minimize the required switching energy and thermal mass of the system. The architecture also allows for Joule heating of the small VO₂ regions by passing current through the metasurface structure. The device exhibits a 500nm resonant wavelength change and a modulation depth of 81%. We also observe continuous spectral tuning which is primarily caused by the grain inhomogeneity in the VO₂ patch. We envision that this type of metasurface could be utilized in spatial and spectral light shaping for applications including holography, adaptive optics, anti-forgery devices, and recognition in security systems.

Interest in the active control of plasmonic properties by external means, such as applied voltages, has led to an intensely growing interest in new plasmonic materials such as doped semiconductors, 2D materials, and graphene. While graphene plasmonics has been well-studied in the IR, shifting the plasmon resonance of graphene to the visible region of the spectrum would require extremely small graphene structures with dimensions smaller than can be fabricated by the best currently available top-down fabrication methods. This is the length scale of polycyclic aromatic hydrocarbon (PAH) molecules, which can be regarded as picoscale versions of graphene, edge-passivated with hydrogen atoms. Recent theoretical studies have predicted that charged PAH molecules with partially filled orbitals can possess molecular plasmon resonances. While neutral PAH molecules exhibit large energy gaps, rendering them transparent in the visible region of the spectrum, the addition of removal of one or more electrons leads to strong absorption features in the visible wavelength range.[1] Experimentally, PAHs can be incorporated into planar device geometries where they show outstanding potential as low-voltage, fast electrochromic media suitable for applications ranging from nanoscale optical components to large-area, color-changing walls or windows.
[1] Adam Lauchner, Andrea E. Schlather, Alejandro Manjavacas, Yao Cui, Michael J. McClain, Grant J. Stec, F. Javier García de Abajo, Peter Nordlander, and Naomi J. Halas, “Molecular Plasmons”, Nano Lett. 15, 6208-6214 (2015).

Metasurfaces are ultrathin flat surfaces that produce abrupt changes in a light beam over a sub-wavelength length-scale. The 2D metasurface also enables the fabrication of tunable optical elements, making them attractive for the design of reconfigurable photonic devices. To demonstrate the concept of a tunable metasurface, we have developed several mechanically tunable Berry-phase metasurfaces operating in the visible frequency using a stretchable polydimethylsiloxane (PDMS) substrate. We first demonstrate that the anomalous refraction angle and therefore the optical wavefront can be continuously tuned by mechanical stretching of the substrate. We then demonstrate proof-of-concept prototypes of tunable metasurfaces with potential real-world applications: a 1.7× ultrathin flat optical zoom lens and a switchable metasurface hologram.
The tunable metasurface structures composed of an array of Au nanoantennas with different orientation angles are designed and optimized using finite-difference time-domain simulations. The designed metasurfaces are first fabricated on a Si substrate using HSQ/PMMA bilayer e-beam lithography and e-beam evaporation of Au and then are transferred to a PDMS film. The fabricated metasurfaces are optically characterized for various stretch ratios. In a tunable metasurface with a constant phase gradient, measured anomalous refraction angles agreed well with calculations using the generalized law of refraction and successfully demonstrated that the refraction angle of the metasurface can be tuned from 11.4° to 14.9° by changing the stretch ratio of the substrate.
We designed, fabricated, and optically characterized the first ultrathin flat stretchable metasurface zoom lens, proving that our tunable metasurfaces can control an optical wavefront for practical applications. By reconstructing captured light intensity profiles of the transmitted beam with the opposite circular polarization, longitudinal beam profiles are obtained. The results showed that the focal length of the lens increases from 150 to 250 μm as the stretch ratio changes from 100% to 130% successfully demonstrating the optical zoom function of the flat lens.
We will also discuss our efforts to design a mechanically switchable hologram as another example of tunable metasurface operating in the visible region. To obtain the phase profiles required for the switchable hologram, a phase-only computer generated hologram was designed with a multi-plane hologram generation technique and a metasurface that produced the obtained phase profile was fabricated and transferred to a PDMS film. Experimentally, the metasurface hologram switches the image when the PDMS film is stretched or relaxed by a certain amount.
Tunable metasurfaces demonstrated in this work will be useful for a variety of applications in information technology, biomedical sciences, integrated optics, optical communications, imaging, flat displays, and wearable consumer electronics.

Alternative plasmonic materials have attracted considerable attention due to their advantages compared to conventional noble metals, including CMOS compatibility, wide tunability of optical and structural properties, reduced optical losses and high-temperature resilience. In particular, CMOS-compatible transparent conducting oxides (TCOs), including indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), and gallium-doped zinc oxide (GZO) have shown promise for ultrafast integrated electro-absorption modulation and nonlinear optics applications. Under applied voltage, the permittivity of TCOs can be tuned by actively adjusting the carrier density in the accumulation layer at the gate dielectric/conducting oxide interface, which makes these materials respond to applied electric signals with controllable absorption modulation. In this work, we discuss the synthesis of widely tunable TCO materials and experimentally demonstrate an electrically tunable device that enables dynamic electrical control of the free carrier concentration in ITO active layers, thus inducing a significant change in its dielectric permittivity. Different device configurations will be analyzed and discussed. Based on these results, we propose a high-confinement hybrid plasmon waveguide ITO modulator and demonstrate large extinction ratio (ER) (>6dB) with a small device footprint (length<1.5µm) at optical communication wavelength. The engineering of ITO-based modulation devices will provide new opportunities for optoelectronic applications in ultrafast Si-compatible plasmonic materials, tunable metasurfaces and absorbers for signal modulation, wavefront manipulation, sensing, and energy harvesting.

Nanostructures and metamaterials incorporating semiconductor quantum dots (SQDs) and metal nanoparticles (MNPs) have been intensively studied. Such hybrid structures take advantage of physical properties of different materials, which can be useful for chemical and biological sensor, solar calls, optical and optoelectronic devices. For instance, self-assembled InAs SQDs are excellent light emitters due to localized excitons, suitable for lasing, single photon sources and other applications. At the same time silver MNP can enhance local electromagnetic field and provide good coupling of electro-magnetic waves with localized plasmon excitations.
In this contribution we report on experimental realization of a coupled plasmon-exciton system of As MNPs and InAs SQDs.
The InAs SQDs were produced using Stranski-Krastanow growth mode on GaAs substrates by molecular-beam epitaxy. We formed 5 layers of InAs SQDs separated by 5 nm thick GaAs spacers. As a result we produced vertical stacks of InAs QDs with lateral size of about 20 nm. The upper layer of SQDs was capped by 3nm-GaAs/3nm-AlAs/4nm-GaAs layer sequence. A thin silver layer was deposited in a vacuum chamber on the top of SQD-containing wafer. A subsequent annealing transformed the Ag film into a system of isolated silver nanoparticles with size and density depending on the annealing conditions. So, the systems of Ag MNP with sizes from 20 to 100 nm were formed above the layer of buried InAs QDs.
Optical reflection, absorption and photoluminescence spectra were studied to document the optical properties of plasmons in Ag MNP and excitons in InAs and reveal their interaction. The optical plasmon resonance appeared to be centered near 1.4 eV. It showed a large inhomogeneous broadening due to substantial size dispertion in the MNP system. The InAs SQDs showed a strong emission near 1.1 eV with 0.07 eV in width. An enhancement and spectral transformation of this emission gave clear evidence of interaction of SQD excitons with MNP plasmons.

We study the thermal excitation of surface plasmons on graphene nanodisks. Using a semi-analytical model based on the electrostatic approximation, fast modeling of near-field interactions between two parallel disks enables the evaluation of radiative heat transfer for different geometric configurations and material properties. We also calculate the plasmonic waveguiding properties of graphene disks chains, which show long decays lengths for the dipolar modes. The analysis provides non-dimensional solutions that can be extended to a large number of cases. We then evaluate the potential of plasmonic waveguide modes as heat carriers by determining the effective thermal conductivity of the chain. Our results demonstrate the potential of graphene surface plasmons as radiative heat carriers, which, combined with their tunability, opens up new opportunities for the development of thermal switchers, thermophotovoltaic devices, and near-field transducers.

The ability to confine light to nanometer length scales using plasmonic nanoparticles has led to disruptive, paradigm-changing, ideas including lenses that image below the diffraction limit, transformational optics, conformal antennas and nanoscale lasers. Yet, creating these intricate nanostructures while simultaneously providing high-throughput (~Avogadro’s number) for material applications remains challenging. Here we experimentally demonstrate by using an electrostatic-based molecular assembly approach how to controllably concatenate gold nanorods end-to-end into discrete dimers, forming a capacitively coupled plasmon (CCP) resonance along the long axis of the dimer. Irradiating these suspensions with femtosecond laser pulses, at the CCP dimer resonance wavelength, selectively welds only the CCP dimers together, bridging the nanorods with gold nanojunctions, producing large, high-quality yields of welded dimers [1]. Macroscale absorbance measurements reveal a charge transfer plasmon (CTP) resonance arising from these welded dimers with relatively large peak magnitudes and small full-width-at-half-maximum. We show by controlling the aspect ratio of the welded dimers that the CTP resonance wavelength can differ significantly from a single nanorod with similar aspect ratio, demonstrating the ability to modulate the effective depolarization factor of the dimer structures [2]. Three-dimensional finite element simulations were also carried out showing the CTP absorbance wavelength is inert to changes in the contact point connecting the dimers and relative orientation. [1] J. Fontana et al, ACS Photonics 3 (5), 904-911 (2016). [2] J. Fontana and B. R. Ratna, Applied Physics Letters 105, 011107 (2014)

Plasmons in graphene nanostructures cover the important mid-infrared wavelengths ranging from a few to tens of microns and have been explored extensively. However, graphene nanostructures usually exhibit a weak and narrow absorption peak which limit their applications in mid-infrared light manipulation and detection. Here, we study the coupling among different graphene plasmonic modes. We show that by engineering the coupling, we can not only enhance the light-graphene interaction, but also further achieve large absorption across a wide desirable wavelength range. We develop a theory to account for the observed coupling behaviors and also guide the design of different coupled plasmonic structures. Leveraging the concept of plasmonic coupling, we realize a hybrid 2-layer graphene nanostructure which absorbs 5 to 7% of the mid-infrared light within 8 to 14 microns (~ 700 to 1300 cm^-1), covering the important atmosphere “infrared transmission window”. Thus, such coupled hybrid graphene plasmonic structures may find applications in sensing and free-space communications.

Acknowledgement:
We thank the Office of Naval Research (N000141410565) and the National Science Foundation (ECCS 1552461) for the support of this work.

Optical sensors measure frequency shifts of resonances caused by the environmental changes, and require interactions of light with the target material over long distances to accumulate large enough phase change that translates into the measurable frequency shift. However, a phase of light is a cyclic variable, which is undefined at the point of complete destructive interference (a point of complete darkness), and varies rapidly in the vicinity of this point. The vortex optical powerflow around phase discontinuities increases the light-matter interaction distance without increasing the device footprint. Accordingly, light interference within a nanostructure that is accompanied by fast phase variations can be used to improve the sensitivity of bio(chemical) sensors with optical transduction.
We demonstrate through modeling and experiment planar multilayered photonic-plasmonic structures that are designed to exhibit zero reflection due to the excitation of Tamm plasmon modes on the interface between the metal surface and a one-dimensional photonic crystal. The existence of Tamm plasmon states is protected by the topological properties – manifested as geometrical Zak phases – of the PhC optical bands rather than by the Jordan’s theorem as for effective-index-matched metamaterial perfect absorbers. We will discuss the use of Tamm plasmon sensors for non-contact temperature-monitoring. We will also demonstrate a new optical sensing scheme based on the singular-phase detection, which offers superior sensitivity to refractive index and temperature changes. The Tamm plasmon sensors can be easily designed to operate at a chosen frequency, can be composed of a variety of plasmonic and dielectric materials, are amenable to fast and large-scale fabrication, and have high tolerance to fabrication imperfections.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering Award No. DE-FG02-02ER45977.

Metasurfaces, which are equivalent to photonic phased array antennas consisting of very thin metal and/or dielectrics, have been intensively developed to demonstrate various functions such as one directional beam bending, achromatic focusing, and chirped pulse compression etc. If the thickness of the metasurface is thin enough, those functions could be coherently controlled by dual facing normal input beams. If the standing wave of their interference makes an antinode or node at the center of the metasurface, the function can be coherently emphasized or transparentised, respectively. Therefore, if we apply it for a beam bending device, it works as a very thin linear optical switching element. However, the beam bending efficiency (BBE) is limited as low as 10 – 20% in conventional metasurfaces due to the intrinsically small interaction with the thin metasurface and the polarization conversion coming from the asymmetric antenna design. On the other hand, reflection type high-efficiency metasurfaces have been also developed by putting a spacer and an optically-thick mirror underneath the metasurface. The mirror generates the interference between the input and reflected beam, and also eliminates the transmission ports. As the result, BBE is drastically improved up to 90%. However, such a metasurface with thick mirror cannot be controlled coherently.

In this paper, we propose and demonstrate the coherent-controllable beam bending metasurfaces with keeping high-efficiency by optimizing the spacer and mirror thicknesses. BBE could be maximized up to 52% with the silica spacer and gold bottom mirror thickness of 65 nm and 32 nm. For the design of the metasurface itself, we employed a trapezoid shape antenna array for easy fabrication. And we have also confirmed that the BBE is kept > 50% under ±5% of fabrication errors. The device was fabricated on a quarts substrate through the e-beam evaporation for the gold bottom mirror, electron cyclotron resonance sputtering for the silica spacer deposition, e-beam lithography, and lift-off process for the metasurface layer. The fabricated device was measured by using free-space optics to perform the dual facing normal input with various relative phase differences. Consequently, we have observed the intensity modulation of the bended beam output with the phase difference. 48% of BBE and 7.8 dB of the extinction ratio have been evaluated under the symmetric input. This is the first experimental demonstration of the coherent control against beam bending metasurfaces. Because of the asymmetricity of this device, ER can be improved with the asymmetric input up to > 50 dB. This device can also work as a specific optical AND gate operating with the combination of an amplitude bit (0, 1) and a phase bit (0, pi). For instance, the combination of (1, 0) only exhibits ~1 as the output, otherwise ~0.1 – the binary contrast can be > 10 dB which exceeds the half mirror case of ~ 6 dB. This work was supported by CREST, JST.

Coupling between surface plasmons and excitons in organic media can result in strong light-matter interactions, resulting in the formation of hybridized "plexcitonic" modes. To-date, most studies have investigated plasmon-exciton coupling between dyes or J-aggregates and either localized surface plasmon resonances (LSPRs) using a core-shell geometry, or surface plasmon polaritons (SPPs) via adsorption of dye molecules onto planar metallic thin-films. The optical response of J-aggregates can be described by a Lorentzian line shape, comprised of a single oscillator with a very sharp linewidth and a small high-frequency relative permittivity. For many applications, including plasmon-enhanced photovoltaics, light-emitting diodes, waveguiding, and SPASERs (i.e., surface plasmon amplification by stimulated emission of radiation), coupling between surface plasmons and excitons within conjugated polymers is of great interest. Contrary to J-aggregates, the optical response of a typical conjugated polymer has a broader line shape with multiple vibronic features and a relatively large high-frequency relative permittivity. We have previously demonstrated that significant light-scattering can occur red-shifted from the absorption edge of various conjugated polymer-coated plasmonic metasurfaces (Petoukhoff and O'Carroll, Nature Communications, 6, 2015), which we attributed to electromagnetic coupling between the optical transitions of the absorber and the scattering modes, called "Absorption-Induced Scattering". Many reports on plasmonic-enhanced photovoltaics have described the largest absorption enhancements within broadband absorbers occurring at wavelengths just red-shifted from the absorption edge of the absorber. Here, we computationally investigate the coupling between conjugated polymers and LSPRs using a conventional core-shell geometry. We employ spherical Ag nanoparticles as the core, and vary the parameters of the surrounding conjugated polymer shell. We model the optical response of the conjugated polymers using a summation of Lorentzian oscillators, where the variables include the shell thickness, resonance wavelength, linewidth, number of oscillators, and high-frequency relative permittivity. We demonstrate that strong plasmon-exciton coupling can be achieved for thick shells (greater than 25 nm thick), and for large, yet reasonable oscillator strengths with large high-frequency relative permittivities. We compare our model to relative permittivity values for real conjugated polymers and show that, within the strong coupling regime, splitting between the hybridized modes of more than 1 eV can be achieved, and, for a sufficiently large high-frequency relative permittivity, the low-energy hybrid mode dominates the scattering response.

A novel technique for the selective photochemical synthesis of silver (Ag) nanoparticles (NPs) on ZnO nanorod arrays is established by combining ultraviolet-assisted nanoimprint lithography (UV-NIL) for the definition of growth sites, hydrothermal reaction for the position-controlled growth of ZnO nanorods, and photochemical reduction for the decoration of Ag NPs on the ZnO nanorods. By controlling the UV-irradiation time, Ag NPs of various size distributions can be selectively deposited on ZnO nanorods. Due to the small structure mismatch between the (0002) planes of ZnO and the (111) planes of Ag, and also the preferential adsorption of citrate ions on the (0002) surface of ZnO, the tip of the ZnO nanorods is more populated with Ag NPs. Epitaxial growth of Ag NPs is also observed on the sidewalls of ZnO nanorods. Based on our experimental results, the photochemical mechanism for the formation of Ag NPs on ZnO nanostructures by photochemical reduction in the presence of a Ag precursor solution is proposed. In addition, tuning the ratio of the visible emission to the UV emission of ZnO nanorods can be realized by varying the UV irradiation time, i.e., varying the resultant size contribution of Ag NPs on ZnO nanorods. The ratio of the visible emission to the UV emission of the ZnO nanorod arrays was increased by 20.5 times by introducing Ag NPs by photochemical reduction. From the results of finite difference time domain (FDTD) simulations, a strong field enhancement can be observed on the Ag NP. This apparently shows that Ag NPs act as efficiently radiative antennas coupling incident light to surface plasmons (SPs). The enhancement of the visible emission is believed to associate with the SP effect of Ag NPs. The Ag NP-decorated ZnO nanorod arrays show significant SP-induced enhancement of yellow-green light emission, which could be useful in optoelectronic applications. The technique demonstrated here offers an alternative approach to the Ag NP-decorated ZnO nanorod arrays, and it is compatible with low temperature fabrication (120 °C and lower) on flexible substrates. Also, the attached Ag NPs can be further functionalized and conjugated with various molecules for tuning broader properties. The Ag NP-decorated ZnO nanorod arrays may find applications such as various chemical and biological sensors.

For many catalytic reactions, the Au/TiO₂ interface acts as an active site. It exhibits an extraordinary high activity for low-temperature catalytic combustion, partial oxidation of hydrocarbons, hydrogenation of unsaturated hydrocarbons, and reduction of nitrogen oxides and so on. In order to contribute to the understanding of the catalyst, we investigate the electronic state of the Au/TiO₂ (320) interface by second harmonic generation (SHG) technique. SHG is a well-established surface-specific probe of centrosymmetric media. In the dipole approximation, SHG is forbidden in the bulk of a medium having inversion symmetry, while at the surface inversion symmetry is broken and SHG is allowed. As the surface steps show noncentrosymmetric behavior, Au/TiO₂ steps should generate a SHG signal. The target of this study is to detect this signal. We fabricated an Au thin film on a stepped TiO₂ (320) substrate in a UHV chamber at a pressure of 2x10^-7 Torr. The thickness of the film was 2 nm. The azimuthal angle and polarization dependent SHG intensity from Au/TiO₂ (320) interface and bare TiO₂ (320) were observed by using both 1064 nm and 532 nm wavelength pulsed laser light. When using 1064 nm, we found isotropic response from both the Au/TiO₂ (320) interface and bare TiO₂ (320). This behavior is dominated by the X⁽²⁾₃₁₁ and X⁽²⁾₃₂₂ nonlinear susceptibility elements. Here, 1, 2 and 3 represent [2-30], [001] and [320] directions respectively. When using 532 nm, we found anisotropic behavior from both Au /TiO₂ (320) and bare TiO₂ (320). For Au deposited TiO₂ (320) sample, the Pin-Pout SHG pattern showed clear anisotropy to the [2-30] direction. The bare TiO₂ (320) sample also revealed anisotropy. This anisotropic behavior is dominated by X⁽²⁾₁₁₃ nonlinear susceptibility element. The anisotropic behavior was only found using wavelength 532 nm as incident light. The SHG intensity patterns from Au/TiO₂ (320) interface and bare TiO₂ (320) surface had different anisotropic behavior and their dominating nonlinear susceptibility element χ⁽²⁾ were also different. It is thought that a resonance between the electronic states of the Au covered step with the energy of the 266 nm near UV second harmonic photon may be responsible for our observations, and that these electronic states may be responsible for the catalytic activity of Au/TiO₂.

Metamaterials, artificially fabricated materials composed of sub-wavelength unit cells, have enabled unprecedented and extraordinary phenomena such as invisibility cloaking, negative refraction and super-resolution imaging. Especially working frequencies of metamaterials are determined by the size of unit cells called meta-atoms. By shrinking the size of meta-atoms, the operating frequency of metamaterials have moved to higher frequencies. Now, thanks to nanometer scale fabrication technologies, we can fabricate "optical metamaterials" working at near-infrared (NIR), visible, or ultra-violet (UV) regions. So far, most of the optical metamaterials have been fabricated on 2D substrates in forms of planar nanostructures called metasurfaces. Various interesting physics and applications of metasurfaces have been reported, but there are several drawbacks arising from their own 2D structures, such as anisotropic and inhomogeneous properties, polarization dependency and incident angle dependency. Those issues are critical obstacles for practical applications of metamaterials and metadevices. In order to tackle these problems, we are now fabricating a full 3D nanometer scale structures using electron beam lithography (EBL) overlay process. In this abstract, I will discuss our efforts in realizing 3D metamaterials and metadevices for practical applications. First, I will present the method of high-accuracy EBL overlay process, which exhibits alignment errors below 20nm and 60% yield in each overlay process. Second, I will present designs and fabrication methods of two 3D metamaterials. One is 3D stacked metamaterials for asymmetric transmission of linear polarization like optical isolator. The other one is 3D chiral induced negative index metamaterials working at optical frequencies. Finally I will present design and fabrication method of 3D metadevices, which are mode-multiplexed nanophotonic modulators and waveguide system. The mode-multiplexed modulators can suggest new strategy for enhancing functionality per area in the integrated photonic and optoelectronic devices. I believe our efforts for making a full 3D metamaterials and metadevices will lead to practical applications of metamaterials in optics, nanoscale science and engineering.

