Nanoimprint lithography (NIL) is seen as a promising technology for the cost effective fabrication of sub-micron and nano-patterns on large areas. Real world conditions such as substrate bow and particle contaminants rule out the use of hard/rigid stamps over wafer scale areas. Hence soft- rubber-like stamp are required. Substrate Conformal Imprint Lithography (SCIL), a full wafer scale NIL technique will be introduced. SCIL combines the low cost, flexibility and robustness of PDMS rubber working stamps, with the nm resolution and low pattern deformation of small rigid stamps. This allows us to research large area photonic and plasmonic applications with a route towards industrialization. We will demonstrate that large area nano-pattens can deliver enhanced performance for LEDs, lasers, thin film - and wafer based photo-voltaics.
White light LEDs use a phosphor layer to partly convert blue light from the LED chip. We demonstrate a very large improvement in phosphor emission (60-fold directional enhancement for unpolarized emission) using nanophotonic structures. This is attained by coupling emitters with high (80%) quantum efficiencies to collective plasmonic resonances in large area periodic arrays of aluminum nanoantennas.
Vertical cavity surface emitting lasers or VCSELs are a type of low cost laser manufactured in high volume and used in fiber optic data communications, optical tracking and sensing applications. Due to the planar technology the VCSELs polarization direction can flip. To lock the polarization a grating is applied on one of the laser mirrors. We imprint gratings on 3” GaAs wafers which contain the final laser stack, etch the semiconductor and process into VCSELs. By replacing traditional e-beam patterning with SCIL we improve performance and yield as it enables using a smaller grating pitch, which reduces optical losses and simultaneously lower cost.
The trend in photovoltaics in towards high efficiency cells. We used two approaches to increase light absorption and improve efficiency. First plasmonic back reflectors for thin-film amorphous hydrogenated silicon (a-Si:H). A single imprint on a glass substrate replicates regular and random pillar arrays on which the cell was grown. The plasmonic cell traps light in an only 90nm thick intrinsic a-Si:H layer and reaches an efficiency of 9.6% under AM 1.5 illumination (400nm pitch).
To reduce the reflectivity of single crystal Si wafers we used front side resonant Mie scatterers. These sub-wavelength cylinders trap light over a broad wavelength range and scatter the light preferentially in the high index silicon. The solar spectrum weighted reflection over 450-900nm is reduced to only 1.3%, compared to 3.4% for a standard textured solar cell. Importantly, this approach is applicable to any high index material and is compatible with very thin wafer concepts where traditional AR approaches fail due to the required feature height.

We describe, for the first time, a strategy based on soft-nanoimprinting lithography for the fabrication of colloidal Metal Organic Frameworks (MOF) nanopatterns with periodicity as small as 400 nm and their utilization for sensing applications.
Despite the fact that this new family of hybrid, porous, crystalline materials have attracted immense attention because of their unique physicochemical properties (1) just a few reports demonstrated their patterning at micrometric scale (2); the nanopatterning of MOFs, that would allow their integration into nanophotonic devices, still remains a challenge.
We developed a technique consisting in the direct nanoimprinting from a colloidal solution of preformed nanoparticles of ZIF-8 material, having size ranging from 35 and 50 nm. The nanoparticle were synthetized by simple precipitation of zinc nitrate and imidazole linker and dispersed in ethanol. At first, when the PDMS stamp was applied on a colloidal solution droplet the patterned nanocavities were naturally filled by capillarity. Then, the evaporation of the solvent through the PDMS stamp lead to the replication of the features and to the homogeneous packing of the colloidal network. Several features (squared, linear,..) with nanoscale resolution were demonstrated over large surface up to 1 cm2. This technique does not need any embossing machine and the ZIF-8 nanostructures could be replicated on both rigid and flexible transparent substrates since no pressure was applied to the stamp. The embossed structures were characterized by electronic microscopy while the crystallinity of the material was investigated by GI-WAXS.
Since porous ZIF-8 showed excellent selectivity in vapor adsorption (3) a simple optical sensing device for organic solvents and based on nano-imprinted diffraction gratings was developed. The replicated ZIF-8 system consisted in array of lines with periodicity equal to 400 nm. Due to the relevant change in ZIF-8 refractive index when exposed to organic solvent vapors (from 1,2 to around 1,45 at 700 nm), a sensitive increase in color intensity of the diffracted light could be observed. The easy-detection system was based on the evaluation of the color intensity change that could be probed by a simple camera (or even a smart-phone) without need of complex optical measurements. The -sorption properties of the system exposed to organic and water/organic solvent mixtures were compared with in-situ environmental ellipsometric investigations.
1. N. Stock et al, Chem. Reviews, 112, 933 (2012)
2. P. Falcaro et al, Adv. Mater. 24, 3153-6 (2012)
3. A. Demessence et al, J. Mater. Chem. 20, 36 (2010)

