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
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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)
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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.

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[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.
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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 d