References
1. N. Liu, H. Guo, L. Fu, S. Kaiser, H. Schweizer & H. Giessen, Nature Materials 7, 31-37 (2008)
2. S. Zhang, J. Zhou, Y.S. Park, J. Rho, R. Singh, S. Nam, A. K. Azad, H.T. Chen, X. Yin, A. J. Talyor & X. Zhang, Nature Communications 3, 942 (2012)
3. B. Kante, Y.S. Park, K. O`Brien, D. Shuldman, N.D. Lanzillotti-Kimura, Z. J. Wong, X. Yin & X. Zhang, Nature Communications 3, 1180 (2012)
4. Z.G. Dong, J. Zhu, J. Rho, J.Q. Li, C. Lu, X. Yin & Xiang Zhang, Applied Physics Letters 1001, 144105 (2012)

In this work, we revisit and developed smart chemical sensing platforms utilizing graphene-mediated surface-enhanced Raman scattering (GERS). The graphene-family nanomaterials (GFN) include conventional mono-/bi-, few- and multi-layer graphene, reduced graphene oxide (rGO) and mesoporous 3D graphene foam. These sensing platforms will be designed such that GFNs will be decorated with Ag and Au nanoparticles for methylene blue (MB) and Rhodamine 6G (R6G) molecular detection in parameter space of a. varying nanoparticle diameter and b. number of layers for varying concentration of these chemical molecules. The results will elucidate that these noble metal nanoparticles significantly enhance cascaded amplification of SERS effects on more than one layer, multilayer graphene and mesoporous 3D graphene foam as compared with mono- and bilayer flat graphene supports. We determined enhancement factors for both the AgNPs/GFNs and AuNPs/GFNs and the chemical molecules, which is found to be three to four orders of magnitude larger than those of NPs directly on Si substrates. In addition, the sensitivity of the sensors could be tuned by controlling the size of NPs and the layered architecture. The highest GERS was found to be for NPs of size ~30-40 nm and for multilayer graphene and mesoporous 3D graphene foam. Moreover, the sensors are capable of detecting these chemical molecules over a wide range of concentration to sub nM to 100 microM. Therefore these AgNPs/GFNs and AuNPs/GFNs are highly promising SERS substrates for chemical molecule detection with ultrasensitivity and also tested for generic bio-diagnostics. We gratefully acknowledge financial support from NASA KY EPSCoR, NSF KY EPSCoR, Gatton Academy and WKU Research Foundation grants.

How to obtain an ultrasensitive, uniform, stable surface enhanced Raman spectroscopy (SERS) response is always a bottleneck for practical application of SERS. It has been widely recognized that the SERS phenomenon is mainly based on the laser excitation inducing localized electromagnetic field enhancement (the area with high electrical field was called ‘hotspots’) on the surface of roughed or patterned noble metals. Among reported metal patterned structures, the nanogaps are ideal nanostructure to produce high ‘hotspots’ and so has gained much attention. However, the fabrication of sub-10 nm features meets the limits of top-down nanofabrication and has been a challenge. Photolithography is limited by the diffraction of light with only advanced deep UV reaching sub-100 nm features. Electron beam lithography can typically produce features only on the order of tens of nanometers. To overcome these limitations, various methods have been developed, such as glancing angle deposition, focused ion beam milling, electromigrated break junctions and mechanically break junctions, which have their respective advantages and also with some problems in large-area fabrication, stability and repeatability.
This work has developed a technique way that advances the mechanically break method in order to produce nanojunctions with the width of the nanogap well controlled below 10 nm. This mechanically break method can realize the large-scale, stable and repeatable fabrication for the nanogaps. First, a single electron-beam lithography (EBL) step was used to define the pattern of the nanojunction array on a (100) silicon substrate with a 200 nm low-pressure chemical vapor deposition silicon nitride. Then the pattern was transferred into the silicon nitride film, creating a nanobridge array of 100 nm wide by 250 nm long junction connecting two pads. After the other part of silicon nitride film being etched out, the junction mechanically broken and formed a nanogap with sub-10 nm in width due to the tensile stress, in which the control of nanogap is dependent on the width and length of the nanobridge. Final, the metallization was carried out by magnetron sputtering of a 10 nm Ag directly to the nanojunction with a proper nanogap. As-fabricated metalized nanogap arrays can be used a perfect SERS substrate, and here a water soluble protein cytochrome C is chosen as a detected material. The distinguished Raman signal from ultra-low concentration of protein molecules elucidate that the preponderance of the nanojunction with nanogaps in plasmonic based detection.

We demonstrate the tunability of the dielectric function of titanium nitride (TiN) by analyzing nitrided titanium dioxide (TiO₂) films. Anatase phase TiO₂ nanoparticle films were created by spin-coating and annealed to decrease air voids. These films were then nitrided by flowing ammonia gas at 1000^oC for 0, 2, 4, or 6 hours. All treated samples show the change in X-ray diffraction (XRD) pattern from tetragonal anatase to cubic phase, indicating conversion from TiO₂ to TiN. X-ray photoelectron spectroscopy (XPS) sputtering shows atomic concentration of nitrogen approximately equivalent to titanium. Atomic concentration values of nitrogen up to 41% for 6 hour treatments are observed and remain fairly constant through the entire depth of the film. Using spectroscopic ellipsometry, we observe a change in the real permittivity over a range of wavelengths. For 0 and 6 hour treatments, the real permittivity changes from 3.7 to –1.36 (at 750 nm). We also nitride patterned TiO₂ gratings and multilayer structures and demonstrate their structural integrity after treatment. TiN has potential use for many plasmonic and metamaterial applications as a stable, tunable material.

The chemical industry relies on heterogeneous catalytic processes to manufacture products, such as fuels and fertilizers, critical to sustaining human life and development. Current catalytic reactions like carbon dioxide hydrogenation are almost exclusively driven by thermal energy. High operating temperatures allow practical reaction rates; however, the high thermal energy input ultimately deteriorates the catalyst lifetime due to sintering. The implementation of plasmonic metal nanoparticles with strong light absorption capabilities is a strategy to efficiently harness light energy and create hot carriers to drive chemical reactions. Rhodium nanoparticles are promising materials for photocatalysis due to their versatile catalytic properties and excellent tunable plasmonic properties. However, major drawbacks include its rarity and low absorption ability. To overcome these limitations, the overgrowth of rhodium on gold nanoparticles creates bimetallic nanostructures exhibiting a columnar rhodium surface for increased light absorption. Our focus on plasmonic nanoparticle design and synthesis, as well as mechanistic studies of plasmonic photocatalysis in CO₂ hydrogenation help to demonstrate a new catalytic platform.

The Cu-Zn system has long been studied as the prototypical representative of a binary system that satisfies the Hume-Rothery rules, however, there is less understanding of the connection between the crystal structure, electronic structure and optical properties in these materials. We consider here whether this system contains compositions suitable for application in plasmonic devices. Several phases are observed in Cu-Zn system: ranging from a-brass (up to 39 wt.% Zn), and then, at successively higher Zn contents, β' , γ, δ , ε and, finally, η- phase (basically just Zn). These phases have different crystal structures, and hence different electronic structures and different dielectric functions. A range of alloys were manufactured in both bulk and thin film forms and then the dielectric functions were determined using ellipsometry. In general, as Zn is increased the optical absorption edge is blue-shifted. This causes the reddish colour of pure Cu to be changed through the bright yellow of brass and then through a series of grey-silver hues as the content of intermetallic compounds increases. The measurements reveal that dielectric function of the β' alloy is quite suitable for plasmonic applications and further work on this material would be worthwhile.

In line with the Marcus theory (R. Marcus, J. Chem. Phys., 1956), local dielectric permittivities control the rates of chemical reactions, since they affect the polarization energies of the reacting and surrounding molecules. We have recently demonstrated that the rates of chemical reactions, e.g. photo-degradation of the p3ht polymer, can be influenced by non-local dielectric permittivities in the vicinity of metallic films and lamellar metal-dielectric hyperbolic metamaterials (V. Peters et al., Scientific Reports, 2015). At this time, we study the effect of the vicinity to metallic films on the rate of the chemiluminescence reaction involving (i) PMMA polymeric films doped with Bis(2,4-dinitrophenyI) Oxalate (DNPO) and Rhodamine 6G dye (R6G) and (ii) vapor of 0.1% H2O2 in H2O2-EtOAc. We demonstrate that the metallic (Ag) film separated from the PMMA:DNPO:R6G film by ~35 nm of MgF2 reduces the rate of the chemical reaction. This finding as well as the above-mentioned p3ht result are in line with the recent report (Lee et al., arXiv:1510.08574, 2016).

The growing prevalence of diabetes in the global population has made detection of insulin in vitro and in vivo an extremely important problem in diabetes therapy and research. A label-free sensing platform that could provide a fast, easy, and quantitative way to quantify insulin can reduce the amount of insulin used to treat diabetic patients, prevent life-threatening hypoglycemic episodes, save unnecessary lab expenses, and accelerate diabetes research and drug discovery. As a potential way to address this need, we have utilized a gold nanoparticle (Au-NP)-coated zinc oxide (ZnO) nanowire substrate and quantified insulin directly adsorbed to the surface using surface-enhanced Raman spectroscopy (SERS). The concentration of successfully quantified insulin ranged from 1 mM down to 10 nM, which were within the physiologically observed values.
The SERS substrate was prepared using a hydrothermal method. To grow ZnO nanowires (NWs), we applied ZnO-texture-seed solution onto a silicon (Si) substrate, annealed at 350 °C for 0.5 hour, and immersed the substrate in a ZnO-NW precursor at 95 °C for 2.5 hours. Next, to decorate the grown ZnO nanowires with Au-nanoparticles (Au-NP), we dipped the substrate in a Au-NP precursor at 90 °C for 2 hours. This Au-NP creation process was repeated about ten times to maximize the uniformity of the coverage and the Raman enhancement.
For measurements, we used purified human recombinant insulin from Sigma-Aldrich. The substrates were then incubated in insulin solutions of various concentrations prepared in phosphate-buffered saline (PBS), rinsed with DI water, and dried using an air gun. Raman spectra were obtained using the Renishaw inVia Raman Microscope. We set the wavelength of the laser at 785 nm, and the laser power delivered to the sample was 0.93 mW. Each spectrum captured was integrated for 100 seconds. The Raman intensity increased linearly as the concentration of insulin solution was varied from 10 nM to 10 µM. Even down at the 10-nM insulin concentration, we were able to clearly track the Raman peaks, and the SERS enhancement factor characterized at this concentration was experimentally calculated to be 5×10⁵. For insulin concentration above 10 µM, the intensity of Raman-emission peaks started to saturate, and this was believed to originate from the surface crowding caused by the high concentration level of the analyte molecules. We are currently in the process of improving the measurement speed and accuracy, and distinction between insulin and other biomolecules that may be present in the background. In addition, we are also looking into electromagnetic and chemical enhancement mechanisms responsible for the observed insulin SERS.

Gold nanoparticles (AuNPs) play a central role in modern nanotechnology research in the fields of bio-medicine, sensors, electronics, fuel cell and catalysis. We present a facile, one-step synthesis of AuNPs at room temperature with a novel tetrathiafulvalene (TTFV) compound, which simultaneously acts as reductant, stabilizer and redox active functional group decorating the AuNPs. The TTFV-AuNPs show apparent surface enhanced Raman scattering (SERS) activity and unique redox properties when used as electrode materials. We also demonstrate that the physical, chemical and electrochemical properties of the TTFV-AuNPs can be easily adjusted by tuning the functionality of the TTFV reagent. For example, carboxylating the TTFV greatly enhances its solubility in water, allowing the synthesis to be carried out in aqueous solution and subsequently produce water-dispersible AuNPs. The carboxylation also renders the TTFV-AuNPs a pH controlled self-assembly through the intermolecular H-bonding between the carboxylic acid groups at the interfaces of AuNPs.

Using numerical simulations, we demonstrate that the dipolar plasmonic resonance of a single metallic nanoparticle inserted in the core of a dielectric waveguide can be excited with higher order photonic modes of the waveguide only if their symmetry is compatible with the charge distribution of the plasmonic mode. For the case of a symmetric waveguide, we demonstrate that this condition is only achieved if the particle is shifted from the center of the core. The simple and comprehensive analysis presented in this contribution will serve as basis for applications in integrated nanophotonic/metamaterials devices, such as optical filters, modulators and mode converters.

Tip enhanced Raman scattering (TERS) amplifies the intensity of vibrational Raman scattering by employing the tip of a probe interacting, in ultra close proximity, with a surface. Although a general understanding of the TERS process is still to be fully elucidated, scanning tunneling microscopy (STM) feedback is often applied with success in TERS to keep a noble metal probe in intimate proximity with a noble metal substrate. Since such STM TERS is a common modality, the possible implications of plasmonic fields that may be induced by the tunneling process are investigated. In addition, TERS of a 2D resonant molecular system, a MoS₂ bilayer crystal and a 2D non-resonant, lipid molecular bilayer are compared. Data with multiple excitation wavelengths and surfaces for the resonant system in the near- (TERS) and far-field are reported. An interpretation based on weak coupling interactions within the framework of conventional resonance Raman scattering can explain the observed TERS enhancements. The non-resonant molecular lipid system, on the other hand, requires strong coupling for a full understanding of the reported observations.

Double gyroidal gold nanostructures are engineered from self-assembled templates such that interconnectivity is maintained in three dimensions over distances of 10-100 microns. These structures are synthesized by first converting a self-assembled polymer blend into an ordered porous ceramic template. (Susca, et. al. Chemistry of Materials 28.7 2016: 2131-2137) These templates are then seeded with Au nanoparticles. Methods to achieve homogenous distribution of seeds will be presented. Under common reaction conditions, periodic precipitation occurs in the form of Liesegang rings. Liesegang rings are pervasive across fields from geology to crystal growth in gels, but we believe this is the first report of them by 3D electroless templating. After a rational change of reaction conditions, Liesegang ring formation is prevented and homogenous growth of seeds across hundreds of microns can be accomplished. The 3D plasmonic metamaterials that are attained from this synthesis contain two interwoven, highly-branched, chiral networks separated by ~10 nm gaps. The networks are expected to be highly coupled to each other and may result in bulk negative index materials at optical frequencies. (Hur, et. al. Ange. Chem. Int. Ed. 50 2011: 11985-11989) We will present preliminary studies of plasmon resonance on polished metasurfaces made from this material. Early evidence suggests broadband absorption, two-photon photoluminescence, and a plethora of non-linear optical phenomena. (Petros, et al. Physical Review Applied 2.4 2014: 044002)

The combination of Plasmonics with amplified ultrafast lasers is of great promise to the field of nonlinear plasmonics, metamaterials, and novel biomaterials. Ultrafast regeneratively amplified laser pulses of 35-fs duration, and with energies up to 7.5 mJ are used to excite localized surface plasmon resonances (LSRP) in synthesized gold nanorods, initiating nonlinear effects and morphological changes. The resultant materials are investigated using UV-VIS and Near-infrared spectroscopy (NIRS), Dynamic Light Scattering (DLS), and electron microscopy (EM). Our experimental findings indicate that alteration in morphology is accompanied by significant changes in the observed plasmon resonances. The morphology and spectral alterations depend on the peak power. Simulations suggest inhomogeneity in the spatial heat maps. The findings provide support to an observed preferential orientation in the initial stages of the morphological changes in the gold nanoparticle systems, and in materials processing by ultrafast lasers for a new generation of materials of interest for functional plasmonics.

Transparent conductive oxides (TCOs) have emerged as alternative plasmonic materials in recent years to replace noble metals. TCOs have unique advantages including CMOS compatibility, tunability of optical and structural properties, reduced optical losses and they can withstand high temperatures compared to noble metals. In recent years, TCOs have been proposed as a novel and promising platform for linear and nonlinear nano-photonics applications. In this work, we demonstrate that post-deposition annealing of Indium Tin Oxide (ITO) thin films in oxygen atmosphere leads to the tuning of their Epsilon-Near-Zero (ENZ) point throughout the near-infrared and short-wavelength mid-infrared (i.e., from l=1 mm to 4 mm) bands with reduced losses. In this talk we will discuss the synthesis of ITO thin films with tunable ENZ wavelengths and the fabrication and characterization of 2D periodic arrays of ITO nano-disks, which exhibit tunable plasmonic resonances in the mid-infrared range from l= 4.0 µm to 10 µm. Our findings demonstrate the critical importance of post-deposition annealing treatments, as well as the power of the sputtering process, to provide both the tunability of optical properties from near- infrared to mid-infrared and the reduction of the optical losses in ITO thin films and plasmonic structures. The ability to engineer and tailor Si-compatible ITO nanostructures with tunable plasmonic resonances in the mid-infrared can open novel opportunities to achieve a more efficient photonic-plasmonic integration of active devices and metamaterials on the CMOS compatible platform.

Optical nanoantennas and nanoantenna arrays are highly innovative approaches for optical signal transmission. Transmitting the signal via a free-space link (power law signal decay) instead of plasmonic waveguides (exponential signal decay) allows realizing low-loss optical communications links without sacrificing deep sub-wavelength field confinement at the transmitting and receiving points. This is a promising route for reconciling the size mismatch between diffraction-limited integrated photonics and integrated electronics. Control of the phase difference of the signal that drives the elements in an array of optical nanoantennas gives an additional degree of freedom beyond the optical properties of the individual elements to engineer the radiation pattern.

Here we report on realizing optical wireless links using phased nanoantenna arrays [1,2]. To realize optical antenna links operating at the near-infrared wavelength λ = 785 nm we fabricate plasmonic nanoantennas using electron beam lithography and embed the structures in a homogeneous dielectric environment. To observe transmission of optical power from the transmitting nanoantenna array we position fluorescent nanoparticles or deposit a thin film of fluorescent material around the nanostructure. The fluorescence is excited by the light transmitted by the antenna array allowing us to map the intensity distribution around the structure under study. In this way we have realized steerable wireless nanoantenna links where we can route the transmitted power to different receivers depending on the phase of the signal driving the elements of the array. Additionally, we have demonstrated polarization dependent unidirectional radiation where circularly polarized light is emitted in opposite directions depending on the handedness of the polarization state. These structures are a promising route to realizing reconfigurable low-loss wireless links between nano objects.

[1] D. Dregely et al., Nat. Commun. 5, 4354 (2014).
[2] K. Lindfors et al., ACS Photonics 3, 286 (2016).

Gradient metasurfaces enable a large degree of control over reflection and transmission of light, while using low-profile and low-loss patterned surfaces. Here, we demonstrate near-unity back reflection with a gradient metasurface tailored for wavelength of 700 nm and incoming angle of 35.7 degrees. The experimental work is based on the Huygens-Fresnel principle for tailoring light scattering at will through engineered phase fronts. The optimized geometry is capable of efficiently redirecting s-polarized light into the first negative diffraction order, towards the source and over a broad angular range and bandwidth. The theoretical geometry was experimentally realized over a 2x2 mm² area, with angle-resolved optical measurements confirming the high predicted efficiencies.
The metasurface consists of a non-symmetric, subwavelength, 1D grating comprised of two high-index (n=2.26) TiO_x bars in each unit cell (periodicity of 605 nm), placed on a Ag mirror, and is fabricated by electron beam lithography. Quantitative optical measurements show that an absolute coupling efficiency of s-polarized light to the first negative diffraction order above 85% is achieved at 700 nm for +/-(20 – 60) degrees. For the Littrow configuration (700 nm, 35.7 degrees), we measured 88% efficiency. The efficiency is primarily determined by the quality of the evaporated mirror, with the unpatterned Ag layer producing nearly identical absorption losses.
A remarkable property of this device resides in the broad operational bandwidth, which can be explained by the non-resonant nature of the TiO_x scattering elements comprising the metasurface. Simulations show that the metasurface functions over an extremely broad wavelength range (490 – 940 nm) with a coupling efficiency above 50% for angles between +/-(24 – 51) degrees. Another interesting feature is the angularly symmetric optical response of the asymmetric surface, which stems from reciprocity considerations.
Based on this work, a 2D design was also developed. This design can enable highly efficient back reflection for non-polarized light. This approach leads to an interesting application for ultrathin CIGSe solar cells, enabling broadband light trapping leading to higher currents and overall efficiencies.

Metasurfaces are typically constructed with 2D arrays of ultrathin metal/dielectric nanostructures with delicately designed geometries to modulate the local phase and/or amplitude of the scattered light. Metasurfaces have attracted immense attention due to their various groundbreaking optical functionalities such as background-free wave plates, broadband vortex beam generation, dual-polarity planar metalenses, helicity-dependent surface plasmon polariton unidirectional excitation, holographic-related technologies and so on, showing the great flexibility of wavefront modulation and spin controlled phenomena.
Holographic multiplexing is capable of recording multiple images in the same area to increase the information capacity and make the optimum use of the space-bandwidth product. Unlike volume holographic recording systems with Bragg-based selectivity, holographic multiplexing based on metasurfaces is a quite distinct field. Here, by combining the synthetic spectrum method with a geometric phase metasurface that can generate a helicity dependent phase profile, we achieve hybrid channels for holographic multiplexing. We experimentally demonstrate circular polarization, position, as well as angle multiplexing in the visible and near IR range with pairs of superimposed images. Furthermore, the broadband wavelengths reconstruction abilities of such metasurfaces holograms are experimentally characterized. In addition, numerical analysis of the factors that affect the multiplexing capacity are presented. The metasurface holographic multiplexing may serve as a platform for large capacity optical data storage, pattern recognition, spatial-temporal filter and information processing owing to its unique advantages of parallel recording and plenty of multiplexing methods.

Metasurfaces are thin layers of 2D arrangement of subwavelength nanoantennas for local manipulation of electromagnetic waves. They have enabled a variety of superior functionalities in a flat and highly integrated manner, making them great candidates for replacing the conventional bulky optical devices. On the other hand, there is a strong need to tune the performance of existing devices without changing their physical geometries. The 2D nature of metasurfaces makes them naturally suitable to be transferred onto the existing devices and to modify their electromagnetic responses, especially when the surfaces are not flat.
Here, we study the all-dielectric conformal metasurfaces for light manipulation at visible range. The metasurface inclusions are silicon thin disks embedded in a flexible polydimethylsiloxane (PDMS) layer. They show desired characteristics in terms of high efficiency, strong field localization, and weak angular dependence compared to their plasmonic counterparts and other dielectric inclusions. Engineering of the fundamental electric and magnetic dipole modes by tuning the disk diameter provides 360 degree phase shift for either transmission or reflection mode. The conformal metasurface composed of such disk-shaped nanoantennas with space-variant diameters is designed to coat a commercial cylindrical lens for spherical aberration compensation. The results show substantial reduction of the beam width and the beam depth at the focusing spot. In addition, it is also designed to wrap around a rough metallic surface to cloak any 3D objects under it at 532 nm. The aforementioned functionality is achieved by locally reconstructing the wave amplitude and phase with proper disk inclusions to mimic reflection from a flat ground plane. An efficient modeling technique based on the field equivalence principle is developed to solve large arrays of metasurface nanoantennas and on conformal platforms. The proposed metasurfaces can be easily scaled to other visible frequencies and for various desired applications, as will be discussed in detail.

We demonstrate plasmonic metasurfaces fabricated by an AFM tip-induced pressure on a single material. By appropriately utilizing the phase diagram of samarium sulfide (SmS), a strongly correlated or quantum material system, we are able to controllably “write” the insulator to metal phase transition with ~20nm resolution, resulting in golden metallic metasurfaces on the optically black semiconductor and a large spectral weight shift in the infrared. By altering the lithographic spacing and metallic fill fraction, we are able to control the response from the far infrared to the visible spectral region. This technique, which has never before been reported, opens the possibility of photonic devices with incredibly broadband and tunable responses.