Light management schemes have known a steady development in the past decade as a solution envisioned to enhance the performance of optical and optoelectronic systems. However, to avoid additional processing costs, it is important to propose simple and scalable fabrication techniques to integrate such schemes within functional devices. In this project, we demonstrate highly efficient and tunable light trapping structures prepared using simple and scalable Sol-Gel chemistry, spin coating processing, and Nanoimprint lithography techniques. First, Distributed Bragg Mirrors in the form of periodic dielectric stacks are fabricated with dense TiO2 layers as the high index material (2.08 in the visible range), and macroporous silica layers as the low index counterpart (1.24 with 50% of porosity). In our approach, a single high temperature annealing step is used to create the porosity in the full stack, highlighting its simplicity and potential for industrial deployment. A defect-free semi-transparent 9-layer stack is obtained and shows a high specular reflectivity up to 96% at normal incidence in its prescribed bandwidth. We also demonstrate the flexibility of our process by making highly reflective DBRs with a tunable reflection range from UV to IR (from 400nm to 1300nm). To demonstrate the versatility of our approach, we optimized a 8-layer DBR and integrated it within in a a-Si:H thin-film solar cell, which resulted in an increase of efficiency by 14.6%. More over, we demonstrated an enhancement of the light emission from an Europium-doped luminescent silica layer that is deposited on a Bragg reflector. Finally we will present our latest results on the possibilities opened by the integration of diffraction gratings fabricated by soft Nanoimprint Lithography, with our DBR structures.

The integration of optical elements on elastomeric substrates can pave the way to the realization of a novel class of stretchable photonic systems with the ability of changing their optical properties upon modification of their shape due to tensile or compressive strain and to be highly conformable to complex surfaces [1, 2].
Here we present an effective approach to the fabrication of nanocomposite-based deformable and stretchable optical, photonic systems by means of Supersonic Cluster Beam Implantation (SCBI) of metal nanoparticles in PDMS [1]. A beam of electrically neutral nanoparticles, accelerated by a pressure difference and focused with a very low divergence by aerodynamical effects, is directed towards the polymeric substrate and gain sufficient kinetic energy to get implanted. The extremely good resilience of the nanoparticles layer embedded in the polymer upon deformation of the so obtained nanocomposite allows to maintain extremely good optical performances upon substantial deformation of the material and a large number of deformation cycles [1]. Moreover the low energies involved in the SCBI implantation process avoid the deterioration of the physical and chemical (and thus the dielectric) properties of the PDMS substrate [3-5].
SCBI metal polymer nanocomposites exhibit interesting plasmonic properties, because of the nanoparticles dynamics inside the PDMS matrix, and represent an extremely useful tool for the understanding and control the macroscopic opto-electronic properties of nanocomposite-based devices. Surface plasmon resonance (SPR) of Ag/PDMS and Au/PDMS nanocomposites with a continuous gradient of implanted nanoparticles doses and thermal annealing treatments is well described by Maxwell Garnett and Shell-Core models. This helps in improving the high tunability and control the optical response of the material by playing on the implantation process parameters and post-implantation treatments.
Moreover, preliminary results of the SPR behavior of the nanocomposite under stretching show a high sensitivity of the SPR response to deformations, suggesting the exploitation of plasmonic nanocomposites for sensing applications.
[1] C. Ghisleri et al., Laser & Photon. Rev. 7, 1020 (2013)
[2] J.L. Wilbur et al., Chem. Mater. 8, 1380-1385 (1996)
[3] C. Ghisleri et al., Accepted by J. Phys. D (2013)
[4] R. Cardia et al., J. Appl. Phys. 113, 224307 (2013)
[5] G.Corbelli et al., Adv. Mater. 23, 4504 (2011)