We take advantage of the large spectral weight shift and orders-of-magnitude conductivity change that accompanies the phase transition in a correlated material system to create metamaterials comprised of a single material rather than using costly and time-consuming fabrication methods to apply a good metal to a dielectric or correlated material substrate. By selecting a material with a relatively low pressure-induced phase transition, we are able to utilize the precision, control, and ease of an AFM to lithographically write the metasurfaces. The patterns are then characterized with near-field and far-field spectroscopic methods over a wide range of frequencies to demonstrate their functionality.

Recent developments in optical near-field spectroscopy have enabled measurements with optical resolution significantly below the diffraction limit (λ/1000), providing access to the metal/semiconductor phase separation on the nanoscale. By probing the local (~20nm) response of the metasurfaces, we are able to characterize the conductivity change directly induced by the AFM lithography – verifying we are indeed able to precisely control the phase transition and the resultant far-field response.

The approach described in this work can easily be extended to other pressure sensitive material systems such as Mott insulators and high Tc superconductors.

Optical properties of dye molecules and plasmonic nanostructures can be drastically modified by the molecule-plasmon interactions in the hybrids. It has been known that spontaneous emission of dye molecules near a plasmonic nanostructure can be enhanced through the plasmon-enhanced electric fields or plasmon-induced resonance energy transfer (PIRET). Surface plasmon resonances (SPRs) in the plasmonic nanostructures can be tuned by molecular switching. Such molecule-plasmon interactions have been harnessed for various functional hybrid devices for applications in biosensing, information processing and light harvesting.

Herein, we report our recent progresses in exploring the molecule-plasmon interactions for two types of applications. One focuses on using Au nanorods (AuNRs) as donors in the resonant energy transfer (RET) for biosensing and bioimaging applications. Using single-nanoparticle scattering spectroscopy, we have demonstrated that AuNRs in the AuNR-dye hybrids can serve as donors to excite the fluorescence of dye molecules as acceptors. Due to their large absorption cross-section and overlap between the optical absorption and emission peaks, AuNRs enable RET with an incident light of the longer wavelength and lower intensity than the dye-molecule-based donors do. Therefore, bioimaging and biosensing techniques based on the PIRET can obtain high signal-to-noise ratios and reduce the photodamage to organelles at the targeted areas. The other application is for the information processing. Specifically, we have realized the large modulation of hybrid plasmonic waveguide mode (HPWM) based on the hybrid molecule-plasmon nanostructures by harnessing the strong molecule-plasmon coupling. The HPWM features advantages of both plasmonic nanostructures for manipulating light at the nanoscale and of dielectric waveguides for low optical loss. Moreover, we have demonstrated the rewritable hybrid plasmonic waveguides with switching function where photoswitching the dye molecules modulates the propagation of HPWM in a large depth. The rewritable waveguides and switches will become integral components in the future rewritable optical circuits for novel applications.

Surface plasmon polaritons (SPPs), propagating electromagnetic waves bound to the surface of electron-rich materials such as metals, are potentially useful for on-chip applications such as integrated optical circuitry and sensing. However, their susceptibility to energetic losses have hindered their use in practical applications. Colloidal quantum dots (CQDs), with their high quantum yield, photostability, color-tunability, and strong emission and absorption, are an intriguing option as a gain medium for loss-compensation in plasmonic structures.

Exploiting the solution-processability of these materials, we use electrohydrodynamic NanoDrip printing as well as large-area dropcasting to place high-quality CdSe-based CQDs onto ultra-smooth Ag surfaces between lithographically defined plasmonic block reflectors. With ultrafast optics, we investigate the stimulated emission of SPPs by CQDs into stable plasmonic modes defined by the reflectors. We demonstrate SPP gain in both the conventional exciton and biexciton as well as higher-energy multiexcitons at higher-energies. Through modification of the geometry of our reflectors, we demonstrate the controlled extraction, amplification, and nanofocusing of the intense, coherent, and narrow-band SPPs generated by the plasmonic cavity. Our findings are highly favorable for the integration of colloidal nanomaterials as a whole into full-chip integrated plasmonics.

Plasmonic nanostructures can be excellent light-driven nanoscale heat sources. In this work, we investigate the details of such plasmonic heating by creating a new type of plasmonic thermistor based on different gold plasmonic structures (nanoholes and nanodisks). Devices are prepared varying the shape, size and distribution of the plasmonic structures and are characterized by optical and morphological techniques. Their capabilities as sources of heat are studied by means of experimental (optical/electrical/thermal measurements) and theoretical (Lumerical/COMSOL simulations) studies. The response of the devices under illumination shows a clear dependence on the type and concentration of the plasmonic features, which is discussed with respect to resonance conditions and optical absorption efficiency. The use of plasmonic nanostructures as heat sources opens a wide range of applications, including energy harvesting and photothermal catalysis.

Highly doped wide band gap metal oxides nanocrystals have recently been proposed as building blocks for applications as transparent electrodes, electrochromics, plasmonics, and optoelectronics in general. Here we demonstrate the application of gallium doped zinc oxide (GZO) and aluminum doped zinc oxides (AZO) nanocrystals as novel plasmonic sensors for the detection of hazardous gases including hydrogen (H₂) and nitrogen dioxide (NO₂). GZO and AZO nanocrystals are obtained by non-aqueous colloidal heat-up synthesis with high trasprency in the visible range and strong localized surface plasmon resonance (LSPR) in the near IR range, tunable with dopant concentration (up to 20% mol nominal). Thanks to the strong sensitivity of the LSPR resonances to chemical and electrical changes occurring at the surface of the nanocrystals, such optical features can be used to detect the presence of toxic gases. By monitoring the changes in the dopant-induced plasmon resonance in the near infrared, we demonstrate that GZO and AZO thin films prepared depositing an assembly of highly doped colloids are able to optically detect both oxidizing and reducing gases at mild (< 100 °C) operating temperatures. Trivalent dopants within ZnO nanocrystals enhance the gas sensing response compared to undoped ZnO. Moreover, improved sub-ppm NO₂ gas sensitivity is achieved by activating the sensors response through combined purple-blue light (λ=430nm) irradiation and mild heating at 75 °C

Topological Insulator-based devices can transport electrons/photons at the surfaces of the materials without any back reflections even in presence of obstacles. In this contribution, we showed that cyclotron resonance of semiconductors can be utilized in Topological Insulator-based devices to break the time-reversal symmetry for one-way edge modes propagation in THz frequency range. We used semiconductors in which we can achieve a cyclotron resonance frequency that is comparable with plasma frequency by applying a small external magnetic field. The required magnetic field with metallic-based structures is usually prohibitive. To demonstrate the concept, we proposed some devices based on Topological Insulator such as tunable power divider. We expect that these results lead to integrated devices with low-loss and high functionality based on topological effects.

The work presented is part of an ongoing effort to model nanostructure photonic behavior and surface plasmon coupling using a finite difference time domain (FDTD) method, with the goal of optimizing scintillator light extraction. This method is well-known and well-tested for modeling surface plasmon resonances and coupling effects. The simulations are designed to predict the enhancement that may be achieved using coatings of varying morphology and composition. Test specimens will also be fabricated to experimentally validate the predictions. Initial work focuses on modelling the near-field effects between a layer of SiO₂ structures etched onto the scintillator surface and then coated in a layer of metallic nanoparticles. The primary objective of this modelling step was and is to maximize the predicted outgoing light intensity first as a function of nanostructure and then as a function of metal nanoparticle type, shape, and distribution on the nanostructure.

In parallel, design of experiments is underway to validate the models. The out-going surface of commercially-available bismuth germanate (BGO) crystals will be coated with the prescribed SiO₂-nanoparticle layer via reactive ion etching (RIE) and electron beam deposition. The light output of the plasmon-enhanced BGO crystal under X-ray excitation will then be compared to that of an as-received crystal to quantify the improvement in performance. These BGO crystals are an ideal test material because they are relatively inexpensive and air stable, which greatly simplifies processing. The emission behavior of this composition is also well established, and so the light emission enhancement achieved can be easily quantified.

We report strong coupling of vibronic transitions of a crystalline organic emitter, Diindenoperylene (DIP), to surface plasmon polaritons on a silver substrate. Angle resolved reflectivity measurements in Kretschmann configuration show multiple avoided crossings in the system’s dispersion curve. The resultant hybrid modes are present between the E_0-0 and E_0-1 vibronic transitions and show a Rabi splitting of 93meV and 105meV respectively. The experimental results are compared to simulated dispersion of the system calculated using transfer matrix formalism. Such hybridization of vibronic transitions in molecular solids through surface plasmons presents an attractive platform for realizing systems with long range spatial coherence.

A variety of plasmonic nanostructures working as nanoantennas, which is capable of enhancing excitation and emission rates and controlling the polarization and directionality of emitters nearby, have been tailored. Remarkable enhancements of light emission properties of emitters such as organic dyes and II−VI and IV−VI semiconductor quantum dots (QDs) have been achieved utilizing nanoscale gaps in plasmonic structures, which can enhance the performance of the fluorescence-type bio-sensing and imaging. However, silicon (Si) QDs coupled to such structures have been scarcely studied in spite of their high compatibility with biological substances.
The purpose of this work is to demonstrate the enhancement and control of light emission from Si QDs by plasmonic nano-gaps. We propose a structure in which Si QDs are coupled to metal nanoparticle (NP) over the metal film (MNOF) configuration. The MNOF structure exhibits strong electromagnetic fields localized at the gap between a metal NP and a metal film.[1] Si QD monolayer placed at the gap works as a nano-spacer in MNOF structures and the emission properties are strongly modified by the coupling with the gap-modes.
In this work, we employ all-inorganic colloidal Si QDs that exhibit air-stable and size-tunable light emission (700-1400 nm) in solution and also in air. [2] A monolayer of Si QDs (3 and 4 nm in diameter) is formed on a Au film with amino-terminated surface. A citrate stabilized Au NP with 80 nm in diameter is placed on the QD monolayer for the formation of the MNOF structure. We first characterize the light scattering properties of individual Au MNOF structures with Si QDs of different sizes (3 and 4 nm). In 3 nm Si QDs, the scattering spectra of AuNOF peaked at 700 nm well overlap with the PL spectra of QDs. We find strong modification of the PL spectral shape and the significant enhancement of PL intensity compared to the case without Au NPs. The enhancement factor is estimated to be more than 1000 for Si QDs at the largest electric field region. The PL spectral shape is very similar to that of the scattering spectra and the width reduces by a factor of 2 by the coupling with the gap-mode. Au MNOF structures made from Si QDs with different sizes exhibit different properties. We will discuss in detail the mechanism of the coupling with numerical simulation results of the structure.
[1] C. Ciraci, et al., Science 337,1072 (2012), G. Akselrod, et al., Nature Phonics 8, 1, (2014)[2] H. Sugimoto, et al., J. Phys. Chem. C, 117, 11850 (2013)

Plasmonic effect of colloidal gold nanoparticles is investigated for enhancing the emission intensity of quantum dot light emitting devices. The gold nanoparticles synthesized by colloidal method have a plasmonic peak in the absorbance spectrum at 535 nm. The PL spectrum of the CdSe/ZnS has the emission peak at 540 nm. The gold nanoparticles were coupled with the CdSe/ZnS by mixing them in different ratios. The PL spectrum of the CdSe/ZnS quantum dots coupled with gold nanoparticles enhanced. This is due to the resonance between the localized surface plasmon of the nanoparticles and the emission of CdSe/ZnS quantum dots. In other words, the quantum yield of the quantum dots increased when coupled with the gold nanoparticles. Devices were fabricated with CdSe/ZnS quantum dots coupled to gold nanoparticles as the emissive layer and metal oxide charge transport layers. The electroluminescence of the device enhanced when quantum dots were coupled with the gold nanoparticles.

Noble metal nanoparticles (MNPs) and quantum dots (QDs) play an integral part in the development of next generation nanophotonic devices. The synergetic interaction between these plasmonic and excitonic nanostructures often modify the exciton formation and recombination dynamics of QDs. This study will summarize our earlier work directed towards elucidating the effect of plasmon resonance on multiexciton emission of single CdSe/CdS QDs near 120nm Au NPs coated with a dielectric layer of variable thickness. We observed a significant change in multiphoton emission behavior of single QDs when they are in near proximity to Au NPs. We also aim to describe here some of the current results of our continuing studies into incorporating the effect of excitation wavelength on the biexciton emission enhancement and photoluminescence lifetimes of single QDs when they are adsorbed onto Au surfaces separated by a 20nm dielectric alumina spacer.It was observed that the biexciton quantum yield of single QDs significantly increased accompanied by much faster photoluminescence decay when they were excited at wavelengths closer to the localized surface plasmon resonance of the Au NPs and the photon antibunching behavior of single QDs transformed into photon bunching state characterized by super-Poissonian statistics. An electrodynamics model was applied to quantitatively evaluate the degree of emission intensity enhancement/quenching as well as lifetimes and quantum yields of excitons and biexcitons. The findings provide valuable insight into manipulating the multiexciton dynamics of single QDs in presence of plasmonic nanostructures that can be implemented to transform multiple areas of quantum communication, photocatalysis and optoelectronic devices.

Light rarely interacts. To counter this, either the number or the strength of light-matter interactions should be enhanced. High-Q optical resonators achieve this by storing light for an extended period of time, which increases the number of light-matter encounters before light escapes. Micro-resonators achieve the same feat by concentrating light into small modal volumes (low-V), which increases the strength of light-matter-coupling. In plasmonics such light concentration is typically achieved using localized plasmon resonances (LSPRs). Both have disadvantages: High-Q optical resonators only work well with narrow emitters and have slow operation speeds. Plasmon resonances, particularly localized plasmons, are extremely lossy (Q~10), which results in broad resonances. Here, we combine the two approaches by exploiting the enhanced interaction from plasmonics while maintaining sharp resonances. To achieve this, we exploit highly reflective, plasmonic reflectors (>90% reflectivity) and confine propagating surface plasmon polaritons (SPPs) on a high-quality silver surface to form plasmonic Fabry-Pérot resonators. In particular, we design stable, aberration-corrected plasmonic resonators with high-Q plasmonic modes in the visible (Q>400 at 630 nm). Such low-loss stable resonators can provide efficient feedback for loss-compensating media such as quantum dots. In a demonstration, we integrate colloidal quantum-dots onto our platform to show on-chip amplification, adiabatic nanofocusing, and spasing in the visible domain. The flexible design and facile, wafer-scale fabrication of these low-loss resonators with propagating modes should allow us to proceed towards integrated and loss-compensated plasmonics.

Rare-earth doped upconversion nanoparticles, which convert near-IR light to visible light, have a potential for applications as wavelength conversion layers in solar cells and fluorescent-labels in bioimaging. However, the upconversion efficiency is usually not high enough for the practical applications due to the small excitation cross section and the low quantum efficiency of rare-earth ions. A promising approach to overcome the problem is utilizing the surface plasmon resonance of metal nanostructures. Recently, nanocomposites consisting of upconversion nanoparticles and metal nanostructures have been proposed. In order to maximize the upconversion efficiency of nanocomposites, the metal nanostructure has to satisfy several criteria. They should have two surface plasmon resonance peaks to achieve simultaneous field enhancements at the incident and emitted light wavelengths and the resonance wavelengths could be controlled by the structural parameters. Furthermore, the electric field should be enhanced in large regions of upconversion nanoparticles. As one of the metal nanostructures satisfying the criteria, we focus on a metal nanocap. A metal nanocap has several surface plasmon resonance modes due to the breaking of the rotational symmetry of metal nanoshell, and the resonance wavelengths can be controlled by the thickness and the coverage in a wide wavelength range.
In this work, we have succeeded in preparing Er and Yb codoped Y₂O₃ (Y₂O₃:Er,Yb) upconversion nanoparticles having Au nanocaps with different coverages. The structure of the nanocomposites, especially the coverage of Au nanocaps, were analyzed in detail by scanning transmission electron microscope (STEM) observations and energy dispersive x-ray spectroscopy. We measured light scattering spectra and upconversion spectra of individual upconversion nanocomposites. The scattering spectra were compared with the results of simulation using boundary element methods and the electric field distributions of resonant modes were analyzed. We studied the correlation between the surface plasmon resonance wavelengths and the upconversion properties for many single nanocomposites with different nanocap coverages and found that there is clear correlation between the upconversion enhancement factors and the shape of the nanocap. In optimized structures, upconversion enhancement of more than 50 folds has been achieved.

The interaction of swift electrons with a material can generate far-field electromagnetic radiation that is coherent to the evanescent field associated with the moving electrons. This effect is well-known as electron-induced emission. The analysis of various interactions between electrons and materials is a substantial source of inspiration for advanced electron microscopy and cathodoluminescence emission. Recent breakthroughs in artificially engineered metamaterials and nanotechnology manifest new opportunities to tailor the interaction of the electron with matter and explore novel optical devices. Here, we demonstrate that designer Babinet metasurfaces composed of C-aperture resonators offer a powerful control over the polarization state of the Smith-Purcell emission, which can hardly be achieved via traditional gratings. By coupling the intrinsically non-radiative energy bound at the source current sheet to the out-of-plane electric dipole and in-plane magnetic dipole of the C-aperture resonator, we are able to excite cross-polarized light thanks to the bianisotropic nature of the metasurface. The polarization direction of the emitted light is aligned with the orientation of the C-aperture resonator. Furthermore, the efficiency of the Smith-Purcell emission from Babinet metasurfaces is efficiently increased by 84%, in comparison with the case of conventional gratings. These findings not only open up a new way to manipulate the electron-beam-induced emission in the near-field region, but also promise compact, tunable and efficient light sources and particle detectors.

The optical properties of metallic nanoparticles with nanometre dimensions exhibit features
that cannot be described by classical electrodynamics. In this quantum size regime, the near-
field properties are significantly modified and depend strongly on the geometric arrangements.
However, simulating realistically sized systems while retaining the atomistic description re-
mains computationally intractable for fully quantum mechanical approaches. Here we discuss
our recent efforts on developing atomistic electrodynamics models that provides a unified de-
scription of bare and ligand-coated nanoparticles, as well as strongly interacting nanoparticle
dimer systems. We will discus implications of atomistic electrodynamics models for simulating
surface-enhanced spectroscopies.

Color filters, which selectively transmit or reflect light in visible range, have been utilized for imaging and display devices for several decades. Recently, structural color filters based on light-matter interaction in the nanoscale have been actively studied. Periodic structures made of metals or high index dielectrics may possess optical resonances induced by the morphology, and thus allow controlling wavelength selectivity vial structural designs. Though the majority of designs so far suggested rely on some form of resonances, metallic and dielectric based designs differ significantly in the nature of the resonance and resultantly possess different strengths and weakness. Most of the structural color filters made of metallic elements utilizes surface plasmon resonances. While several designs have been suggested, such as metallic nanohole array, metal-insulator-metal (MIM) strips and nanorod arrays. These filters all have broad spectral bandwidth and relatively low transmittance or reflectance due to intrinsic optical loss of metallic materials at visible wavelengths.
Using patterned silver nanostructures rather than gold that is commonly used in literature can reduce optical loss. However, due to oxidation of silver in air, additional passivation is needed. And the passivation of nanopatterned silver is not straightforward and the spectral characteristics would depend very sensitively on slight oxidation that may still result.
On the other hand, many dielectric-based designs rely on guided mode resonances (GMR). As there are high index dielectric materials with very low loss such as Tantalum Pentoxide, the optical loss of the system is negligible and its spectral linewidth can be made very sharp. But, resonance wavelengths for GMR filter is in general sensitive to incident angle, limiting its wide application.
Here we demonstrate a hybrid color filter, which is similar to GMR filters in terms of structure a slab waveguide with periodic perturbations but utilizes metallic thin film as the waveguide structure. The metal region is flat and the perturbing bumps are made entirely with a dielectric, allowing high angle tolerance and fabrication process reliability.
The resonance wavelengths and transmission-reflection spectra were calculated numerically by adopting Finite-Difference Time-Domain (FDTD) method. Additionally, particle swarm optimization (PSO) was used to obtain the optimal structural design.
Compared to GMR color filter, of which wavelength shift is over 50nm for the incident angle range considered, it show much higher angle tolerance, almost five-fold improvement.
With flat metal films rather than patterned metallic structures, the passivation is potentially much easier and more reliable. Moreover, the suggested color filter can be realized by single-step patterning process such as nanoimprint lithography. The simple process and high angle tolerance can facilitate commercialization of structural filters.

Weixiang Ye^1,2, Christina Lambertz¹, Ariadna Martos³, Andreas Henkel¹, Andreas Neiser¹, Torben-Tobias Kliesch⁴, Andreas Janshoff⁴, Petra Schwille³ and Carsten Sönnichsen<sup>1

1</sup>Institute of Physical Chemistry, University of Mainz, Duesbergweg 10, 55128 Mainz, Germany
²Graduate School Materials Science in Mainz, Staudingerweg 9, 55128 Mainz, Germany
³Department of Cellular and Molecular Biophysics, Max Planck Institute of Biochemistry, Am Klopferspitz 18, D-82152 Martinsried, Germany
⁴Institute of Physical Chemistry, University of Goettingen, Tammannstr. 6, D-37077 Goettingen, Germany
E-mail: [email protected]

Plasmonic gold nanorods are widely used as sensors to detect the changes in refractive index of the surrounding medium.¹ The small detection volume around gold nanorods allows to study local attachment/detachment of proteins with high precision on single nanoparticle level.^2,3 The measurement of these dynamics requires a combination of high time resolution and spectral precision, which we achieve with an optical dark-field spectroscopy setup, where the scattering spectrum of an individual gold nanorod is detected by a charge-coupled device (CCD) camera coupled to a transmission spectrometer.

We use single particle plasmon sensors to investigate the Min system proteins attaching and detaching from lipid coated gold nanorods with an unprecedented bandwidth of 100 ms time resolution and one hour observation time. The long observation reveals small changes of the oscillation period over time. Averaging many cycles yields the precise wave profile that exhibits the four phases suggested in previous reports. Furthermore, our results show coverage oscillations on top of a static background that is absent in fluorescence-based studies and is indicative of a layer of permanently bound proteins. Hence, our single particle plasmon sensor is an attractive tool for studying dynamic processes at the molecular scale as it enables us to access the development of the protein surface coverage over time without any manipulation of the proteins for fluorescence labeling.

References
[1] Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58 (1), 267.
[2] Ament, I.; Prasad, J.; Henkel, A.; Schmachtel, S.; Sönnichsen, C. Nano Lett. 2012, 12 (2), 1092.
[3] Zijlstra, P.; Paulo, P. M. R.; Orrit, M. Nat. Nanotechnol. 2012, 7 (6), 379.

Nanoparticles have a novel optical property applicable for biosensor, photo-induced catalyst, and photo-thermal therapy. In speculating the commercialized nanoparticles (gold, silver, etc), the scale-up manufacturing process is also faced with the heterogeneous activity of the nanoparticles. Thus, the homogeneous control and validation of nanoparticles are high-priority research. Recently, in-situ measurement at the single particle level is issued to probe the intrinsic heterogeneity and the optimized particle condition in evaluation process. Here, we focus on two subjects: synthesis of intriguing nanoparticle and correlative AFM/optical analysis for morphology-dependent activity.

The localized surface plasmon resonance (LSPR), which is one of the main optical properties, is drastically changed by the morphology, size, and composition of nanoparticles. To approach the single particle correlative analysis, we introduce the single particle imaging system including electron microscopy, correlative atomic force microscopy and dark-field hyperspectral microscopy for individual nanoparticles (~100 nm in diameter). We also investigate the nanoparticle activity using home-made photo-thermal reflectance microscopy (based on 4-bucket method, STD ~0.1 K) and super-resolution fluorescence microscopy (based on single molecule localization microscopy, FWHM ~18 nm). In summary, we figure out that the optical and thermal properties are coincidentally improved at specific condition such as pump laser, refractive index, and thermal diffusivity. The nanoscale mapping of catalytic activity shows that our nanoparticles have an advantage for high efficiency and homogeneous control.

Recently we developed magnetic-plasmonic Ag@FeCo@Ag core@shell@shell nanoparticles (NPs) for biomedical separation/imaging applications. The Ag@FeCo@Ag NPs were synthesized by combination of a polyol method and a multi-step hot injection method. Initially, each shell (i.e. FeCo intermediate shell and Ag outer shell) was thought to be formed in sequence when each precursor was sequentially added to the reaction solution. However, our observation indicated that the FeCo shell was formed after the Ag precursor for the Ag outer shell was injected. Hence it turned out that the formation mechanism of Ag@FeCo@Ag NPs is not as simple as we initially thought. In this study we illuminated the formation mechanism of Ag@FeCo@Ag NPs by comparing the synthesis results of Ag@FeCo and FeCo NPs. Based on the results, we elucidate the formation mechanism of Ag@FeCo@Ag NPs through two well-known phenomena: (1) competition between size focusing and Ostwald ripening and (2) size-dependent catalysis of Ag NPs. That is, the final injection of Ag precursor causes size focusing of the Ag cores resulting in monodispersed Ag cores having a larger size than the critical size, in which electrons can be transferred from polyol to Ag cores and Ag cores to Co or Fe ions, and then the FeCo shells were formed on the Ag cores uniformly. During the formation of the FeCo shell, Ag atoms were simultaneously incorporated in the FeCo shell and the surface segregation of Ag took place spontaneously, resulting in the formation of a Ag outer shell.

We demonstrate the optical modulation of gold nanocrystal@polyaniline (PANI) core/shell nanostructures with gold nanocube (AuNC) and gold nanorod (AuNR) cores by using two different stimuli of electrical potential and pH. A significant wavelength shift of the core localized surface plasmon resonance (LSPR) peak can be achieved with high reversibility and stability by changing the medium refractive index through an applied electrical potential or change in pH. In this system, the chemical structure of the PANI shells can be reversibly changed between emeraldine salt (ES), pernigranline base (PB), and leucoemeraldine base (LB) forms depending on the chemical environment, thus providing three different complex refractive indices. Furthermore, the PANI shell acts as an intrinsic spacer layer to the system and prevents undesirable plasmon coupling between cores even at high surface coverages. Up to a 24 nm LSPR peak shift is realized for AuNC cores with 37 nm thick PANI shells with an applied electric potential. Notably, a larger electrically induced 42 nm LSPR peak shift is realized for AuNR cores with only 8 nm thick PANI shells, and a huge LSPR modulation of 107 nm can be achieved for these nanostructures upon changing the pH from 2 to 11. These hybrid gold nanocrystal@PANI core/shell nanostructures with a high degree of modulation can be used for various applications including photovoltaics, nanophotonics, and optoelectronics.

We report on the optical properties of silver nanodisk (Ag ND) Langmuir Blodgett monolayers that were transferred to substrates in different coupling regimes. Ag NDs deposited in the liquid expanded-gaseous (Le-G) phase demonstrated individual plasmon resonance behavior while Ag NDs deposited in the liquid condensed-liquid expanded (Lc-Le) and solid-liquid condensed (S-LC) phases exhibited plasmon coupling between closely packed adjacent nanoparticles, which caused a red shift in the localized surface plasmon resonance (LSPR) spectra of the Ag ND monolayers. Atomic force microscopy (AFM) imaging showed the presence of excess polyvinylpyrrolidone (PVP) surfactant micelles on the Ag ND monolayer surfaces, which could be eliminated by first compressing the Ag ND monolayer to a high surface pressure. This step led to micelles depleting into the subphase, and the resulting transformation caused a blue shift in the extinction spectra of deposited Ag ND monolayers due to a decrease in the medium refractive index. Ag ND monolayers were then used in conjunction with a conjugated poly(3-hexylthiophene-2,5-diyl) (P3HT) medium to reversibly modulate the LSPR by changing the local refractive index around the nanoparticles. The highest reversible LSPR shift of 26 nm under an applied ±500 mV electropotential and a refractive index sensitivity (RIS) of 138 nm/RIU was found for monolayers deposited in the Lc-Le phase due to an increased presence of hot spots when compared with monolayers deposited in the Le-G phase. Monolayers deposited in the S-Lc phase exhibited lower RIS due to the onset of delocalized plasmon resonance behavior. This systematic study on LB-fabricated monolayers reveals the dependence of the LSPR peak shift and RIS of Ag ND monolayers on interparticle plasmon coupling and can pave the way for more sensitive and optimized assemblies.

Nobel metal nanoparticles are well known for their striking optical properties, and ordered nanoparticle arrays have been heavily studied in pursuit of nanoparticle-based devices. DNA-programmable assembly is a robust tool for synthesizing crystalline nanoparticle architectures that exhibit great tunability over nanoparticle size, shape, lattice structure, and crystal habit. Here, superlattice crystals composed of octahedral nanoparticles, and the corresponding crystals composed of spherical nanparticles, are studied to determine the effect of nanoscale structure on the far-field optical properties of the superlattices. Both spherical and octahedral nanoparticles can be assembled into body-centered-cubic superlattices with a well-defined rhombic dodecahedral crystal habit and their backscattering spectra measured and simulated. Both plasmonic and photonic modes are observed in such structures, and both can be tuned by controlling the nanoscale and microscale structure. Superlattices composed of octahedral nanoparticless exhibit polarization dependent backscattering; an effect that is not observed in analogous sphere-based superlattices. Electrodynamics simulations show that this polarization dependence is mainly from the anisotropy of nanoparticles, and is only manifested if the octahedral nanoparticles are well-aligned in the superlattice. This shows the robustness of DNA-programmable assembly, and the introduction of anisotropy by using nanoparticle building blocks further expands the platform of DNA-programmable assembly as a tool to build metamaterials.

Plasmonic metal nanostructures which exhibit a characteristic optical property have the potential to produce a new analytical technology and optical technology. There are many simulations of the electric field enhancement caused by the plasmon resonance, few research reports to observe the electric field enhancement directly. Comparison of the simulation and experimental results is essential. Thus we focus on the Kelvin probe force microscopy (KPFM), that can measure the surface potential on nanostructure. Using the KPFM, we may observe the electric field enhancement directly and may compare them to the simulation results.
In this study, incident-light wavelength dependence and structural dependence were examined on gold nanodisks (500 nm diameter and 30 nm thickness) prepared on Si substrate using electron beam lithography and lift-off method. Two types of array were prepared; one was “Regular array”, all distance between edge-to-edge of disks were 500 nm, and another was “Quadrupole array”, Quadrupole was comprised of four disks, the distance between edge-to-edge of disks were set to 50 nm in the x direction and y direction, and the distance between quadrupoles were set to 500nm in the x direction and y direction.
For each arrays, reflection spectra of the portion containing the gold disk were measured by UV-Vis spectra connected to optical microscope for a range of a circle with a 3 mm diameter and Si substrate was used for the reference. Also, KPFM measurements were carried out on each arrays at two conditions: unirradiated state and under monochromatic light irradiation.
From the UV-Vis spectra, broad dips were observed around 650 nm on both structure. And interestingly, strange behavior of surface potential was observed at 650 nm light irradiation on both structure. In addition, the surface potential of Regular array shifted to negative by light irradiation, on the contrary, the surface potential of Quadrupole array shifted to positive by light irradiation. These strange phenomena would be discussed at our presentation from the viewpoint of electron movement.

Magnetic-plasmonic hybrid nanoparticles (NPs) have attracted increased attention as an indispensable building block for future NP-based biomedicine such as hyperthermia treatment, magnetic drug targeting or magnetic immunodiagnostics. In this study, we chemically synthesized novel magnetic-plasmonic hybrid NPs, Au@FeCo core-shell NPs, which could be expected to be one of the best hybrid NPs for future biomedical applications based on their dual functional properties. However, the key issue for the practical application of the NPs is without doubt, to provide the NPs with robustness and durability. Although much research has been conducted so far, there is still plenty of room to further improve the long-term stability in terms of magnetic properties. Thus, in order to deepen our understanding on the chemical stability of these hybrid NPs, we studied the aging behavior of Au@FeCo core-shell NPs by systematically measuring physico-chemical properties of the NPs as a function of aging time using various analytical techniques including STEM-HAADF, UV-Vis, XRD, XPS and SQUID. This presentation focuses on the synthesis and chemical stability of the Au@FeCo NPs, and then is followed by a discussion on their magnetic-plasmonic characteristics.

Chiral gold nanoparticle superstructures have attracted much interest because of their potential applications in novel optical devices and chiroptical sensors. Assembly methods are required to i) construct these materials and ii) carefully tune their structural parameters in order to optimize properties. Peptide-based methods for controlling the assembly of nanoparticles into chiral superstructures have been developed.^1-2 These methods employ peptide conjugate molecules that consist of a gold-binding peptide (PEP_Au=AYSSGAPPMPPF) attached to an organic tail (e.g. aliphatic chain or π-conjugated molecule); these constructs associate to the nanoparticles and direct their assembly. In this work, we prepare a series of divalent peptide conjugate molecules, C_x-(PEP_Au)₂ (x = 14-22). We examine peptide conjugate assembly behavior using FTIR and CD spectroscopy as well as X-Ray diffraction; we identify important features of the soft assembly that can directly affect nanoparticle binding and nanoparticle size; we show how the peptide conjugates can direct the assembly of chiral single-helical gold nanoparticle superstructures. In particular, we study the effect that aliphatic tail length has on the assembly of the conjugates and ultimately the assembly and structural parameters (e.g. pitch, nanoparticle diameter and aspect ratio, etc.) of the gold nanoparticle single helices. We determine that the structure of the nanoparticle assembly can be directly correlated to the structure of the peptide conjugate assembly. Importantly, we show that fine adjustment of and control over peptide conjugate assembly allows for careful control over the structure of the chiral nanoparticle assemblies.

References
1. C.L. Chen, P. Zhang, and N.L. Rosi. J. Am. Chem. Soc. 2008, 130, 13555-13557.
2. C. Song, M.G. Blaber, G. Zhao, P. Zhang, H.C. Fry, G.C. Schatz, and N.L. Rosi. Nano Lett. 2013, 13, 3256-3261.

Attenuated total reflection (ATR) condition can be obtained with simple optical configuration using prism. Chemical biosensors based on surface plasmon spectroscopy under the ATR contidion have the advantage of high sensitivity, leading to label-free detection. Most of plasmonic sensor sytems by the ATR method have been arranged in Kretschmann configuration. In this case, surface plasmon polariton is excited at the interface between metal with a finite thickness and dielectric with an infinite thickness. The plasmonic surface is typically formed by deposition of metal thin layer onto glass and plastic plates with comparable refractive index to the prism. The glass and plastic plates need to be transparent to the incident wavelength in terms of the configuration. Otto configuration also provides the plasmonic field on metal surface. In this case, surface plasmon polariton is excited at the interface between dielectric with a finite thickness and metal with an infinite thickness. If the dielectric gap between the prism and the metal surface can be formed, surface plasmon polariton can be excited at the metal surface on opaque substrates.
Paper is a candidate of the substrate for chemical biosensing in Otto configuration. The advantages of the paper are cheap, light weight, easy handling, and combustibility, even though the surface roughness is larger than those of glass and plastic used in Kretchmann configuration. When biological samples are used as analytes, the used paper substrate can be easily incinerated without autoclave treatment. In this work, we demonstrated excitation of surface plasmon polariton using gold-deposited opaque papers in Otto configuration. Calendered paper made from wood pulp and artificial paper made from synthetic resin were used as the substrate. Gold layers with 100 nm of thickness were formed onto the papers by vapor deposition with an appropriate deposition rate. To form the dielectric gap, silica was deposited at the area except the center on a glass plate with a same refractive index as a prism. The reverse of the silica-depsited glass was coupled with the prism. The silica “flame” was coupled with the gold surface on the paper. The center area between the glass plate and the gold surface is the dielectric (air) gap. When the incident wavelength is 632.8 nm and thickness of silica is ca. 500 nm, excitation of surface plasmon polariton was observed by angular scan of reflectivity under ATR condition. When the dielectric gap was replaced from air to water, angular scan of reflectivity showed the shift of the resonance angle to higher angle. In addition, we successfully achieved molecular recognition between biotin and streptavidin on the gold surface chemically modified with a biotin derivative using an artificial paper.

We present a simple and robust crystal growth technique, which yields large area single-crystal films of silver ideally suited for fabricating plasmonic devices and high-finesse metamaterials. The silver films with thickness of 110 nm were grown on mica substrates at 500 °C by using a sputtering method. We confirmed the high quality of the silver thin films by measuring the surface roughness and the optical constants. The quality factor of the surface plasmon polaritons estimated from the measured optical constants is 6000 at the wavelength of 1 µm, which is about 20 times higher than the epitaxial single-crystal gold thin films on LiF substrates.¹

Plasmonic devices and metamaterials exploit surface plasmons which exist at metal-dielectric interface, for instance, a metal film in air. A film of silver or gold is generally employed for the plasmonic devices because they have an abundance of free electrons. Metamaterials are a class of artificial materials designed to interact with light in ways no natural materials can.^{2, 3} The response of the metamaterials is very sensitive to the presence of dissipative losses in the metallic resonators, especially in the visible and ultraviolet (UV) ranges. The several approaches to overcome the losses including the search for better plasmonic materials among metal alloys, heavily doped semiconductors and conductive oxides,^4-6 as well as direct compensation of losses by combining metamaterials with various optical gain media⁷ were reported. Those solutions, however, aim to minimize Joule losses, while in practice dissipation rates are often much higher than expected from the Ohm’s law. The additional drawback associated with surface roughness and grain boundary scattering due to polycrystalline nature of metal films was reported,⁸ and therefore employing single-crystals of silver can make a major contribution to the reduction of plasmonic losses.

We demonstrated that single-crystal gold/silver thin films epitaxially grown on LiF substrates had smooth surfaces with root mean square roughness less than 0.1 nm and nanostructured metamaterials fabricated with these films had strong resonant response in the near-IR spectral range.⁹ In this work, we have developed a single-crystal silver thin film growth technique in order to reduce the losses further and extend the response wavelength to the near-UV range. This silver growth technique allows us to employ inexpensive and low-loss plasmonic material in various applications.

References
[1] V. A. Fedotov et al, Optics Express, 20, 9545 (2012).
[2] V. G. Veselago et al, Nature Mat. 5, 759 (2006).
[3] N. I. Zheludev, Science 328, 582 (2010).
[4] D. A. Bobb et al, Appl. Phys. Lett. 95, 151102 (2009).
[5] M. G. Blaber et al J. Phys: Cond. Mat. 21, 144211 (2009).
[6] A. Boltasseva et al, Science 331, 290 (2011).
[7] K. Tanaka et al, Phys. Rev. Lett. 105, 227403 (2010).
[8] M. Kuttge et al, Appl. Phys. Lett. 93, 113110 (2008).
[9] T. Uchino et al, MRS Meeting, Boston (2015).

Photonic crystals (PCs) are periodically ordered microstructures, which exhibit distinct structural colour arising from the interaction of light with the dielectric and metallo-dielectric materials. Through rational design, and smart tuning of the PC periodicity, it is possible to tailor the colour exhibited by these materials.
The objective of this study is to design and assemble stimuli-responsive composite PC structures by self-assembly of polymeric and ceramic colloidal suspensions, in order to study their optical response and photonic band-gap tuning.
Based on the methodology reported by Hatton [1], silica inverse opals were produced by vertical self-assembly of polymer colloids. The resulting species exhibit angle-dependant coloration, which depends on the lattice spacing of the microstructure and can be selected using different sized polymer colloids (240-1000 nm). Additionally, it was found that coupling a metallic silver layer on the surface of this structure, by sputter coating, switches the optical response and enhances reflectivity.
The exhibited colour can be tuned by altering one or more physical parameters of the system, such as lattice spacing, symmetry, induction of defects, and refractive index contrast. Here, we show how the exhibited colour can be tuned by a change in the refractive index of one of the phases. For example, in a 240 nm spaced structure, colour changes from bright orange to dark blue when infiltrated with ethanol, which can be reversed by evaporation of the solvent.
The use of smart materials in photonic structures allows reversible tailoring of colour by external physical or chemical stimuli, such as temperature changes, solvent infiltration, application of an electromagnetic field, etc. These novel materials are promising systems for applications in optoelectronics and photonics, photovoltaics, optical sensing, and colour display devices [2].
In particular, thermo-responsive polymer gels such as poly(N-isopropylacrylamide) (PNIPAM) have been widely studied for tuneable photonic devices [3]. However, most of the research have focused on lattice spacing changes due to swelling/shrinkage of the polymer gel near the lower critical solution temperature (LCST). In this work, present our findings on the effect of refractive index change in the polymer gel phase of silica inverse opals infilled with PNIPAM, due to micro-phase separation of the polymer hydrogel when undergoing hydrophilic/hydrophobic transition at the LCST.
The ultimate goal for this project is to produce large-scale tuneable colour display devices for applications in sensors, adaptive camouflage and cloaking.

References:
[1] B. Hatton, L. Mishchenko, S. Davis, K. H. Sandhage, J. Aizenberg, Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 10354.
[2] J. Ge, Y. Yin, Angew. Chem. Int. Ed. Engl. 2011, 50, 1492.
[3] J. M. Weissman, H. B. Sunkara, a. S. Tse, S. a. Asher, Science (80-. ). 1996, 274, 959.

The geometrical parameters of metallic nano-structures determine their optical responses. It is appealing to think about a universal light device, in which the plasmonic modes at different frequencies are excited onto a large piece of nano-structure metallic network and harness the electromagnetic (EM) field. However, fabrication of such a metallic network is challenging because it demands fine structures at the nanoscale over a large-scale. Herein we report on a direct strategy to prepare pure, scalable nanoporous metallic networks by physical vapor deposition (PVD). Such nanoporous networks are made of interconnected pure metallic nano-size ligaments of about10-100 nm and multimodal connective nano-pores. Several metallic and metal-oxide networks have been fabricated among them Cu, Ag, Au, Al, Pt, Ti, Fe and TiO₂. The coinage metallic networks are colored and translucent. We characterize their opto-electronic responses, and demonstrate reduction process of molecules adsorbed onto the network due to generation of hot-electrons

Plasmonic nanostructures with a chiral shape, i.e. not superimposable on its mirror image, have been shown to exhibit strong chiral optical effects. Shapes that exhibit broadband far-field chiral optical properties are important for circular polarization devices. Circular dichroism (CD), the differential extinction between left- and right-handed light, is commonly used to characterize the far-field chiral optical properties of these nanostructures. Tailoring the shape of plasmonic nanostructures to maximize broadband CD is necessary for practical circular polarization devices but both experimentally time-consuming and computationally intensive.

Here, we show that the chiral coefficient¹, a purely geometric quantity, facilitates the rapid optimization of the shape of planar plasmonic chiral nanostructures for maximum broadband CD. The geometric parameters defining dimers, triangles, and L shaped plasmonic Al, Au and Ag nanostructures are rapidly optimized in minutes for broadband CD by maximizing the chiral coefficient.² Optimization using rigorous electromagnetic field simulations over the entire geometric parameter space would take significantly more computational resources and time. Rapid optimization of the nanostructure shape, using the purely geometric chiral coefficient, is attributed to reducing the dimensionality of the problem (e.g. 3D to 2D) and avoiding the wavelength dependance of electromagnetic simulations of highly dispersive plasmonic materials.

We will discuss the strong correlation between the geometric chiral coefficient and the integrated far-field chiral optical properties of various planar plasmonic chiral nanostructures.

References

(1) Gilat, G. J. Physics A: Mathematical and General 1989, 22, L545.
(2) Wilson, J.C. and McPeak, K.M. et al. in preparation 2016.

Metamatetrials, in particular metamaterials with hyperbolic dispersion have been shown to control the scores of physical phenomena including spontaneous and stimulated emission, van der Waals interactions, energy transfer and rates of chemical reactions. Many applications of metamaterials and plasmonic systems suffer from absorption loss in metal. The loss can be often offset by optical gain in an adjacent dielectric medium, provided by for example excited dye molecules or quantum dots. However, high concentration of dye molecules causes luminescence quenching, limiting the magnitude of the population inversion and the optical gain.
At this time, we study the effect of lamellar hyperbolic metamaterials, localized surface plasmon resonances in metallic nanoislands and metal/dielectric interfaces on luminescence quenching in thin polymer (poly (methyl methacrylate)) films doped with high concentrated HITC dye molecules. We experimentally show that vicinity of metal strongly inhibits the rate of the concentration quenching, which is determined by a combination of quenching center formation, energy transfer and enhancement of spontaneous emission. The full scope of the experimental findings and the discussion of the results will be presented at the conference.

Colloidal crystals made of silica sub-micron particles, synthesized by sol-gel process, were deposited on silicon wafers and glass substrates by Langmuir-Blodgett technique, which permits to control both the degree of compactness of the crystals and the number of the deposited layers. Monolayers of colloidal crystals were submitted to colloidal lithography, which consisted in dry etching of colloidal crystals by means of Ar⁺ ions accelerated at 500 eV. Colloidal lithography resulted in the pattern transfer of the colloidal crystals onto the underlying substrates by shadowing mechanism. Ion etching at oblique incidence introduces a direction of anisotropy in the pattern transfer. If the substrate is azimuthally rotated during the ion etching the structure gains a chiral patterning with a handedness that depends on whether the rotation is clockwise or counterclockwise. To promote plasmon resonance phenomena, the etched samples were coated with a thin layer of gold.
Polarimetric measurements were performed on the fabricated samples and a remarkable circular dichroism was observed in samples with a chiral pattern. Chiral samples with opposite handedness show mirror image circular dichroism and circular birefringence effects. Additional polarimetric measurements were made at several different angles of incidence in order to check for spatial dispersion effects in the nanostructures and to study their effect on the plasmon resonance and plasmon coupling. The morphology of the samples before and after etching was also observed by scanning electron microscopy (SEM).