Surface-enhanced Raman scattering (SERS) nanoprobe consisted by metallic nanostructure and capturing antibody offers unprecedented opportunity for ultrasensitive detection of antigen due to its strong signal generation arising from metallic nanostructure. Though many strategies have been developed that can precisely control metallic nanostructure for uniform SERS signal generation, most of antibody conjugation process are still relied on the conventional methods which lack controllability of antibody orientation and also conjugating various kinds of antibody remains as challenging work. Here, we demonstrate a new M13 virus based SERS nanoprobe (viral probe) that has strong SERS signal and possesses single antibody with controlled orientation enabling quantitative immunoassay. M13 virus is composed of 5 different proteins (p3, p6, p7, p8, and p9) with filamentous shape. Each protein can be easily modified via genetic engineering of M13 genome and this enables rational design of virus for SERS nanoprobe. The long p8 part was served as a template for assembling gold nanoparticles thereby generating SERS signal. By expressing gold nanoparticle binding sequence on p8, gold nanoparticles could be assembled closely along the virus. For antigen detection, an antibody was expressed on p3 protein. As the antibody can be expressed with orientation, the epitope can be fully exposed providing excel antigen capturing ability. Additionally, since all the viruses have the same capturing ability, exploiting this virus is desirable to quantitative immunoassay. Also, p3 part has been utilized for discovering antibody by constructing various antibodies on p3. Therefore, such easy expression of antibody on p3 facilitates fabrication of numerous types of SERS nanoprobe that capture diverse antigen. The formation of assembled gold nanoparticle on the virus was confirmed by UV-vis spectroscope, scanning electron microscope, and dark field microscope and these results clearly showed the aligned multiple nanoparticles along the virus. The SERS activity of fabricated viral probe was evaluated by Raman measurement on each probe. Strong and uniform SERS signal was generated from the viral probe originating from the assembled nanoparticles. We demonstrated the validity of viral probe in quantitative antigen detection by performing immunoassay. Prostate specific antigen (PSA) was detected by antibody in viral probe and the amount of PSA was quantified by SERS signal generating from gold nanoparticles in viral probe. From the increasing SERS intensity with the raise of PSA concentration, it is concluded that viral probe have selectivity for detecting antigen and can generate SERS signal corresponding to captured antigen. The M13 virus based SERS nanoprobe can pave the way for quantitative detection of antigen in infinitesimal range and offer great potential for improved medical diagnostics.

Surface Plasmon Polaritons (SPPs) are fluctuations of the free electron density in metals coupled to electromagnetic waves. SPPs at optical frequencies show a significant momentum mismatch with respect to the light incident on a flat metal/dielectric interface, therefore coupling strategies generally rely on prisms (Kretschmann configuration) or metal gratings to excite them.
In this talk I will show alternative methods to generate SPPs at optical frequencies using light diffraction by individual nanocorrugations etched in metal films. In particular, I will show how nanometer scale slits, grooves and holes can be used as efficient, localized sources of SPPs.
Spatial localization of the source of SPPs allows for control of the SPP propagative phase, thus enabling researchers to perform "plasmonic interferometry," i.e. optical interferometry at the nano- and micro-scale using SPPs as the interfering waves.
By properly varying the nanoscatterer separation distance and in-plane distribution, the optical interference of SPPs can be spatially modulated and spectrally tuned. This property, together with the highly confined nature of SPPs, can be employed to enhance the optical absorption in thin film solar cells, and improve the sensitivity and selectivity of high-throughput, real-time biochemical sensors.
I will also discuss how Plasmonic Interferometry can be turned into a powerful tool to measure the coherence length of light sources, as well as determine the dispersion of optical constants of dielectric materials in a broad wavelength range.

We demonstrate a novel way to distinguish coherent and incoherent radiation from silicon and metallic nanostructures by using a nanoscale electron probe to generate light emission. We use angle-resolved cathodoluminescence imaging spectroscopy (ARCIS), in which a 30 keV electron beam in a scanning electron microscope is used as a broadband point-like excitation source. The transient electric field about the electron trajectory excites coherent transition radiation from the surface which is a direct measure of the local dielectric constant. In addition, incoherent radiation is excited by the recombination of electron-hole pairs generated by electrons penetrating into the silicon.
In our experiments we use a half-parabolic mirror placed between the electron column and the silicon sample to collect the electron beam generated coherent and incoherent emission. We collect angle-resolved radiation profiles over a spectral range from 400-900 nm. The data are fitted to a model that assumes dipolar radiation profiles for the transition radiation component and Lambertian profiles for the incoherent emission. Excellent agreement between the angular data and the theory is observed. The analysis enables partitioning the collected emission spectrum in coherent and incoherent parts. The coherent part serves as a nanoscale probe of the local dielectric constant, a property that can not be detected using far-field techniques at nanoscale resolution. Moreover, we reconstruct the incoherent radiation spectrum over the entire 400-900 nm spectral band.
The measurements are complemented with studies on thin-film and bulk samples of Au, Ag, Cu and Al. These plasmonic metals are all efficiently excited by the electron beam and their transition radiation spectrum is collected. Good agreement with theory is observed, based on dielectric constants measured using ellipsometry.
Overall, the ARCIS technique serves as a nanoscale probe of optical resonances in dielectric and plasmonic nanostructures, and enables measurements of optical constants at length scales that are inaccessible with any other technique.

Surface plasmon resonances in metallic nanostructures are attracting great interest due to their property to enhance the electromagnetic field of light in the nanostructure nearfield. Recently, much interest is given to the plasmonic properties of nanoparticles separated by small gaps or connected by junctions of varying size. These structures are platforms to investigate the coupling between electronics and plasmonics as relevant for example for sensing and molecular switching [1].
We present surface plasmon resonance measurements on conductively and capacitively coupled nanowire dimers by electron energy-loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM). The investigated dimers are created by adjusting the synthesis conditions during pulsed electrodeposition of AuAgAu segmented nanowires and the subsequent acidic dissolution of Ag. The dimers have lengths in the µm-range and diameters of about 100 nm. A capacitively coupled dimer is formed by two nanowires separated by a small gap, the smallest gap being about 8 nm. For the conductively coupled dimers, two wires are connected by metallic junctions with varying diameters. Our results are compared to finite element simulations using CST Microwave Studio.
Our EELS-STEM measurements visualize for capacitively coupled dimers the generation of bonding and antibonding mode pairs up to the third multipole order. [2] For the conductively coupled dimers, we find that an increase in junction size does not shift significantly the antibonding modes. However, it causes a strong blue shift of the bonding modes, leading to an energetic mode rearrangement compared to the mode arrangement of a capacitively coupled dimer with similar dimensions. [3] The shift of the bonding modes is analysed as a function of connection size and multipole order.
In addition, the influence of the dimer symmetry on the resulting EEL spectrum is discussed.
[1] Perez-Gonzalez, O., Zabala, N., and Aizpurua, J., New J. Phys 13 (2011) 083013.
[2] Alber, I., Sigle, W., Mueller, S., Neumann, R., Picht, O., Rauber, M., van Aken, P. A., Toimil-Molares, M. E. ACS Nano 5 (2011), 9845-9853.
[3] Alber, I., Sigle, W., Demming-Janssen, F., Neumann, R., Trautmann, C., van Aken, P. A., Toimil-Molares, M. E., ACS Nano 6 (2012) 9711-9717.