Biochemical sensors based on plasmonic interferometry have shown unparalleled sensitivity and ultra-small device footprint. However, current architectures require a highly coherent, precisely aligned external light source which poses technical challenges for on-chip integration. Here, we demonstrate an alternative sensing scheme that eliminates the need for coherence and alignment by directly embedding an ultra-thin layer of broadband light emitters onto the surface of an array of plasmonic interferometers, consisting of nanoapertures and shallow grooves [1]. A fraction of the excited emitters in the nanoaperture can couple into surface plasmon polaritons (SPPs), which propagate toward the grooves and then are partially reflected back toward the nanoaperture to interfere with the directly emitted light. As a result, the fluorescence spectra transmitted through the subwavelength aperture of each plasmonic interferometer can be modulated by SPPs-mediated interference. By proper design of the grooves (i.e. width, depth, pitch and number), the spectral modulation depth can reach up to a factor of 2. Small changes in the refractive index of the dielectric material above the metal surface can affect the interference condition (i.e. the relative optical path), thus leading to a pronounced spectral modification of the fluorescence emission through the nanoaperture, and providing for a novel sensing scheme. The proposed architecture allows for spectroscopic fingerprinting of chemical analytes while eliminating the need for coherence and alignment of an external broadband light source, as well as periodic calibration of the sensors. More interestingly, such a sensing scheme allows for the open system detection, such as in a micro-droplet without using a microfluidic channel, which significantly increases the stability and simplifies the sensing system. This advantage may also help accelerate the rate of parallel target screening used in drug discovery, among other high volume and high sensitivity applications.
[1] D. Li, J. Feng, and D. Pacifici, “Nanoscale optical interferometry with incoherent light”, Scientific Reports, 6, 20836.

Their exquisite sensitivity to the local environment combined with their dramatic alteration of the local photonic mode density makes plasmon-resonant nanoparticles ideal candidates for use in stimulus-responsive materials with potential applications ranging from biosensing to smart optics. In this talk, we explore the synthesis and characterization of reconfigurable plasmonic metachromophores that change their structural organization, and hence their collective optical properties, in response to external stimuli such as light. We present a variety of approaches to actuate interparticle couplings in response to external triggers, including both photoswitch-modified DNA, and stimulus-responsive polymers, and explore ways to generate positive and negative feedback between the stimulus and response.

Single-molecule surface-enhanced Raman spectroscopy (SERS) offers new opportunities in exploring the complex chemical and biological processes difficult or impossible to comprehend using conventional ensemble techniques. However, precise positioning of a single molecule of interest within a hot spot, which enables its analysis at the single-molecule level, has been challenging. Here we present a novel strategy for locating and securing a target analyte at a specific site within a plasmonic nano-junction. The “smart” hot spot is generated by employing a thiol-functionalized cucurbit[6]uril (thiol-CB[6]) as a molecular spacer between a silver nanoparticle (AgNP) and substrate, and hence it can tightly grasp any target molecule conjugated with a strong binding motif, such as spermine, through unique host-guest chemistry of CB[6]. The position of the analyte can be further controlled by varying the carbon-chain length between the analyte and binding motif. Polarization-dependent SERS measurements and precise spatial control of the analyte allow us to reveal the orientation of the analyte and the enhancement factor distribution within the hot spot. The SERS enhancement is found to decrease approximately by one over the square of the distance from the center of the hot spot. The details of this work will be presented.

The viral membrane fluidity and liquid order has been shown to influence the infectivity. Liquid order of membranes is a function of lipid composition and especially the cholesterol content. Increasing cholesterol content enhances the membrane liquid orders and decreases its fluidity. Therefore, being able to measure the cholesterol content and membrane fluidity in a single-virus level would make us able to study the infection mechanisms with unprecedented accuracy. We have developed a new technique which takes advantage of the high scattering cross section of silver nanoparticles by temporally tracking them on the membrane of a virus. The rotational motions of nanoparticles attached to the virus membrane, and therefore their scattering polarization fluctuations over time, is correlated with the membrane fluidity. We have shown, through this technique and independently through a Laurdan assay, that changes in the cholesterol content of liposomes and virus-like particles, which result in changes in membrane fluidity, affect the polarization fluctuation signals. Here, I'll present our latest analyses of these results through autocorrelation decay time and rotational diffusion calculation for finding the viral membrane cholesterol content and fluidity.

Photothermal conversion based on plasmonic nanostructures has been explored to induce localized heating of the host medium, which is highly attractive for a broad range of applications including biomedicine, steam generation, optofluidics, and acceleration of chemical reactions. The heat transfer from a plasmonic nanoparticle to the host medium starts with the absorption of photons via localized surface plasmon resonance and conversion of part of the photon energy to heat, which is then transferred to the surrounding medium. With gold nanocages as the plasmonic nanostructures, we have demonstrated the use of photothermal conversion to induce phase transition in pNIPAAmco-pAAm copolymers from a hydrophilic to a hydrophobic state for controlled release. In a follow-up study, we further took advantage of the photothermal effect to induce a phase-changing material to change from a solid to a liquid state so drugs could be released from the melted phase via diffusion. Most recently, we demonstrated that plasmonic heating could be used to drive the phase transition from ferroelectric to pyroelectric in PVDF thin films and thus enbale the fabrication of near-infrared detectors. In this talk, I will cover both materials synthesis and device fabrication.

Super-resolution fluorescence microscopy is a powerful tool for biological research, but obtaining multiplexed images for a large number of distinct target species in whole cells and beyond remains challenging. Here we use the transient binding of short fluorescently labeled oligonucleotides (DNA-PAINT, a variation of point accumulation for imaging in nanoscale topography) for simple and easy-to-implement multiplexed super-resolution imaging that achieves sub-10-nm spatial resolution in vitro on synthetic DNA structures. We report a molecular counting approach, called qPAINT, that relies on the predictable kinetics of DNA hybridization to count integer numbers of spatially unresolvable targets. Finally, we demonstrate whole cell imaging using DNA- and Exchange-PAINT and optical sectioning, now allowing DNA-based super-resolution imaging deep inside cells, away from the glass coverslip.

Arranging functional nanoparticles into networks with deterministic pattern is of particular interest for diverse plasmonic and optical metamaterial applications. Herein, we utilize the unprecedented programmability of DNA origami to assemble gold nanoparticles (AuNPs) into networks of clusters, 1D chains, and 2D crystals, with tetragonal or hexagonal patterns. Silver is grown onto such as-organized AuNP networks to fabricate gold-silver core-shell nanoparticle networks with increased particle size, decreased inter-particle distance, enhanced plasmonic coupling, and thus emergent plasmonic properties, such as antiferromagnetism, as revealed by dark-field scattering microscopy and numerical simulations. We believe this work provides a general strategy toward fabricating complex plasmonic nanodevices, which hold great promise for a large diversity of applications.

Introducing plasmonic resonant scatterers in photovoltaic devices is a promising way to increase energy conversion efficiencies by trapping incoming light in ultra-thin solar cells. Colloidal plasmonic oligomers are obtained following a cost-effective self-assembly strategy and incorporated in organic-based cells produced using spin-coating techniques in ambient air conditions. An interesting increase is observed of both external quantum efficiency (EQE) and short-circuit current for solar cells loaded with plasmonic oligomers compared with reference organic cells with and without isolated gold nanoparticles. Theoretical calculations demonstrate that the wavelength-dependent EQE enhancement is a resonant process due to the increased scattering efficiency in plasmonic antennas allowed by a chemically controlled 1 nm nanogap. This method opens the way towards roll-to-roll fabrication of efficient plasmonic ultra-thin photovoltaic devices.

The nano-gap antennas are linked at a controlled distance of a few nanometers by Dithiothreitol molecules. The spacing molecules ensure a minimum distance that plays a fundamental role in the formation of intensity hot spots in the nanogap as well as large and red-shifted scattering peaks. This OPV device, realized in ambient air condition, exhibited an efficiency 14% higher than the reference one showing a relevant enhancement in the red part of the EQE measurements.

Francesco Pastorelli, Sebastien Bidault, Jordi Martorell, Nicolas Bonod, DOI: 10.1002/adom.201300363

Developing efficient energy storage systems remains one of the biggest challenges in the transition towards a fully circular economy. Many energy-storage processes rely on phase transformations of nanomaterials in reactive environments. Compared to their bulk counterparts, nanostructured materials exhibit fast charging and discharging kinetics, resistance to defects formation, and thermodynamics that can be modulated by size effects. However, in ensemble studies of these materials, it is often difficult to discriminate between intrinsic size-dependent properties and effects due to sample size and shape dispersity. Here, we use a wide range of in-situ transmission electron microscopy techniques to reconstruct the absorption of hydrogen in individual palladium nanocrystals exposed to increasing H₂ pressures [1,2]. Using electron energy-loss spectroscopy, we find that while palladium single crystals undergo sharp transitions between the metallic and hydrogenated states, multiply twinned particles load over a range of H₂ pressures. Furthermore, electron energy-loss spectrum-imaging of hydrogenated particles reveals how hydrogen atoms are excluded from regions subject to high compressive strains. Finally, by coupling dark-field imaging and electron diffraction, we are able to reconstruct the spatial distribution of hydride phases within a single nanoparticle. Our results highlight the importance of surface effects and structural defects in energy storage systems and provide a general framework for monitoring phase transitions in individual nanocrystals in a reactive environment.
[1] A. Baldi, T. C. Narayan, A. L. Koh, and J. A. Dionne, In situ detection of hydrogen-induced phase transitions in individual palladium nanocrystals, Nature Materials 13, 1143–1148 (2014)
[2] T. C. Narayan, A. Baldi, A. L. Koh, R. Sinclair, and J. A. Dionne, Reconstructing solute-induced phase transformations within individual nanocrystals, Nature Materials 15, 768-774 (2016)

Colloidal inorganic nanocrystals (NCs) are among the most exploited nanomaterials to date due to their extreme versatility. Research on NCs went through much advancement in the last fifteen years. New exciting directions have been uncovered recently through the development of plasmonic semiconducting nanoparticles and by the possibility to chemically adjust the density of free carriers in them. The present talk will highlight the recent progress by our group in the areas of advanced synthesis, assembly and in the study of chemical and structural manipulations in NCs (doping, ion exchange, etc.) with focus on materials absorbing in NIR. Applications range from laser hyperthermia, to electrochromic devices and heavy metal recovery.

The advancements to sustainable and environmentally benign chemical processes necessitate heterogeneous catalytic reactions with low operating temperature and preferential formation of targeted products. Current thermocatalytic reactions require a high operating temperature to overcome the large activation energy and perform reactions at practical rates. The high operating temperature not only demands high thermal energy input, but also deteriorates the catalyst lifetime due to the sintering of catalyst nanoparticles. To enhance reaction rates and reduce operating temperatures, plasmonic metal nanoparticles are demonstrated to be a new family of photocatalysts to couple light with chemical reactions and exhibit distinctly different characteristics from semiconducting photocatalysts, including the positive effects of increasing light intensity and temperature on the quantum yield of photocatalytic reactions. Hence, heterogeneous photocatalysis by plasmonic metal nanoparticles promises an alternative approach to enable important chemical reactions at reduced temperatures. We recently demonstrated the tunable plasmonic properties of rhodium nanoparticles. Together with their versatile catalytic activities, rhodium nanoparticles represent compelling candidates as plasmonic photocatalysts. This presentation reports our recent comparative study on rhodium and gold nanoparticles as plasmonic photocatalysts. Specifically, photocatalytic carbon dioxide hydrogenation is observed using both rhodium and gold nanoparticles, with reduced activation energies and product selectivity controlled by metal and illumination. Methane production is strongly and preferentially enhanced when rhodium nanoparticles are illuminated, while carbon monoxide is the exclusive product from gold nanoparticles whether illuminated or not. The different interactions between metals and reaction intermediates determine the product selectivity. The photocatalytic reaction on rhodium nanoparticles without external heating is highly selective to methane production and at a rate comparable to the thermocatalytic reaction rate at 350^oC, suggesting that plasmonic hot electron generation could represent a significant step toward sustainable and practical catalysis by reducing or eliminating heating requirements. This work emphasizes that both the optical and chemical properties of plasmonic metal nanoparticles are of critical importance to the understanding and control of their catalytic activity and selectivity. Rhodium nanoparticles, with their plasmonic properties and well-known capability to catalyze important chemical reactions, could open up new opportunities in plasmonic photocatalysis.

Scaffolded DNA origami has proven to be a versatile method for generating functional nanostructures with prescribed sub-100 nm shapes. Programming DNA-origami tiles to form large-scale 2D lattices that span hundreds of nanometers to the micron-scale could provide an enabling platform for diverse applications ranging from metamaterials to surface-based biophysical assays. Toward this end, here we design a family of hexagonal DNA-origami tiles using computer-aided design and demonstrate successful self-assembly of micron-scale 2D honeycomb lattices and tubes by controlling their geometric and mechanical properties including their inter-connecting strands. Our results offer insight into programmed self-assembly of low-defect supra-molecular DNA-origami 2D lattices and tubes. In addition, we demonstrate that these DNA-origami hexagon tiles and honeycomb lattices are versatile platforms for assembling optical metamaterials via programmable spatial arrangement of gold nanoparticles (AuNPs) into cluster and superlattice geometries.

Tailoring the light-matter interaction at the nanoscale is imperative for the field of plasmonics. This requires precise fabrication of structures few nanometers in dimensions with complex architectures yet void of imperfections at the nanoscale. Conventional top-down fabrication techniques lack the resolution, precision and flexibility necessary to realize these objectives. To achieve such structures, in this work, we utilize engineering of surfaces and interfaces to direct assembly of chemically synthesized nanostructured building blocks into larger functional entities employing an integrative bottom-up approach. In this scheme, chemical surface modifications of nanomaterials coupled with physical templating drive formation of atomically smooth plasmonic structures. This process is further facilitated by the use of externally applied optical, thermal and mechanical forces. Utilizing this approach, we have achieved three-dimensional structures based on metallic particles with nanometer-thin plasmonic junctions that can be electrically probed and activated. Through integration of physically reconfigurable materials we have also achieved mechanical tunability of these structures with controlled sub-nanometer motion. This results in mechanical tunability of plasmon resonance which we have employed to develop active plasmonic devices that can serve as on-chip switches and nanoscale metrology tools.

Degenerately doped metal oxide semiconductors exhibit plasmonic resonance at near and mid-infrared wavelengths tunable by varying their composition. Nanocrystals of many such materials have now been synthesized and applications are emerging that leverage the responsiveness of their localized surface plasmon resonance (LSPR) to electronic charging and discharging. We are developing methods to tune their LSPR spectra and thereby design plasmonic oxide nanocrystals deliberately for different applications, ranging from localized photothermal heating to enhanced spectroscopies for molecular detection. Such applications depend on the nature of the plasmonic modes and the damping of plasmonic excitations, characteristics which are also reflected in LSPR line shapes. For example, to maximize the potential for near-field enhancement, we must minimize electronic scattering. Our results indicate that engineering the dopant distribution within the nanocrystals and selecting dopants to minimize electronic hybridization with the conduction band are two effective strategies to this end, resulting in dramatically reduced LSPR linewidths. Tuning anisotropy of metal oxide nanocrystals offers another route to engineer their plasmonic modes, thereby tuning the spatial distribution and intensity of near-field hot spots surrounding the nanocrystals. We show that in semiconducting metal oxides, both shape and the underlying crystalline anisotropy collaborate to produce tunable multi-modal LSPR properties.

Deterministic placement and dynamic manipulation of individual plasmonic nanoparticles with nanoscale precision feature an important step towards active nanoplasmonic devices with prescribed levels of performance and functionalities at optical frequencies. Bottom-up self-assembly approaches, which often involve molecular recognition characters, offer large-scale fabrication, biochemical compatibility, and many other benefits. Among a variety of materials for self-assembly, DNA represents one of the most attractive building blocks largely due to its unprecedented programmability. In particular, the DNA origami technique allows for rational 3D organization of metal nanoparticles with nanometer precision, allowing for engineering complex plasmonic architectures with tailored optical response.
First, we demonstrate hierarchical assembly of plasmonic toroidal metamolecules, which exhibit tailored optical activity in the visible spectral range.¹ Each metamolecule consists of four identical origami-templated helical building blocks. Such toroidal metamolecules show stronger chiroptical response than monomers and dimers of the helical building blocks. We also demonstrate that given the circular symmetry of the toroidal metamolecules, distinct chiroptical response along their axial orientation can be uncovered via simple spin-coating of the metamolecules on substrates.
Next we show the creation of active plasmonic systems, in which plasmonic nanorods can execute directional, progressive and reverse nanoscale walking on two or three-dimensional DNA origami.^2,3 Such a walker comprises an anisotropic gold nanorod as its ‘body’ and discrete DNA strands as its ‘feet’. Specifically, our walker carries optical information and can in situ optically report its own walking directions and consecutive steps at nanometer accuracy, through dynamic coupling to a plasmonic stator immobilized along its walking track. We demonstrate that two gold nanorod walkers can independently or simultaneously perform stepwise walking along a shared DNA origami track.³
Our studies exemplify the power of plasmonics, when integrated with DNA nanotechnology for realization of advanced artificial nanomachinery with tailored optical functionalities.

REFERENCES
Urban, M. J., P. K. Dutta, P. Wang, X. Duan, X. Shen, B. Ding, Y. Ke and N. Liu, “Plasmonic Toroidal Metamolecules Assembled by DNA Origami,” JACS, Vol. 138, No 17, 5495-5498, 2016.
Zhou, C. M., X. Duan and N. Liu, “A plasmonic nanorod that walks on DNA origami,” Nat. Commun., Vol. 6, 8102, 2015.
Urban, M. J., C. Zhou, X. Duan and N. Liu, “Optically Resolving the Dynamic Walking of a Plasmonic Walker Couple,” Nano Lett., Vol. 15, No. 12, 8392-9396, 2015.

Single nanopores in thin membranes are powerful tools for studying single biomolecules without the use of labels. DNA and other molecules can be detected one by one as they traverse the pore and thereby modulate the pore’s ionic conductance. We have added plasmonic functionalities to these nanopores by positioning the pore right at the gap of a bowtie nanoantenna.¹ Based on light-induced local heating of the nanoantenna, we introduced the concept of plasmonic thermophoresis, which could assist the delivery of molecules to the sensor and enable measurements at lower concentrations.² More recently, molecular dynamics simulations showed that the optical hot spot of the antenna may be used to control the translocation process of a DNA molecule while optically reading its sequence. Combined with simplified fabrication based on plasmonic dielectric breakdown³ the concept may be developed into a new methodology for single-molecule DNA sequencing.⁴

References:
1. Plasmonic Nanopore for Electrical Profiling of Optical Intensity Landscapes
MP Jonsson and C Dekker.
Nano Letters 2013, 13, (3), 1029-1033

2. DNA Translocations through Solid-State Plasmonic Nanopores
F Nicoli, D Verschueren, M Klein, C Dekker and MP Jonsson.
Nano Letters 2014, 14, (12), 6917-6925

3. Self-Aligned Plasmonic Nanopores by Optically Controlled Dielectric Breakdown
S Pud, D Verschueren, N Vukovic, C Plesa, MP Jonsson and C Dekker.
Nano Letters 2015, 15, (10), 7112-7117

4. Plasmonic Nanopores for Trapping, Controlling Displacement, and Sequencing of DNA
M Belkin, S-H Chao, MP Jonsson, C Dekker and A Aksimentiev.
ACS Nano 2015, 9, 10598-10611

Control of near-field emitter coupling to surface plasmon polaritons (SPPs) is of interest for thin-film, edge-emitting light sources. Here, we investigate emitter-SPP coupling in insulator-semiconductor-metal-insulator (ISMI) waveguides, containing an organic semiconducting (i.e., conjugated) polymer film on a range of Ag metal film thicknesses, as a function of optical excitation pump power and collection polarization. The waveguide structure consists of a SiO₂ substrate, an Ag film of varying thickness (0, 35, 45, 50, 55, 65 and 100 nm), a 100-nm-thick polyfluorene (PFO) conjugated polymer film, and a SiO₂ superstrate.

The observed trends in the peak emission intensity, the threshold for amplified spontaneous emission, and the emission peak width of the semiconducting polymer emitter are attributed to variations in both Ag film thickness and roughness between the samples. However, plasmonic coupling evidence is obtained from analysis of the polarization-dependent spectra. Increasing transverse-magnetic (TM)-polarized emission peak intensity with decreasing metal film thickness is observed from ISMI waveguides at high excitation powers, suggesting more efficient emitter-SPP coupling for thinner Ag films when the polymer undergoes stimulated emission. The lack of a trend in TM-polarized peak intensity at low excitation powers suggests that SPPs are not propagating far enough in the sample to be emitted from the edge. Additionally, the transverse-electric (TE)-polarized emission peak intensities do not vary significantly among the samples, suggesting that emission coupling to photonic modes is not affected by metal film thickness. Furthermore, emission dichroic ratio values (defined as the ratio of TE to TM polarized emission) are analyzed to help minimize experimental variables. Emission dichroic ratios are larger when the polymer undergoes stimulated emission, compared to when it undergoes spontaneous emission, for all metal films apart from the thinnest (35 nm). This indicates that gain in the semiconducting polymer film reduces the extent of emitter coupling to SPP modes for thicker metal films. However, once the metal thickness is below a critical value, gain improves emitter-SPP coupling. These results are consistent with theoretical calculations which show that the dominant SPP mode exhibits a greater propagation length for thinner metal films, suggesting greater near-field overlap between the semiconductor film and SPP modes.

The design of ultra-thin, highly absorbing metallic nanostructures has become increasingly important for a variety of applications ranging from photo-detection to photo-catalysis. Indeed, it has been demonstrated that, despite their short mean-free paths (10-20nm), hot-carriers generated in a metal can be transferred to either an adjacent semiconductor or an adsorbed molecule leading to detectable photo-currents and measurable reaction products, respectively. Typically, plasmonic nanostructures are utilized for achieving broadband or narrowband, efficient light absorption at the small dimensions required for the efficient transfer of plasmonic hot-carriers¹. However, relying on nano-patterned designs, challenges the scalability of these structures for practical applications, in particular for solar-energy conversion devices.

Here, inspired by the Salisbury screen concept, we propose a different paradigm and we study light absorption in ultrathin (10-15 nm thick) planar metal films. Indeed, we demonstrate both theoretically and experimentally that broadband or tunable narrowband, angle insensitive near-unity light absorption is possible in unpatterned, ultrathin metallic systems which could then be easily upscaled.

Our absorbers are comprised of a low-loss noble metallic backreflector (we choose silver and gold), a subwavelength transparent dielectric (such as SiO2), typically 40-50nm thick, and an ultrathin (10-15 nm) metallic active absorbing layer. As a thin absorbing metal layer we consider several different materials, including Al, Ti, and Cu. We deposit such a layered structure with the help of e-beam evaporator, and characterize the grown films with AFM and ellipsometry. In an Ag - 80 nm SiO2 – 15 nm Cu we observe a near unity light absorption in a range of frequencies from 450 nm to 550 nm. For Au – 70 nm SiO2 – 10 nm Ti we find that over 90% light absorption is achieved from 350 nm to 900 nm. Finally for an Al film we observe a narrowband near-unity absorption that may be tuned across visible range of frequencies by controlling the thickness of the dielectric spacer. We explore theoretically the angle dependence and characterize the amount of light absorbed in the metal.

Our work opens new pathway for efficient light collection and enables a simple platform for photon assisted hot carrier harvesting.
References:
1. M. L. Brongersma, N. J. Halas, and P. Nordlander, Nature Nanotechnology 10, 25–34 (2015)

Optical resolution beyond the fundamental diffraction limit is highly anticipated in various fields such as electronics, biotechnology and nano-fabrication. The most reliable optical technique for high resolution imaging so far is near field scanning optical microscopy (NSOM) which provides a point-by-point image of the object. The technique is relatively slow and has its own complications due to the tip convolution function and mechanical movements. Here we propose the theory of a motion-free alternative high resolution imaging device using graphene gratings with tunable plasmonic resonance. In contrast to a mechanically moving sharp tip, we utilize a stationary graphene sheet with spatially variable surface conductivity profile via electronic gating. By virtue of diffraction and plasmonic resonances, the spatial conductivity profile enables conversion of evanescent spectrum decaying in the near-field into propagating waves. Surface conductivity profiles are then reconfigured via biasing to efficiently convert each specific part of the evanescent spectrum to propagating waves that can be detected at a far-field distance. The scattered light from the objects through the graphene sheet excited with various conductivity profiles is calculated in the far-field. Numerical optimization and image reconstruction techniques are employed on the far-field measurements (calculations) to recover the amplitudes of the evanescent spectrum enabling reconstruction of images with resolutions of the order of wavelength/10. With more two-dimensional materials emerging with plasmonic properties in different frequency regimes, the proposed technique has potential to be an alternative to NSOM based imaging without the complicated and slow-paced mechanical motions more suitable for real time imaging.

The integration of upconverters into solar cell devices has attracted much attention owing to an extraordinary optical properties exhibited by a sequential photon pumping. Despite of a few studies confirming the expectation, fundamental explanations for a real contribution of upconversion to photovoltaic efficiency are still requested. In this regard, we demonstrate poly(3-hexyl thiophene) (P3HT) sensitization associated with upconversion event by virtue of Kelvin probe force microscopy (KPFM) for the purpose of visualizing the existence and the quantity of near infrared (NIR)-induced charge generation. Along with the upconversion study in P3HT layer, a ternary system which is composed of Au nanoparticles/upconverters/P3HT was also investigated using the same technique. Based on the KPFM results, the upconverted light enabled the formation of charge carriers in P3HT and the integration of Au nanoparticles further promoted the process, doubling the difference in the value of surface photovoltage (SPV), from 38.5 mV between neat P3HT and upconverters/P3HT films to 76.3 mV between upconverters/P3HT and Au nanoparticles/upconverters/P3HT films. The specific interaction between upconverters and Au nanoparticles also increased a hole collection, which can positively impact on device performance in organic solar cells (OSCs).

The promising optical and catalytic properties of gold nanoparticles, and in particular Au nanorods, have made them a major area of research. Gold nanorods are typically synthesised via a seed mediated approach, using Ag ions and halides as surfactants. Despite intense interest, there remain many outstanding questions regarding the mechanism(s) that drive anisotropic growth and control the final shape and surface atomic structure of Au nanorods. For example, what causes an essentially spherical seed particle with a cubic unit cell, to break symmetry and develop a preferential growth direction? What effect does this symmetry breaking event have on the final size and shape of the nanorod? What is the orientation and relative stability of the surface facets and how do these evolve during growth into a mature nanorod?

Here we address these questions through a series of experiments examining the atomic scale structure and morphology of the nanocrystal from the embryonic stages of nanorod growth through to the mature rod. We develop and apply aberration-corrected transmission electron microscopy (TEM) methods to determine the shape, facet orientation and relative facet stability at the atomic level.

To identify the symmetry breaking event and the factors controlling it, Au seed particles are slightly overgrown to the size range at which symmetry breaking occurs (4-6 nm). In this size range, small, asymmetric truncating surfaces with an open atomic structure become apparent¹. This symmetry-breaking event is observed to occur at discrete sizes between 4nm and 6nm, depending on the Au:AgNO₃ ratio. We also find the size and aspect ratio of the final Au nanorod is related directly to the nanocrystal size at which the symmetry breaking occurs. Together these reveal the factors that control the aspect ratio and width of the final rod².

In an effort to understand shape control, we also develop methods for determining the orientation and stability of the nanorod facets. We apply a scanning TEM technique to count the number of atoms in each atomic column orientated in 2 zone axes. Using this method we are able to determine the shape and facet crystallography of the nanorod. By applying this method at successive time intervals, it is possible to determine the relative stability of the different facets and the overall stability of the nanoparticle shape. We measure the shape and facet orientation at different stages of nanorod growth^3,4. For the final mature rod, the side-facets comprise both high {0 1 1+√2} and low {110}, {100} index facets of comparable size, shape and stability⁵.

The implications of these observations for controlling shape will be discussed.

1) Walsh M, Barrow S, Tong W, Funston A, Etheridge J. ACS Nano 2015 9 715
2) Tong W et al. in prep.
3) Tong W et al. in prep.
4) Katz-Boon H, Rossouw, C, Weyland M, Funston A, Mulvaney P, Etheridge J. Nano Lett. 2011 11 273
5) Katz-Boon H, Walsh M, Dwyer C, Mulvaney P, Funston A, Etheridge J. Nano Lett. 2015 15 1635

Over the last couple of years, there has been much interest in the development of single nanoplatforms able to offer potential multifunctionality such as light-induced heating, temperature sensing and luminescent bioimaging for applications in nanomedicine. The two ideal nanosystems fulfilling such functionality are plasmonic nanostructures being the most efficient sources of light-induced heat generation and trivalent lanthanide ions (Ln³⁺)-based ratiometric nanothermometers which are self-referencing, non-contact and accurate for the measurement of the local temperature change at the nanometer scale. However, bringing these two independent systems into a single functional heater-thermometer nanoplatform, e.g., for hyperthermia treatment of tumor/cancer cells, with optimized optical properties of the platform and precise quantification of the heater’s actual temperature rise, is a considerable challenge.

Here, we introduce a new heater-thermometer nanoplatform combining a plasmonic nanoheater (Au nanorod) and a ratiometric upconversion nanothermometer ((GdYbEr)₂O₃ nanorod), operating upon a 980 nm laser excitation. The heater and the thermometer are in contact and their size dispersion is minimized, enabling the thermometer to measure the actual local temperature increase of the heater. The heating efficiency of the heater is strongly increased by bringing its localized surface plasmon resonance band into resonance with the infrared laser excitation. The plasmonic induced temperature rise, 302–548 K (maximum temperature sensitivity 1.22% K^-1, uncertainty 0.32 K and repeatability >99%), is assessed using Boltzmann’s distribution of the Er³⁺ upconversion ²H_11/2→⁴I_15/2/⁴S_3/2→⁴I_15/2 intensity ratio. Heating in the physiological temperature range (302–330 K) is measured at lower laser power densities (8.3–24.8 W cm^-2). The nanoplatforms are biocompatible with MG-63 cells and mapped within cells using hyperspectral imaging.

Gold nanostars have emerged in recent years as powerful plasmonic nanostructures for a wide variety of applications, from sensing to catalysis. We have been particularly interested in the development of surface enhanced Raman scattering (SERS) tags for the identification, localization, and quantification of biologically relevant molecules, from proteins in cell membranes to natural binders in works of art, and relevant literature is starting to emerge that focuses on the application of nanostar-based tags in the medical field. However, only few seminal papers on the fundamental properties of these nanostructures are available, thus impacting our overall understanding of these particles, their plasmonic properties, and their field enhancement power. Herein I will present our recent work on gold nanostars, in which we show how, by coating the particles with silica, and gradually etching the shell away to expose variable degrees of the tips, we are able to tune the SERS signal enhancements made possible by these nanostructures. Interestingly, by comparing the experimental results with the calculated heat losses, it is possible to observe a remarkable correspondence between the two. This result is extremely promising as it provides potentially an even more tunable plasmonic behavior for these nanoparticles, which can be combined with the biocompatibility provided by the silica coating, rendering these nanostars attractive to build effective SERS tags. To support this hypothesis, we have carried out multiparametric toxicity studies and found that gold nanostars do not significantly impact the viability of healthy and cancerous cells, further promoting our goal of applying them to localize and quantify targets of biological nature, particularly for in vivo applications.

Terahertz plasmonic systems has received growing interest in the past decade because of facilitating exquisite features for various applications from biological sensing to the advanced security. In this work, by using a four-member metallic Koch-type antenna consisting of fractal Y-shape blocks jointed to each other, we probed formation of various plasmonic resonant modes across the terahertz spectrum. Employing both numerical analysis and experimental studies, we have examined and verified the spectral response of the plasmonic Koch-type antenna. It is proved that the proposed microscale multiresonant antenna can be tailored to support both strong transmission dipolar and charge transfer plasmons dips as distinct minima at the terahertz band. Utilizing direct shuttle of charges across the antennas’ branches via charge transfer plasmons effect, we provide a method to overcome the regular weak capacitive coupling in terahertz regime. This mechanism allows to induce substantial absorption lines across the terahertz domain with high tunability. We also compared both capacitive coupling and direct transfer of charges in the same system to show the advantages of the proposed technique. The achieved results pave the way for new methods to design integrated and high responsive practical terahertz plasmonic opto-electronic devices.

Formation process of a metal nanostructure array has attracted attention due to capability of enhanced electric field of incident light originating from localized surface plasmon resonance (LSPR). Various applications based on a metal nanostructure array have been proposed, such as surface-enhanced Raman scattering (SERS), solar cells and so on. The performance of the devices is dependent on LSPR properties, and the LSPR properties are decided mainly by the geometrical structures of the metal nanostructure array. Therefore, to improve a performance of the plasmonic devices, controlling geometrical structures of the metal nanostructure array is essential. Until now, various fabrication processes of metal nanostructure arrays have been proposed. However, the efficient fabrication process of geometrically-controlled metal nanostructures has not been established yet due to technical difficulties. In the presentation, the efficient formation process of geometrically-controlled metal nanowire arrays based on mechanically deformation using anodic porous alumina molds will be discussed.
Anodic porous alumina, that is one of typical mesoporous materials, is obtained by anodizing Al in an acidic electrolyte. Advantageous point using anodic porous alumina as a mold is controllability of geometrical structures of the nanoholes [1], in addition, is its high mechanical strength. In the present process, an anodic porous alumina membrane was set onto surface of a metal plate and pressed using oil press. The metal was heated below the melting point during pressing. The metal was mechanically deformed and injected into the nanoholes of the anodic porous alumina. After dissolving the alumina mold, a metal nanowire array was obtained. We demonstrated the formation of nanowires consisting of various metals, such as Au, Ag, Al and so on. Obtained metal (Au) nanowire arrays could be applied to a substrate for SERS measurements [2]. It is expected that the present process can be applied to form not only the metal nanowire arrays but also various functional plasmonic devices requiring geometrically-controlled metal nanowire arrays.
[1] H. Masuda, K. Takenaka, T. Ishii, K. Nishio, Jpn. J. Appl. Phys., 45, L1165 (2006).
[2] T. Kondo, N. Kitagishi, T. Yanagishita, H. Masuda, Appl. Phys. Express, 8, 062002 (2015).

Infrared (IR) photodetectors are highly desired for various applications, e.g., imaging, control, and telecommunication. Recently, graphene attracts people‘s great attentions for developing infrared photodetector due to natural gapless and ultrahigh mobility. However, the responsivity of graphene-based photodetectors has so far been limited hundreds mA/W due to the small optical absorption of graphene, as well as the ultrashort life time of photo-induced carriers. Integration of nanomaterials (quantum dot, carbon nanotube, nanopletes) in the light absorption layer can improve the responsivity of graphene photodetectors, but response speed and spectral range are limited by the nanomaterials. Here, we present a plasmonic-enhanced graphene-based infrared (1.1μm to 2μm) photoconductor with high responsivity and fast response time. The light absorption of graphene is improved by one order through plasmonic effect. Perticularly, a vertical built-in field is employed into prolongs life time the photo-induced carriers in graphene. Thank to the above two mechanisms, the photoresponsivity of the our device is up to 80 A/W with a fast response time less than 600ns at the wavelength of 1.55μm. As we know, it is the highest responsivity among infrared photodetectors based graphene. These results address key challenges for high response graphene-based photodetector and are promising for the development of mid/far infrared optoelectronic applications based on graphene.

We have fabricated highly uniform surface-enhanced Raman scattering (SERS) substrates on 4-inch wafer and used them to conduct precise quantification of analyte molecules. SERS is an attractive technique to analyze materials. The nanoscale features on the substrate amplifies the excitation laser and generate enhanced spectra that show the vibrational energies of the molecules under examination. The most important component in the SERS measurement is a well-designed substrate that enhance the excitation light and Raman emission. Various kinds of SERS substrates have already been developed, and some showed extremely high enhancement. However, their fabrication processes are often complex and costly, and the resulting substrates are unsuitable for quantitative measurements. It has been hard to achieve both high uniformity and high enhancement.
The fabrication process consists of sequential wet chemical processes: hydrothermal synthesis of ZnO nanowires (NWs) and liquid phase deposition (LPD) of Au nanoparticles (NPs). A 4-inch Si wafer coated with a ZnO-seed layer was immersed in ZnO-NW precursor solution and heated to 95 °C. Then, the wafer was immersed into Au-NP precursor solution, heated to 90 °C, and rocked back and forth to continuously mix the solution. This LPD process was repeated 5 times to guarantee sufficient coverage of Au NPs.
When inspected with bare eyes, the substrate looked uniformly black. In SEM images, we observed clusters of 10-20-nm diameter Au NPs efficient stacked using the ZnO NWs as frames. In order to evaluate its SERS performance and spatial uniformity, the substrate was incubated in 1 mM benzenethiol (BT) solution and scanned using 785-nm laser at a step of 500 µm. The SERS spectrum showed clear peak of BT at 999, 1022, 1072 cm^-1. The SERS intensities of those peaks were uniform throughout the entire substrate, with ±10 % standard deviation.
To demonstrate quantitative measurements, we incubated the substrate in varying concentrations (10 nM - 10 µM) of adenine solution. We tracked the large SERS peak of adenine at 735 cm^-1 and obtained the relationship between concentration and intensity as I = 2.45×10⁶ C^0.4302 – 73.38, where I and C are SERS intensity and molar concentration of the solution, respectively. In order to verify accuracy of our approach, we prepared 60 nM, 200 nM and 4 µM adenine solution separately and applied to the SERS substrate. Our method produced the concentration of 63.1 nM, 213.3 nM and 3.71 µM, respectively, matching well with the given concentration with 10% accuracy.
We have fabricated wafer-scale SERS substrates by simple wet chemical processes and verified its high sensitivity and uniformity experimentally. Our substrate can be potentially and widely utilized as commercial SERS substrates for quantitative biological and chemical sensing applications.

The ability to tune the coupling strength between excitons and electromagnetic fields enables the observation and utilization of various physical phenomena ranging from spontaneous emission enhancement via the Purcell effect (in the weak coupling regime) to Bose-Einstein condensation of exciton-polaritons (in the strong coupling regime). For the latter near-infrared emitting single-walled carbon nanotubes (SWCNTs) are a particularly interesting material due to their large exciton oscillator strength and narrow excitonic transitions even at room temperature.
Here, we combine semiconducting SWCNTs with plasmonic crystals formed by periodically arranged gold nanodisks. The corresponding hybrid plasmonic-photonic modes (surface-lattice resonances, SLRs) exhibit high quality factors and sub-diffraction mode confinement of the surface plasmons. For thin (100 nm) layers of sorted (6,5) SWCNTs on such nanodisk arrays, new spectral features follow the SLR dispersion curves due to the angle- and polarization-dependent Purcell effect (i.e. weak coupling regime). Therefore, by setting the interdisk distance to 670, 830 and 1000 nm we can turn random networks of (6,5) SWCNTs that emit nonpolarized light at ~1000 nm into broadband tunable (~1000-1500 nm), directional and polarization selective light sources (Nano Lett. 2016, 16, 3278).
For samples with thick (200-300 nm) layers of (6,5) SWCNTs, we achieve strong coupling and the formation of plasmon-exciton polaritons. We use the coupled-oscillator model to quantify this regime and to describe experimental reflectivity and photoluminescence results. The extracted coupling strength values are around 125 meV, thus placing plasmon-exciton polaritons in the ultrastrong coupling regime. These results further advance insight into plasmonic crystals in the near-infrared as well to the growing field of the plasmon-exciton polaritons, quantum condensation and polariton lasing.

The use of plasmonic nanofluids that contain noble metal nanoparticles (NPs), into the layers of solution processed solar energy harvesting devices promises fabrication of solar cells with increased efficiency and enhanced stability in large scale. However the design of such a solution processed plasmonic solar cell involves several steps and obstacles to be overcome.
In this work these several steps for the design of a successful solution processed plasmonic solar cell are discussed, starting from the synthesis of silver plasmonic nanofluids and ending to the fabrication of organic solar cells with improved performance. The synthesis of the plasmonic nanofluid was made using a combination of Laser Ablation processes in (aquatic or isopropanol) solutions. First the 532 nm beam of a picoseconds laser was used to ablate a silver target and to form silver NPs in the solutions. Secondly, the 532 nm and 355 nm beam of a nanosecond laser was employed to refine the silver NPs size distribution and also to tune the localized surface plasmon resonance (LSPR) of the plasmonic nanofluid. The optical properties of the plasmonic nanofluids were evaluated by optical transmittance measurements in the Visible-UV spectral range, while the NPs size distribution and their concentration in the liquid were evaluated after analysis of the transmission spectra.
The solar cells were fabricated in the inverted structure, the silver plasmonic nanofluid was mixed with PEDOT:PSS and developed on top of the P3HT:PCBM photoactive blend, in order to take advantage of the plasmonic effects. The devices were tested under AM1.5G at 1000 W/m² of illumination, which showed the fabrication of efficient plasmonic solution processed solar cells with improved V_oc, FF and J_sc values and slightly higher power conversion efficiency compared to this of the non-plasmonic/reference ones.

In the past decade, progress in nanofabrication has allowed the experimental demonstration of subwavelength-structured materials exhibiting unusual optical properties. Such artificial materials termed metamaterials can be engineered to manipulate electromagnetic waves in a way rarely observed in nature. Hyperbolic metamaterials (HMMs), a class of metamaterials with uniaxial anisotropy, have been demonstrated to have unique properties including negative refraction, focusing effects and subwavelength imaging. These properties arise due to anisotropy of HMMs that have one orthogonal effective permittivity tensor element different in sign to the other two. HMMs have profound applications in diverse parts of photonics and terahertz technologies such as enhanced spontaneous emission, hyperlens, and terahertz filters. New materials that provide subwavelength focusing effect in terahertz frequencies will substantially improve the performance of terahertz quasioptics and lead to new device applications like terahertz imaging.
We theoretically demonstrate the subwavelength focusing of terahertz waves in HMM based on two-dimensional subwavelength silicon pillar array microstructure. The silicon microstructure with a doping concentration of at least 10¹⁷ cm^-3 offers a hyperbolic dispersion at terahertz frequency range and promises the focusing of terahertz Gaussian beams. We study the focusing effect based on ray tracing based on effective permittivity theory which agree with the results acquired from FDTD simulations. The focusing effect is found to be controllable by adjusting the doping concentration of the silicon pillar array which determines the real part of the out-of-plane permittivity and, therefore, the refraction angles in HMM. The focusing property in HMM structure allows the propagation of terahertz waves through a subwavelength aperture. The silicon-based HMM structure can be realized using microfabrication technologies and has potential to advance terahertz imaging with subwavelength resolution.

We experimentally demonstrate the use of plasmonic nanostructures to enhance the fluorescence of molecular beacons (MB) for sensitive DNA detection. Conventional surface-bound MB probes relies on metal surface for quenching and suffers from insufficient signal-to-noise ratio resulting in limited detection sensitivity. We show here that the fluorescence emission signal can be increased by incorporating MBs on a silver surface with an engraved open-ring nanostructure array (ORNA). The mechanism can be explained that the light emitted from a fluorophore couples to a plasmonic nanostructure located 10-20 nm away from it at a resonant frequency. The coupling induces coherent oscillations of electrons on the structure and forms electric field hot-spots. The electric field hot-spots in turn act as secondary excitation to the fluorophore leading to fluorescence intensity enhancement. The enhancement intensity depends on the distance between fluorophore and metal nanostructure; sitting too close to the metal surface leads to fluorescence quenching. A precise control of the separation distance can improve the detection signal of molecular beacons.
ORN array (ORNA) was engraved 100 nm in a 200 nm-thick silver film deposited on a silicon substrate using focused-ion beam milling. The dimensions of the ORNA structure was designed based on FDTD simulation to achieve high absorption at 520 nm wavelength which matches the emission wavelength of fluorophore. The ORNA device was assembled in a 1.2 μL sized microfluidic cell for DNA detection assay. Fluorescein labeled-MB probes (32 bases including 5-base stem on both ends) were conjugated via a thiol-modified terminal to a silver surface containing both ORNA and plane areas. Fluorescence enhancement was evaluated under hybridization of complementary DNA (cDNA) with various concentrations. After cDNA hybridization, the fluorophore of the MB stays up to ~11 nm from the silver surface. We observed a ~6 times signal to noise ratio at 1 μM cDNA hybridization. Limit of detection (LOD) is measured to be ~ 10 pM defined as the concentration yielding a signal-to-noise ratio of 3 (S/N =3). The signals acquired from plane silver surface are much weaker; a SNR of 3 is projected to occur at ~6.4 μM cDNA concentration. The results indicate that ORNA enhances the LOD by over six orders of magnitude. The sensitive DNA detection technique will benefit the applications in medical diagnostics and point-of-care technology.

Thermoplasmonic effect of noble metal nanoparticles has received increasing attention because of its capability to control local temperature at a nanometer scale. Their applications include such as cancer therapy, ultrasonic generation, and optofluidics. Marangoni flow is one of the phenomena that are sensitive to the local temperature gradient and expected to be controlled by the thermoplasmonic effect. Because the surface tension varies depending on temperature, a shear force is induced on a gas-liquid interface if there is a temperature gradient. This shear force generates fluid motion, which is called “Marangoni flow”.
Recently, we demonstrated stable Marangoni vortex flows controlled by the thermoplasmonic effect of gold nanoisland film [1]. This thin film was prepared on the bottom surface of a microfluidic chamber, which was then filled with water. A microbubble was created in the chamber by focusing a CW laser onto the thin film because of the thermoplasmonic effect. When the laser spot was displaced from the bubble center, two vortex flows were observed next to the bubble. Those vortex flows were controllable by laser spot position, that were found to be useful for driving, collecting, ranging, and sorting microparticles. In our previous study, we focused our attention on the vortex flow generation. Therefore, the fluid motion when the laser spot is set to near the bubble center was not discussed in detail. In this study, we investigate the thermoplasmonic Marangoni flow under the laser irradiation around the bubble center. In addition, we report visualization of the flow around a microbubble from the direction parallel to the surface of the gold nanoisland film.
The gold nanoisland films were self-assembled on glass substrates by using a glancing angle deposition technique. Fluidic chambers with 50-μm-thick and 1-cm-thick were created on the films. The chambers were filled with water in which polystyrene spheres with diameters of 2 and 0.75 μm were dispersed. Then, we focused the 785-nm-wavelength laser on the film and created a microbubble. By keeping the laser spot around the bubble center, the particle motion around the bubble was observed from normal and parallel to the film surface. The particle motion in the thermoplasmonic Marangoni flow changed significantly depending on the relative position of the laser spot and the bubble. The results can be understood by considering the steep temperature gradient around the bubble due to the highly localized heat generation from the gold nanoisland film.
[1] K. Namura et al., Appl. Phys. Lett. 106, 043101 (2015).