The interaction strength of light with metallic nanorods (extinction cross-section) is determined by the strength of the scattering (radiative) and absorptive (non-radiative) processes that occur at the local surface plasmon resonance (LSPR). Since the components of the extinction cross section depend differently on particle volume, independent synthetic control of particle size, composition and shape enables extensive tailoring to address diverse application needs, such as maximal near field enhancement, far-field scattering magnitude, or heat generation. Unfortunately the relationships between structure, composition and extinction, scattering and absorptive cross-sections have not been explicitly demonstrated. The availability of only a few experimental reports is mainly due to challenges obtaining and verifying a series of precise concentrations of compositionally-pure, nanorod dispersions that have an independent variation of particle volume and aspect ratio. Here in, we address this challenge by bringing together synthetic procedures that provide independent tuning of the gold nanorod (AuNR) structure (effective volume, aspect ratio, length, and diameter range of 500x, 3x, 4x and 3x, respectively), with robust characterization of the composition and structural purity. We further fabricate core-shell structures (Ag, Pd, Pt, S,and Se) to selectively enhance the desired optical response. The accuracy and validity of the existing theoretical calculations are verified; for example demonstrating the scaling of extinction cross-section and the relative contribution of scattering on rod architecture and composition. This refined, quantitative structure-property correlation provides a solid platform to engineer plasmonic nanoparticles beyond AuNRs for emerging applications.

We investigate the photo-excitation of localized surface plasmon polaritons (LSPPs) in semiconductor plasmonic resonators at THz frequencies. This is realized by a patterned optical excitation of free charge carriers in thin films of GaAs using a spatial light modulator. This enables full spatial and temporal control of plasmonic resonances. By changing the illumination patterns we are able to excite LSPPs in plasmonic resonators without the need of physically structuring the sample. A single semiconductor layer can be thus used to generate a variety of plasmonic devices. Furthermore, using this concept we were able to observe transfer of surface plasmon polaritons in loaded plasmonic antennas. Moreover, this approach can be used for photo-generating plasmonic waveguides and circuits.

Control of photon absorption and emission is at the core of many emerging photonic technologies, ranging from photovoltaics and sensors to single photon emitters for quantum encryption. Recently, plasmon-exciton coupling has been demonstrated as a means to tailor the efficiency of emission by optimizing the overlap between the emissive state and the localized surface plasmon resonance. Fabrication of such plasmon-exciton structures by colloidal approaches offers numerous advantages relative to lithographic techniques, including controllable sub 5 nm gap size, 3D architectures and higher throughput, if challenges can be overcome, such as heterogeneous products, poor colloidal stability, and unacceptable pattern variability for device registration. Herein we discuss an interface and assembly method using orthogonal chemical interactions that provide pattern arrays with high yield and specificity of quantum dot (QD) gold nanorod (AuNR) architectures. Specifically, poly(ethylene glycol)-grafted-AuNRs in water selectively adsorb to polystyrene (PS) brushes. Number density, spacing and local orientation of the adsorbed AuNRs depend on the dimensions and shape of the chemical contrast pattern based on the PS brushes. The inert polymer and dithiol coupling directs subsequent QD binding to the adsorbed AuNRs, leaving a QD-free surface. Tuning the pattern, assembly and coupling conditions afford excellent control over spacing (1-10 nm), location (ends vs. all around), and number of QDs per AuNR. Following similar coupling approaches, bulk solutions of QD-AuNRs architectures, also with controlled spacing, QD/AuNR ratio and long-term colloidal stability, are demonstrated. The non-resonant photoluminescent enhancement of these structures was 5x, which is almost 70% of theoretical maximum and demonstrating the feasibility for applications ranging from biological sensing to advanced optical communication.