Plasmonic structures are widely used in low-cost, label-free biosensors, and the investigation of how to improve their sensitivity or to widen their range of applications is a central topic in the field of plasmonics.[1,2] The most commonly used plasmonic sensors are based on the concept of surface plasmon resonance (SPR) and, in particular, on the sensitivity of these resonances to changes in the refractive index of the medium surrounding a metallic structure.
In the search for an improved bulk sensitivity of SPR-based sensors, researchers have proposed different strategies. Thus, for instance, it has been shown that the use of the magneto-optical properties of layered systems containing magnetic materials can, in principle, enhance the sensitivity of these sensors.[3,4] Another possibility that is becoming increasingly popular is the use of nanohole arrays or perforated metallic membranes featuring arrays of subwavelength holes. [5,6] These sensors make use of the extraordinary optical transmission phenomenon, which originates from the resonant excitation of surface plasmons in these periodically patterned nanostructures.
We present here a theoretical study that shows how the use of hybrid magnetoplasmonic crystals comprising both ferromagnetic and noble metals leads to a large enhancement of the performance of nanohole arrays as plasmonic sensors. In particular, we propose using Au−Co−Au films perforated with a periodic array of subwavelength holes as transducers in magnetooptical surface-plasmon-resonance sensors, where the sensing principle is based on measurements of the transverse magnetooptical Kerr effect (TMOKE). We demonstrate that this detection scheme may result in bulk figures of merit that are two orders of magnitude larger than those of any other type of plasmonic sensor.[7] The sensing strategy put forward here can make use of the different advantages of nanohole-based plasmonic sensors such as miniaturization, multiplexing, and its combination with microfluidics.

References

[1] O. Tokel, F. Inci, U. Demirci, Chem. Rev. 114, (2014) 5728
[2] M.-C. Estevez, M. A. Otte, B. Sepulveda, L. M. Lechuga, Anal. Chim. Acta 806, (2014) 55
[3] B. Sepulveda, A. Calle, L.M. Lechuga, G. Armelles, Opt. Lett. 31, (2006) 1085
[4] M.G. Manera, et al., Biosens. Bioelectron. 58, (2014) 114
[5] A.A. Yanik, et al., Proc. Natl. Acad. Sci. U. S. A. 108, (2011) 11784
[6] A.E. Cetin, et al., ACS Photonics 2, (2015) 1167
[7] B. Caballero, A. García-Martín, and J. C. Cuevas, ACS Photonics 3, (2016) 203

Nature has remarkable ability to control the shape of inorganic material. Direct contact of biomolecule with inorganic surface at specific site significantly modifies the direction of crystal growth resulting in macroscopic shape change. Inspired from organic modifier in nature, we developed organothiol assisted growth system which is capable of directing morphological development through distinctive interaction between organothiol and gold surface.[1-2] Thiol group in the molecule acts as anchor that tightly binds the molecule onto the surface and various functional groups in organothiol make characteristic attachment onto the metal surface that mediate growth direction making morphology into various shape. Using rationally designed organothiol molecules, here, we demonstrated unprecedented morphologies with exceptional optical properties.
First, benzenethiol group was utilized to direct the morphology of gold nanoparticle. In case of benzenethiol, interaction between gold and molecule can be systematically varied through changing functional group at para position. By varying the functional group, chlorine, hydrogen, and amine, we achieved trisoctahedron, tetrahexahedron, and concave rhombic dodecahedron (RD) respectively. A concave RD gold nanoparticle enclosed by various high index facets, such as (331), (221), and (553), was synthesized for the first time by using 4-aminothiophenol (4-ATP). With the exceptional shape of concave RD, it showed strong surface enhanced Raman scattering enhancement. Additionally, this new shape exhibits superior electrocatlytic activity for the selective conversion of CO₂ to CO in aqueous solution.
Furthermore, by changing spatial configuration of functional group in organothiol molecule, we constructed nanoparticle with strong optical activity at visible range. The nanoparticle exhibits outstanding anisotropy g-factor 5.6 x 10⁻² and optical rotatory dispersion effect. In addition, tunable optical response was achieved through tailoring the structure of nanoparticle modulated by spatial control of functional groups.
This organothiol directed growth paves the way to fabricate numerous types of novel morphologies and provides design rule to create desired shape which will be beneficial to many applications such as optical antenna, imaging, diagnosis, delivery of molecules, chemical sensor, and energy generation.
References [1] Lee, H.-E.; Yang, K. D.; Yoon, S. M.; Ahn, H.-Y.; Lee, Y. Y.; Chang, H.; Jeong, D. H.; Lee, Y.-S.; Kim, M. Y.; Nam, K. T. ACS Nano 2015, 9, 8384. [2] Ahn, H.-Y.; Lee, H.-E.; Jin, K.; Nam, K. T. J. Mater. Chem. C 2013, 1, 6861.

Nanometer-sized metal clusters have the potential to function as prime candidates for photocatalysis based on their tunable unique optical and electronic properties, combined with large surface-to-volume ratio. Due to the very small optical cross sections of such nanoclusters, support-mediated plasmonic activation is most efficient.
A semi-transparent gold film [1] is used as support for the platinum clusters and imaged with optically-assisted scanning tunneling microscopy under 532 nm illumination. Plasmonic excitation and electron transfer enhance the light-induced tunneling current on the clusters relative to their gold support. To investigate the improved catalytic activity, an oxidative decomposition reaction of a methylene blue film is performed. The platinum cluster catalytic activity under illumination exceeds the bare gold surface baseline activity by more than an order of magnitude per active area.

[1] Nienhaus et al. J. Phys. Chem. C 2014, 118, 13196.

Metals that are highly susceptible to oxidation are notoriously difficult to use for preparing nanoparticles with well-defined size and shape. This is even more notable for plasmonic metals such as copper, because the oxidation of the resulting particles significantly degrades the resulting optical properties. As a result, copper nanoparticles have found little use as an optical sensing probe. However, by utilizing a core@shell structure, we are able to successfully create nanoparticle probes that are resistant to oxidation and display the pure plasmonic properties of copper. The phenomenon relies on electron transfer between the core material and the copper shell of the particles, effectively operating as an alloy, even though the core and shell materials are phase segregated. The results provide new information into how to create stable and robust plasmonic nanoparticle probes with non-traditional materials, including metals that are strongly susceptible to oxidation. In this study we explore the effects of using various metals such as gold or platinum as the core material while analyzing the electronic characteristics of the copper shell by using techniques such as XPS. The nanoparticle structures are analyzed using STEM-HAADF and EDS elemental maps to confirm the core@shell nanoparticle structures.

This paper presents a novel hybrid plasmonic cavity design on nonlinear difference frequency generation for THz. Compared with conventional photonic designs, hybrid plasmonic cavity allows sub-wavelength confinement to THz while still keeps high Purcell factor which is proportional to the output power of THz. According to the finite element method, small modal volume that extends far beyond diffraction limit, is allowed in the hybrid plasmonic cavity which gives a high Q/V_m ~ 7.2x10³(λ_THz/n_GaAs)^-3, indicating a strong Purcell enhancement for the THz mode. The output power of the nonlinearly generated THz wave is estimated to be 166.3mW. The small signal and depletion of the pump and saturation cases have been investigated based on coupled mode theory. This hybrid plasmonic design has advantages as strong cavity enhancement, high output power of THz, room temperature operation, and easy optical integration and fabrication.

It was shown theoretically that electromagnetic wave is highly trapped in hyperbolic material which enhance quality factor (Q-factor) of whispering gallery mode (WGM) in small size resonator [1], k_excitationR < 10 or R < 4.875 µm as λ_excitation = 325 nm. This enhancement is demonstrated experimentally in Au – ZnO hybrid spherical resonators (HSRs). Such HSRs are successfully synthesized by hydrothermal technique [2]. The WGM behavior is observed in photoluminescence spectrum. Further analysis show that Q-factor of resonance mode is enhanced due to plasmon – WGM coupling. As a result, not only TE and TM modes but also high order radial mode could be observed in small size resonators (~ 1 µm). In the other hand, WGM can be tuned by adjustment the size (10 nm, 40 nm, 300nm, 1 µm) of Au core inside a HSR. The strength of this plasmonic enhancement is strongly connected to the size of Au nano core. It might be related to either the core/shell size ratio or the formation of multicore inside a HSR.

Reference
[1] C. Wu et. al., Electrodynamical light trapping using whispering – gallery resonances in hyperbolic cavities, PRX 4, 021015 (2014).
[2] R. S. Moirangthem et. al., Optical cavity modes of a single crystalline ZnO microsphere, Opt. Expr. 21, 3010-3020 (2013)

Optical metamaterials typically operate in a narrow bandwidth compared to the resonant frequency of the unit cells comprising the material. Tunable metamaterials enable the dynamic manipulation of the metamaterial properties, extending the effective range of the optical response. Potential applications of such devices include tunable metasurfaces for flat optics or as a switch for an optical signal. Here, we present experiments combining plasmonic optical metamaterials with a two dimensional mechanical metamaterial. Using a MEMS based technique we print plasmonic materials onto 3D-printed polymer structures and subsequently manipulate the metal and polymer geometry, thus tuning the optical properties. This technique allows for the tuning of the individual unit cells and, through the use of auxetic two dimentional structures, the manipulation of the area density of plasmonic unit cell. We discuss our technique for fabrication of such tunable metasurfaces and the results of recent experiments.

There is much interest in the elaboration of “Saturn-like” particles, i.e. spherical dielectric particles designed with a metallic ring at the equator, due to their interesting plasmonic properties. The aim of this work was to design new “Saturn-like” particles, consisting of Au nanoparticles adsorbed on the equatorial surface of 2 µm silica beads, via a bottom-up fabrication route; spin coating and electro-deposition techniques. A three-step approach has been attempted, to synthesize the Saturn-like particles. First, spin coating of silica micro-particles on Au coated glass substrates to have a monolayer of silica on the surface. Second, electrodeposition of three layers of metal; Au, Ni and Au, around the silica beads in order to incorporate a sacrificial intermediate layer that can be later on removed. Third, the exposed silica surface is modified by APTES functionalization to facilitate the adsorption of Au nanoparticles on the exposed equator. Experimental characterization of each step showed that we have been successful in realizing the results we had expected for the first two steps. Silica monolayer has been first optimized on glass substrate, when same conditions has been applied on Au coated glass substrate the silica formed randomly distributed particles. The process of three metal layer deposition around the silica particle can be achieved successfully with controlled thickness for each layer through monitoring the change of surface area with time from current deposition curves, during the electro deposition step. The SEM and TEM assured adsorption of Au NPs on the functionalized silica exposed part. However, experimental characterization of the final step to determine the selective adsorption of Au nanoparticles on the silica equator is so far remained inconclusive.

With their unique capability of manipulating light at the sub-wavelength scale, plasmonic metasurfaces are promising for biophotonic applications, including photothermal therapy, optical manipulation of cells, and molecular sensing. Herein, we demonstrate a new type of moiré plasmonic metasurfaces for multi-functional biomedical applications. We have invented a directed-assembly method known as moiré nanosphere lithography to fabricate the metasurfaces in a high-throughput, cost-effective manner. Using Au as the plasmonic material, we have shown that the moiré metasurfaces exhibit highly tunable multiple plasmonic resonance modes in a wide range of electromagnetic spectrum due to a large number of plasmonic components. Furthermore, we have improved the optical responses and field enhancements of the Au moiré metasurfaces by integrating them into a metal-insulator-metal (MIM) structure. In this MIM structure, the Au moiré metasurface is placed on top of an optically thick Au thin film layer with a dielectric spacer layer sandwiched between them. In particular, the precisely controlled electromagnetic interaction between the moiré metasurface and Au thin film can lead to strong plasmonic resonance modes in two distinct regimes, i.e., near-infrared (~ 1300 nm) and mid-infrared (~5 μm). Harnessing the strong optical interactions in these two spectral regimes, we have demonstrated the multi-functional applications of the MIM structure for photothermal treatment, bacteria trapping, and surface-enhanced infrared microscopy. With the multi-functionality and directed assembly fabrication, the moiré metasurfaces will find a wide range of applications beyond biomedicine, including information technology and solar energy.

The use of "Plasmonics" is one very promising method to improve the emission efficiencies of various light-emitting materials and devices [1]. Next important challenge is to extend this method into UV or IR wavelength regions. Especially, deep-UV light sources are very important for various applications such as sterilization, lithography, optical memory, etc. By using Aluminum films, we already obtained the enhancements of deep-UV light emissions at ~260 nm from AlGaN/AlN quantum wells (QWs) [2] and also unexpectedly large enhancements of green emissions from InGaN/GaN QWs [3].
In this work, we succeeded in fabricating nanoparticles (NPs) by using Aluminum, Indium, and also Tantalum as the new deep-UV plasmonic materials. The NPs were formed by a thermal annealing of deposited metal thin films. We found that the Ta NPs have strong localized surface plasmon resonance (LSPR) peak at around 200 nm, which is the shortest wavelength in any other metal NPs ever reported previously. As long as we know, this is the first report for deep UV plasmonics using Ta NPs. The strong deep UV resonance spectra of Ta NPs were well represented by the simulations of the finite-difference time-domain method (FDTD) and the discrete dipole approximation (DDA) methods.
Moreover, quite recently we observed the peak splitting of extinction spectra when the two-dimensional sheets of silver NPs were placed on a metal substrate [4]. This very unique optical phenomenon should be due to the mode splitting effect by the strong mode coupling. The strong dipole oscillator located near the metal interface can interact with the mirror image of the dipole oscillator, which has the opposite phase. The interference causes the strong mode coupling and the splitting of the extinction spectra. This presents a powerful and useful technique to tune the strong mode coupling effect without any lithographic structures. By using this structure with Al, very strong and sharp resonance peak was obtained at 156 nm by the FDTD simulation. As far as we know, this is the LSPR spectrum which has the shortest peak wavelength at Ultra-Deep-UV region.
These LSPR spectra in the deep-UV regions presented here would be useful to enhance deep UV emissions of super wide bandgap materials such as AlGaN/AlN QWs and also mixed oxide semiconductors. We believe that our approach based on ultra-deep UV plasmonics would bring high efficiency ultra-deep UV light sources and it should lead to new class of several photonic and electronic technologies.

[1] K. Okamoto, Plasmonics for Green Technologies, Advanced Photonic Sciences (Chap. 8), InTech (2012).
[2] A. Takada, T, Ohto, R. G. Banal, K. Okamoto, M. Funato, and Y. Kawakami, 57th JSAP Spring Meeting, 17p-TC-13 (2010).
[3] K. Tateishi, M. Funato, Y. Kawakami, K. Okamoto, and K. Tamada, Appl. Phys. Lett., 106, 121112 (2015).
[4] K. Okamoto1, D. Tanaka, R. Degawa, X. Li, P. Wang, S. Ryuzaki1, and K. Tamada, submitted for publications.

Today’s nanotechnology has enabled the synthesis of metallic nanoparticles with a variety of geometries, greatly advancing the research field of plasmonics. The complementary system of the inverted nanostructure, however, has so far been limited to either 2D holes or spherical voids, owing to the challenge of creating voids with well-defined 3D geometries. Here we examine, both experimentally and theoretically, the localized surface plasmon resonances (LSPR) in fully enclosed aluminium nano-voids (or “anti-nanoparticles”) in the shape of truncated octahedra.

Nano-voids were created in high-purity aluminium using an annealing and quenching process. To characterize LSPRs of these fully buried voids, we employed electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) – an aberration-corrected FEI Titan operating at 80 kV. The lower accelerating voltage causes less damage to aluminium, which is essential to achieve reliable EELS mapping on voids. To compare with experimental observations, electrodynamic EELS simulations were performed based on electron-driven discrete dipole approximation (e-DDA) [1].

Our results show that these aluminium nano-voids exhibit strongly localized field enhancements, with the LSPR energies 10.7 - 13.3 eV (116 - 93 nm), well beyond the conventional LSPR spectrum range. The LSPR tunability can be achieved by tailoring the shape of nano-voids using controlled electron irradiation. Furthermore, owing to the simplicity of the nano-void system, free of aluminium oxidation and supporting substrates, we demonstrate that the intrinsic LSPR properties of pure Al nanoparticles can be revealed from nano-void characterization using the sum rule for the complementary systems. Combining with the unique properties of Al, a cheap, abundant, and mass-producible metal, our results indicate that both the Al nanoparticles and nano-voids can effectively extend the available plasmonic spectrum range to the extreme UV region (≤ 124 nm).

This work was supported by the Australian Research Council (ARC) grant DP110104734 and DP150104483 and a Monash University IDR grant. The FEI Titan3 80-300 S/TEM at Monash Centre for Electron Microscopy was funded by the ARC Grant LE0454166.

[1] N. W. Bigelow, A. Vaschillo, V. Iberi, J. P. Camden and D. J. Masiello, ACS Nano. 6, 7497-7504 (2012).

Collective electron oscillations arising from light-matter interaction in metal nanoparticles (NPs), known as localized surface plasmon resonances, result in strong far-field scattering of the excitation radiation. Most research on individual and collective multiparticle excitation has been focused on interaction with scalar beams, i.e. linearly or circularly polarized light. For single metal NPs with different size and shape, only electric dipolar and sometimes electric quadrupolar resonances exist. On the other hand, both electric and magnetic dipolar resonances could be observed for assemblies of metal NPs, or metal NPs combined with dielectrics. [1] However, it’s difficult to selectively excite particular resonances by linearly polarized light. Furthermore, the spectra require electrodynamic simulations and multipole expansion for identification of individual resonances, especially for higher ordered magnetic multipolar resonances.

Here we present an approach to selectively excite individual multiplolar resonances in single core (dielectric)-NP satellite (metal) structures, by using tightly focused cylindrical vector beams (CVBs). [2] The core-satellite structures consist of silver NPs that are covalently attached to the surface of a SiO₂ dielectric core, with a total diameter of ~300 nm. Scattering spectra were collected by using focused CVBs with azimuthal and radial polarization. The single particle spectroscopy and FDTD simulations show that azimuthally polarized light can selectively excite magnetic multipolar resonances (dipole, quadrupole, and octupole), while radially polarized light mainly excites electric resonances. We show that the strength of the magnetic resonances can be even greater than that of electric resonances in individual structures. One can compare the spectra of each multipolar resonance excited by different type of polarized light. Compared to the spectra excited by linearly polarized light, both the magnetic quadrupolar and octupolar resonances have about 5 times enhancement excited by azimuthally polarized light (~ 3 times in the case of radially polarized light).

Theoretically, these phenomena can be explained by the Maxwell-Faraday’s relation, that is, the curl of the electric field can produce a time varying magnetic field. This time-varying magnetic field at optical frequencies is induced in systems that do not possess spin or orbital angular momentum. This work opens new opportunities for the selective spectroscopic investigation of “dark modes” and Fano resonances in nanomaterials, magnetic modes in engineered metamaterials, and controlling nanomaterial excitations and dynamics.

[1] a) Halas, N. J. et al. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913. b) Dionne, J. A. et al. A Metafluid Exhibiting Strong Optical Magnetism. Nano Lett. 2013, 13, 4137.
[2] Youngworth, K. S.; Brown, T. G. Focusing of High Numerical Aperture Cylindrical-Vector Beams. Opt. Exp. 2000, 7, 77.

Metamaterials with simultaneously zero effective permittivity and permeability offer exotic properties such as uniform spatial phase and infinite wavelength. Additionally, zero index has been proposed for use in a broad range of photonic applications including super-coupling and simultaneous phase matching in nonlinear optics. Implementations of zero-index metamaterials have been recently demonstrated in both out-of-plane and on-chip configurations. Both configurations take advantage of a photonic Dirac-cone dispersion at the center of the Brillouin zone that offers matched impedance and low losses. However, current mass production of photonic devices necessitates integration with CMOS technology. These previously demonstrated configurations are inherently incompatible due to either an out-of-plane configuration or metallic and high aspect-ratio structures, respectively.
Here we present an on-chip zero-index metamaterial consisting of a square array of air-holes in a 220-nm-thick SOI wafer. This all-dielectric structure offers improved compatibility with current integrated silicon photonics platforms. In addition, this design enables mass production of zero-index-based photonic devices at low cost and high fidelity using standard CMOS fabrication technology.
To transition from the high-aspect ratio inverse case of silicon pillars in air, the design is instead intended for a transverse electric (TE) polarization. An increased proportion of silicon allows for improved confinement, especially for thin layers, and produces a Dirac-cone dispersion with better quality factor as compared with the silicon-pillars structure under transverse magnetic (TM) polarization. We optimized the design parameters by iterating the air-hole radius (r) and unit cell pitch (a) to obtain an effective index of zero at 1550 nm. The bandstructure of the metamaterial for TE polarization is shown to have a Dirac-cone dispersion at the center of the Brillouin zone corresponding to the design wavelength. This dispersion is formed through a degeneracy of a quadrupole mode and two dipole modes at the Gamma point. The effective permittivity and permeability cross zero simultaneously and linearly at the design wavelength. Furthermore, the effective impedance shows a finite value of 0.8 at 1550 nm. These results indicate that this metamaterial possesses an impedance-matched, isotropic zero index at 1550 nm.
To experimentally verify that the metamaterial has a refractive index of zero, we fabricated a right-triangular prism measuring twenty unit cells across. The measured effective index of this prism crosses zero linearly at 1630 nm and shows positive and negative indices at short and longer wavelengths, respectively, indicating a Dirac-cone induced zero index. We also simulated the far-field pattern of the prism with parameters measured from the fabricated device. The simulated effective index is in excellent agreement with that of measurement.

Mannan is a polymannose isolated from the cell wall of Saccaromyces cerevisiae and has strong binding affinity to mannose receptors on antigen presenting cells (APCs), such as dendritic cells and macrophages. Mannan-functionalized nanomaterials or nanostructures are thus of high interests in studying immune responses towards fungi. In this talk I will present our recent results on the fabrication of mannan-coated silver nanostructures using a laser-deposition technique. We used a mixture solution of AgNO₃ and mannan to reduce silver ions to nanoparticles. In a different experiment, we used a suspension of silver nanoparticles presynthesized with mannan as the sole reducing and capping agent. Using 405 nm diode laser in a confocal microscope, we successfully fabricated mannan-covered micropatterns by laser-induced photoreduction of silver ions or aggregation of mannan-capped AgNPs. The results show that both starting materials can be applied to deposit micro- or nanoscaled structures that are covered with mannan, which was confirmed by fluorescence microscopy. These silver nanostructure-supported mannan patterns are promising candidates to mimic the fungal membrane and beneficial for immune cell studies.

Plasmonic nanoparticles have become an increasingly common research area as well as becoming a key component in many important applications, such as solar energy harvesting, chemical sensing via surface enhanced Raman scattering, cancer treatment and optical encoding of information to name but a few. The main reason behind their adaptability to these and other prominent applications is their unique optical properties that allow for the manipulation of light below the diffraction limit. This effect, known as Local Surface Plasmon Resonance (LSPR), is a resonance phenomenon occurring when the frequency of the incident photons match the frequency of the surface electrons oscillating against the restoring force of the positive nuclei. The optical identity of the LSPR is extremely sensitive to the particle’s size, shape, distribution and the dielectric functions of the nanoparticle and surrounding medium. The ability of this plasmon resonance to scatter light (Mie scattering), finds great utility in optical and imaging fields. The scattering process can be orientated towards specific angles (primarily specular) but depending on the nanostructuring can be diffuse as well. The separate identification and quantification of specular and diffuse scattering is of great importance in order to design nanoparticle configurations that favour a specific enhanced scattering regime. A prime example of this is to consider the variation to the performance of a photovoltaic device by considering the incident angle onto the device, akin to the angular variation of the sun striking a solar cell throughout a day. To date, the literature demonstrates a theoretical discussion of the angular properties of nanoparticle arrays. Here we present novel work combining a theoretical and experimental investigation into the S-, P- and N-polarisation specular and diffuse reflection of light from laser fabricated plasmonic nanoparticles on various thicknesses of silica on a silicon substrate. These nanoparticles present a single LSPR at normal incidence but a strong variation to colouration when viewed with bare eyes over a wide range of angles. For the full characterisation of these samples we present a custom built a goniometric system capable of illuminating samples at various angles and identifying both the specular, diffuse and polarisation reflection separately. A range of specular angles (0^o – 70^o), in addition to a series of diffuse angles from each specular position were investigated.

The increasing interest in the development of ultra-compact room temperature Terahertz (THz) emitters with wide-range tunability has stimulated in-depth studies of different mechanisms of THz generation in the past decade due to its various potential applications such as biomedical diagnosis, security screening, chemical identification, life sciences and very high speed wireless communication. In this work, we propose and investigate a core-shell nanostructure exploiting plasmonic resonances to achieve enhanced difference frequency generation (DFG) across the THz range. The proposed structure is composed of a second-order nonlinear material aluminum nitride (AlN) as the core, which is encapsulated by a Au layer acting as a plasmonic nanocavity and an SiO₂ outer layer preventing interparticle near-field coupling. The numerical analysis of the structure was carried out in a commercial finite difference time domain method based commercial simulation tool. We engineered the structure to support resonances at pump, idler and signal frequencies required for the DFG of interest. In comparison to the bare nonlinear core, we observed large enhancement of input pump and idler waves in the near-field and efficient coupling of DFG THz radiation to the far-field for the engineered core-shell geometry. Our simulations show that substantially intensified tunable THz output is achieved, when we expose the proposed nanoengineered composite with appropriate near-infrared input waves. The spectral linewidth of the THz radiation can also be tuned by controlling the pulse width of the input waves. The proposed structure opens new avenues to design and fabricate efficient, promising, and on-chip integrated THz structures for all-optical and optoelectronic devices.

Plasmonic nanoantennas confine electromagnetic fields at infrared wavelengths to volumes of only a few cubic nanometers, resulting in huge local fields in the vicinity of the resonantly excited metal particles. These near fields are used to enhance the infrared vibrational bands of molecular monolayers and thus enable a spectroscopic detection with ultra-high sensitivity. [1,2] In the presentation, we will report on fundamental aspects of the vibrational enhancement in surface-enhanced infrared spectroscopy, [3,4] applications to hyperspectral infrared chemical imaging and sensing in life sciences, such as in-situ protein sensing. Additionally, we will present a combination of the above mentioned concept with a high power and broadband mid infrared laser source to further lower the detection limit in infrared spectroscopy. [5]

[1] F. Neubrech et al., Phys. Rev. Lett. 101, 157403 (2008).
[2] D. Dregely et al., Nat. Commun. 4, 2237 (2013).
[3] S. Bagheri et al., Adv. Opt. Mater. 11, 1049 (2014).
[4] S. Bagheri et al., ACS Photonics 2, 779 (2015).
[5] T. Steinle et al., Opt. Express 23, 11105 (2015).

Colloidally synthesized doped metal oxide nanocrystals has recently got great scientific attention for their ability to manipulate the localized surface plasmon resonance (LSPR) by varying their composition. Due to highly variable carrier concentration in these material, it enables plasmon frequency over entire infrared region and has facilitated a new class of tunable plasmonic materials with potential applications in the fields of photocatalysis, sensors, smart windows and surface enhanced Infrared absorption spectroscopy (SEIRA). In particular, SEIRA exploits the signal enhancement to increased the sensitivity of spectroscopic feature exerted by the plasmon resonance of nanostrucrtured/nanocrystal thin films. To date, most of the studies based on plasmon- molecular vibration coupling have been focused on the plasmonic metal nanocrystal. Here, we will present the doped Indium Oxide as prototypical material. Dopant driven nanocrystal shape evolution and thereby plasmon property of colloidially synthesized octahedron , cubo-octaherdron and cubic will be emphasized. We will further describe the size dependent plasmon property of doped Indium Oxide, map the near field enhancement property of single cubic nanocrystals via EELS and quantify both far field and near field plasmon property via COMSOL electromagnetic simulations. Furthermore, we show how C-H molecular bonds (2800-3200 cm^-1) couples to periodic film doped Indium Oxide nanocrystal both experimentally and computationally. This development of metal oxide plasmonic platform for molecular sensing could lead to easy to make, electrically tunable surface enhanced infrared absorption spectroscopy substrates.

Infrared imaging systems display an apparent material temperature that is derived from an assumed, static thermal emissivity. Spatially modulating the emissivity can therefore alter the apparent, local temperature and allow for the construction of any arbitrary thermal image without changing the actual temperature of the object. Recent work has demonstrated emissivity modulation with electrical biasing of active nanophotonic structures using graphene, quantum wells, and vanadium oxide; however, implementing these designs into large-area, spatially-modulated infrared displays can be challenging as they require complex and costly pixelated systems. As an alternative, we fabricate and experimentally demonstrate a dynamic metamaterial thermal emitter that is locally activated by ultraviolet (UV) light resulting in a “pixel-less” infrared display. The UV light is used to generate free carriers in a photoactive material which increases optical losses near the metamaterial. These losses cause the metamaterial to transition from an underdamped system (low emissivity) to a critically damped system (high emissivity), as explained through coupled mode theory. The photo-activated carriers have relatively long lifetimes (milliseconds) due to surface adsorption/desorption processes which allows for low activation power. We show that by projecting UV light on certain areas of the metamaterial, we are able to create large-area, high-resolution infrared display images. This technology could have applications in adaptive thermal camouflage, infrared scene generation, and smart thermal regulation of building materials.

Room-temperature microbolometer arrays with high sensitivity within narrow wavelength bands for spectral imaging in the mid-wave and long-wave infrared are described. The approach is based on vanadium-oxide air-bridge bolometers that are integrated with plasmonic resonant absorbers. The added resonators maintain response speed without increasing processing complexity. Metal-dielectric-metal sub-wavelength resonators contain the semiconducting VO_x IR sensing elements within the dielectric section. The optical constants of this hybrid multilayer dielectric treated as an effective medium are determined by infrared ellipsometry. These data inform analytic design formulas for the plasmonic resonators. Large area arrays of such resonators are fabricated and characterized by infrared reflectance spectroscopy to confirm design predictions. Then these thin structured films are patterned into air bridges with 40 micron lateral dimensions and dual absorber-bolometer function. The bolometers are tested with filtered black body and narrow band quantum cascade or CO2 lasers to determine spectral and temporal response. Super pixels comprising sub-arrays of bolometers with different resonance wavelengths are designed. Applications include high-dynamic-range imaging of hot targets, such as engines and rocket plumes, simultaneously with night-vision on targets closer to ambient temperature.

Infrared (IR) light has broad applications including surveillance, biomedical imaging, sensing, and spectroscopy. In particular, advanced sensing devices operating at IR wavelengths are in a pressing need, because many biomolecules have characteristic vibrational modes in this spectral range. Plasmonic structures are capable of inducing strong local fields to enhance light-matter interactions, which not only promise improved detection sensitivity but also enable label-free diagnosis. As a result, the study of IR plasmonic resonators is a fundamental task of great importance.
In this talk, we will show that self-assembled plasmonic core-shell resonators exhibit unique optical properties in the IR region, revealed by the distinct features in their reflectance spectra. Dielectric-metal core-shell particles can support sophisticated, hybridized plasmonic modes as well as photonic modes (cavity modes). When they are closely positioned, strong interparticle coupling arises, further giving rise to complex distribution of the scattered light. Depending on the number, configuration, and dielectric properties of the constituent particles, the lineshape of the scattering spectrum from a single ensemble varies. In our experiments, we chose dielectric spheres (polystyrene or silica) of 1 or 2 um in diameter, coated with gold shells of controlled thickness varying from 10 to 50 nm. The self-assembly process was conducted on a gold substrate and was driven by the capillary force from the evaporation of water in small droplets of particle suspension. Upon complete evaporation, randomly assembled and suitably separated clusters consisting of different numbers of core-shell particles were located on the substrate, including monomers, dimers, trimers, quadrumers, and heptamers, etc. Fourier transform infrared spectroscopy (FTIR) was then conducted to characterize the reflectance from single clusters. The measured spectra confirmed that in the IR regime, the core size, shell thickness, number and configuration of particles in the clusters all have distinguishable impacts on the optical properties of the assembled resonators. Numerical simulations were also performed and showed quantitative agreement. We expect that further improvement could be applied to achieve plasmonic resonant structures over a larger scale, which may lead to future realization of reconfigurable optical metamaterials.

Weyl points are the degenerate energy states resulting from band crossings of linear dispersion features in three dimensional momentum space. Unlike Dirac points in two-dimensional systems, Weyl points have been shown to be stable and the associated surface states are predicted to be topological with non-trivial Chern number [1,2]. These topologically protected surface states give rise to various interesting phenomena such as backscattering immune unidirectional transport. Gyroid photonic crystals, a triply symmetric crystal with surface containing no straight lines, are predicted to possess Weyl points.
We have synthesized and characterized the first mid-infrared (Mid-IR) gyroid photonic crystals, including both single and double gyroid crystals with Weyl points present, in the Mid-IR region. Full wave simulations of gyroid photonic bandstructures reveal that single gyroid structures have a complete band gap. Double gyroids bring quadratic point degeneracy into the bandgap. Breaking the parity of double gyroids can be accomplished by introducing an air sphere, the quadratic point lifts its degeneracy and form a pair of Weyl points. Simulations reveal that gyroids must be composed of high refractive index materials such as a-Si in order for gyroids to possess such properties.
Two-photon lithography was utilized to write polymer gyroid scaffold with unit cell sizes of 4-6 µm composed of 20x20x5 unit cells, on intrinsic Si substrates. We deposited a-Si coatings on the polymer gyroids via PECVD, removed the crystal sides to facilitate polymer removal, yielding a hollow inorganic a-Si photonic crystal, which was then conformally coated and in-filled with 250nm of a-Si. We characterized the resulting a-Si single gyroid photonic crystals by FTIR, and observed 100% reflectance at 8 µm [3], which agrees with the predicted photonic band gap wavelength from simulation. Comparing a single gyroid with unit cell sizes of 4.5 µm and 5.1 µm, the measured center of reflection peak shifted from 7.5 µm to 8 µm. Double gyroid with unit cell size of 4.4 µm ([101]) and unit cell size of 4.68 µm (oriented 20⁰ along y axis relative to [101]) are fabricated, both with Weyl points at 8 µm and k between 0.3π/a and 0.5π/a. Characterization of double gyroid photonic crystal have been performed by angle resolved spectroscopy with a quantum cascade laser. Photonic crystal bandstructure are constructed from angle resolved reflectance and transmittance spectra. Measured bandstructure of double gyroids are compared with simulated bandstructure projected based on crystal orientation.
1. L. Lu, L. Fu, J.D. Joannopoulos, M. Soljačić, Nature Photonics 7, 294–299 (2013)
2. L. Lu, Z. Wang, D. Ye, L. Ran, L. Fu, J. D. Joannopoulos, M. Soljačić, Science 7, 622-624 (2015)
3. S. Peng, R. Zhang, V. H. Chen, E. T. Khabiboulline, P. Braun, H. A. Atwater, “Three-Dimensional Single Gyroid Photonic Crystals with a Mid-Infrared Bandgap”, ACS Photonics (accepted)

In this contribution we demonstrate all-optical modulation of both the infrared and visible spectrum with indium-tin-oxide nanorod arrays (ITO-NRAs). We show that intraband, on-resonance optical pumping reduces the intrinsic plasma frequency of ITO, which leads to redshifts of two localized surface plasmon resonances (LSPRs) in the infrared range. The intraband pumping also gives rise to a large optical nonlinearity in the visible range, and with it large absolute transmission modulation with favorable spectral tunability and beam-steering capability. Theoretical models are developed to facilitate the quantification of the observed nonlinear optical behavior in both the infrared and visible range. The optical modulation following the intraband pumping is found to be in the sub-picosecond regime, which is an order of magnitude faster than the noble metal counterparts. We further demonstrate another type of optical nonlinearity in the visible range following interband optical pumping, which effectively modulates the full-visible spectrum in a multi-picosecond time scale. Our work highlights that transparent conducting oxide materials, such as ITO, are excellent alternative plasmonic materials for the dynamic control of light from the visible to the infrared.

References: 1. P. Guo, et al, Ultrafast switching of tunable infrared plasmons in indium tin oxide nanorod arrays with large absolute amplitude, Nature Photonics, 2016, 10, 267-273. 2. P. Guo, et al, Large optical nonlinearity of indium tin oxide nanorods for sub-picosecond all-optical modulation of the full-visible spectrum, Nature Communications, 2016, 2nd round of review.

Enantiomer separation is a critical step in many chemical syntheses, particularly for pharmaceuticals, but prevailing chemical methods remain inefficient. Here, we introduce an optical technique to sort chiral specimens using achiral coaxial plasmonic apertures. These apertures are composed of a deeply subwavelength circular dielectric channel embedded in a conducting film and can stably trap sub-20-nm dielectric chiral specimens. First, using both full-field simulations and analytic calculations, we show that selective trapping of enantiomers can be achieved with circularly polarized illumination. Opposite enantiomers experience distinct trapping forces in both sign and magnitude: one is trapped in a deep potential well while the other is repelled with a potential barrier. These potentials maintain opposite signs across a range of chiral polarizabilities and enantiomer-aperture separations. Then, we experimentally probe these nearfield enantioselective optical forces using chiral force microscopy, a novel technique based on atomic force microscopy. Using an achiral tip, we show that optical forces range from 10-65 pN per 100µW/µm² of incident intensity, with the largest measured forces corresponding to the spectral peak in coaxial transmission. As expected, this optical force exponentially decays with increasing tip-aperture separation. Using tips patterned with chiral nanostructures, we observe a force difference of 7pN in both sign and magnitude, depending on the handedness of the optical illumination. Our measurement reveals the nanometer-scale spatial distribution of enantioselective optical forces, and indicates that the interaction of chiral light and chiral specimens can be mediated by achiral plasmonic apertures. More generally, our optical trapping scheme provides a possible route toward all-optical enantiomer separation and enantiopure syntheses.

Chiral structures, which are non-superimposable upon their mirror image, are essential in the selectivity of biological processes [1]. Thus, a great interest exists in enhancing the sensitivity of detection schemes for chiral molecules. The field of plasmonics offers a promising avenue to enhance biosensing, as metallic nanostructures induce highly concentrated electromagnetic near fields. When plasmonic structures have a chiral shape (examples include spirals [2], helices [3] or chiral pyramids [4]) they can greatly enhance the selectivity of chiral sensing by inducing intense, highly twisted chiral near fields. However, to date no direct experimental method exists to quantify the chirality of these fields and thereby assess the potential of a nanostructure for the aforementioned applications.

Recently, we developed a theoretical framework [5] that identifies the optical chirality flux as an ideal physical quantity to meet this goal. Our study shows that the optical chirality flux scattered into the far field of a structure provides valuable information on its highly twisted near fields. Further, we demonstrate that the optical chirality flux is a measurable quantity due to its close connection to the degree of circular polarization.

Here, we will present an experimental technique to detect the optical chirality flux generated by chiral nanostructures. This far-field method reveals information on the magnitude and handedness of chiral near fields. While conventional chiroptical spectroscopy (e.g. circular dichroism) provides information on the geometric arrangement of chiral components, our technique assesses the chirality of the electromagnetic fields generated by a structure. This opens the door to the effective design and evaluation of chiral plasmonic nanostructures for chiroptical applications.


References:

[1] A. Roger and B. Norden, Circular Dichroism and Linear Dichroism, Oxford University Press (1997).

[2] M. Schäferling, D. Dregely, M. Hentschel, and H. Giessen, Tailoring Enhanced Optical Chirality: Design Principles for Chiral Plasmonic Nanostructures, Phys. Rev. X 2, 031010 (2012).

[3] J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, and M. Wegener, Gold Helix Photonic Metamaterial as Broadband Circular Polarizer, Science 325 (2009).

[4] K. M. McPeak, C. D. van Engers, M. Blome, J. H. Park, S. Burger, M. A. Gosalvez, A. Faridi, Y. R. Ries, A. Sahu, and D. J. Norris, Complex Chiral Colloids and Surfaces via High-Index Off-Cut Silicon, Nano Lett. 14 (2014).

[5] L. V. Poulikakos, P. Gutsche, K. M. McPeak, S. Burger, J. Niegemann, C. Hafner, and D. J. Norris, The Optical Chirality Flux as a Useful Far-Field Probe of Chiral Near Fields, ACS Photonics, 3, 1619-1625 (2016).

The control of light propagation in the visible and near-infrared domain using resonant systems such as optical nanoantennas has been a matter of intense research during the last decades. The possibility to create and manipulate nanostructured materials encouraged the exploration of new strategies to control the electromagnetic properties with an external agent. A possible approach is combining magnetic and plasmonic materials, where it is feasible to control the optical properties with magnetic fields in connection to the excitation of plasmon resonances [1].
These nanoantennas have been traditionally made of metallic entities, which have the important drawback of a sizeable absorption. In the case of magnetic resonances based on Babinet inverted magnetoplasmonic structures, it has already been demonstrated that the magneto-optical effect has the ability to manipulate magnetic dipole-like resonances [2].
In the last years, there has been a quest for the so-called magnetic resonances in the visible domain [3]. Linked to it, there has been an increasing interest in the use of high index dielectric nanospheres as optical antennas, in particular for their ability to sustain magnetic resonances and the absence of absorption [4-6].
In this work we introduce the magneto-optical effect in the context of those high index dielectric nanospheres, i.e. a silicon nanosphere with a non-negligible of diagonal element in the dielectric tensor. We will show how the magneto-optical effect is controlled by the internal resonances of the nanosphere, and that the magnetic resonances dominate the spectral dependence of the magneto-optical response, having the electric dipolar resonance a very weak effect. We will establish a clear correlation of the spectral magneto-optical response with the spatial field profile at the interior of the nanosphere that is, in turn, linked to each type of resonance [7].

1. Armelles, G., Cebollada, A., García-Martín, A. and González, M. U., Adv. Opt. Materials, Vol. 1, 10–35, 2013.
2. Armelles, G., Caballero, B., Cebollada, A., Garcia-Martin, A. and Meneses-Rodríguez, D., Nano Lett., Vol. 15, 2045–2049, 2015.
Alu, A. and Engheta, N., Opt. Express, Vol. 17, 5723–5730, 2009.
4. García-Etxarri, A. et al., Opt. Express, Vol. 19, 4815–4826, 2011.
5. Schmidt, M. K., Esteban, R., Sáenz, J. J., Suárez-Lacalle, I., Mackowski, S., and Aizpurua, J., Opt. Express, Vol. 19, 13636- 13650, 2012.
6. Person, S., et al., Nano. Lett. 13, 1806-1809, 2013.
7. de Sousa, N., Froufe, L.S., Sáenz, J.J., and Garcia-Martin, A., submitted, 2016

Nanoplasmonic systems are valued for their strong optical response and their small size. Most plasmonic sensors and systems to date have been rigid and passive. Introducing dynamic properties to these structures opens new possibilities for applications. To that end, we have developed a physical vapour deposition technique for growing many billions of plasmonic nanoparticles with high shape fidelity and broad flexibility in the constituent materials. In this work we will present novel chiral nanoparticles composed of a Au-Fe alloy: the lack of mirror symmetry leads to a chiroptical response; the use of plasmonic Au ensures that the response is strong; and, uniquely, Fe allows the particle orientation to be controlled through an external magnetic field.

We will present dynamic plasmonic nanoparticles that can be used as mechanical sensors to selectively probe the rheological properties of a fluid in situ at the nanoscale and in microscopic volumes. We fabricate chiral magneto-plasmonic nanocolloids that can be actuated by an external magnetic field, which in turn allows for the direct and fast modulation of their distinct optical response. The method is robust and allows nanorheological measurements with a mechanical sensitivity of ~0.1 cP, even in strongly absorbing fluids with an optical density of up to OD~3 (~0.1% light transmittance) and in the presence of scatterers (e.g. 50% v/v red blood cells).

Plasmonic nanoparticles hold great interest as extremely local sensors that promise to deliver modular and low-cost sensing with high-detection thresholds [1]. However their sensing performance, including sensitivity and figure of merit (FOM), has been so far insufficient for the practical use in medical applications [2].
Here we introduce dispersion and shape engineered chiral plasmonic particles that lead to record refractive index sensitivity (1,091 nm RIU^-1 at λ = 921 nm) and FOM (42,800 RIU^-1) [3]. The chiral particle shows polarization-dependent extinction rich in spectral features. In particular zero-crossing serves as a natural point for tracking with very small effective line-widths and thus results in high FOM. Moreover, we for the first time show that it is possible to engineer the dielectric function of individual nanoparticles by the physical shadow growth [4], which allows us to achieve remarkable sensitivity. When the refractive index of the medium surrounding the particle is changed, the large shift is induced in the spectral features that can be readily tracked, even in complex biological media with limited transmission (e.g. optical density, ~3 OD).
In this presentation, the fabrication of designer nanocolloids that are shape and dispersion engineered will be presented and it will be shown how their spectral analysis enables new sensing tasks in complex biological fluids.

[1] Mayer KM, Hafner JH. Localized Surface Plasmon Resonance Sensors. Chemical Reviews 111, 3828-3857 (2011).
[2] D. Howes P, Rana S, M. Stevens M. Plasmonic nanomaterials for biodiagnostics. Chemical Society Reviews 43, 3835-3853 (2014).
[3] Jeong H-H, Mark AG, Alarcón-Correa M, Kim I, Oswald P, Lee T-C, and Fischer P. Dispersion and shape engineered plasmonic nanosensors. Nat Commun, DOI: 10.1038/ncomms11331 (2016).
[4] Mark AG, Gibbs JG, Lee T-C, Fischer P. Hybrid nanocolloids with programmed three-dimensional shape and material composition. Nat Mater 12, 802-807 (2013).