Since mechanoluminescence was discovered by Francis Bacon, a variety of mechanoluminescent materials have been developed. Until now, many materials have been known to emit light by the application of stress even though scientists have not yet arrived at a clear understanding of the effect. However, no practical application has been realized because previous instances of mechanoluminescence have been weak and unrepeatable. To overcome these shortcomings, elastico-mechanoluminescent materials have been developed, which generate the luminescence during deformation of solids without fracture or tribo-reactions. Recently, we designed and demonstrated flexible composite films with highly bright and durable characteristics by focusing on the substance (polydimethylsiloxane; PDMS) that transfers the mechanical stress to the mechanoluminescent materials (copper-doped zinc sulfide, ZnS:Cu).[1] By employing transparent PDMS with a high elastic modulus, we were able to realize in a simple manner a brightness of ~120 cd/m2 and durability over ~100,000 repeated motions.
The development of color manipulation technology would thus open a door for exploiting mechanoluminescence in light source and imaging devices. We demonstrated the color manipulation of mechanoluminescence by controlling the concentration of two independent mechanoluminescent materials (ZnS:Cu,Mn and ZnS:Cu) in a PDMS matrix.[2] We show that the color can be tuned linearly by regulating the weight ratio of the mechanoluminent materials and the color region in the chromaticity diagram can be extended by increasing the stress rate. We also reported that by applying mechanical stress, a colorful patterned imaging device and white lighting source can be produced.
However, one of the major drawbacks in PDMS based mechanoluminescent composite films is a difficulty of blue color demonstration due to its soft matrix. Even when a composite film is fully stretched, the compressive stress is insufficient to emit blue luminescence. Actually, the composite film with blue phosphors shows green luminescence rather than blue. In this presentation, we used combination of alternative stress applying methods and general organic dyes to demonstrate deep blue luminescence. Based on this approach, we also achieved various white mechanoluminescence. We believe that the findings of this color manipulation technology can open a new window for developing new smart systems and opto-mechanical devices.
[1] S. M. Jeong, S. Song, S. -K. Lee, and B. Choi, Appl. Phys. Lett. 102, 051110 (2013).
[2] S. M. Jeong, S. Song, S. -K. Lee, and N. Y. Ha, Adv. Mater. (in press), doi:10.1002/adma.201301679.

In this research we develop novel SERS substrate utilizing flower-like 3D nanoparticles array via IAAL(ion-assisted aerosol lithography) method for uniform and reproducible Raman signal with high field enhancement. We demonstrate the control of hot-spots (in terms of their locations and intensities) as a function of the petal number m (4, 6, and 8). Electromagnetic calculations show that hot-spots that are formed in 3D nanogaps located between adjacent petals exist between petals and their intensities increase with the number of petals. The obtained SERS enhancement factor is ~ 10^7, sufficient for the single molecule detection. Quantitative and qualitative optical behaviors of fabricated 3D multipetal flower assemblies are also studied by measuring dark field (DF) spectra. Enhanced excitation of non-radiative higher order surface plasmon modes for the higher m-petal flowers is also interpreted in terms of m-petal geometry, in agreement with the results of SERS, DF spectra, and rigorously calculated electromagnetic field.

Periodic and hierarchical nanostructures have been intensively studied for a wide range of applications for photonic microsensors. For example, dielectric structures possess photonic bandgap and metallic nanostructure exhibit surface-enhanced Raman scattering (SERS); both properties are highly sensitive to environment, thereby being useful for microsensors. However, fabrication of such hierarchical nanostructures still remains a challenge. Conventional photolithography has limitations in definition of structures due to diffraction limit. Furthermore, to create various designs of patterns, same number of photomasks is required. Although E-beam lithography and focused ion beam lithography enable the preparation of nanostructures, their complex and time-consuming fabrication processes makes the techniques impractical.
Here, we demonstrate a facile and versatile method for creating various designs of hierarchical nanostructures using novel interference lithography. A monochromatic plane wave which propagates through diffraction grating produces three dimensional patterns of light intensity by interference of diffracted beams. This near-field pattern is repeated with a period which is called Talbot distance. When the grating period is comparable with wavelength of incident light, zero- and first- order diffraction are available, which results in the generation of well-defined simple 3D profile of light intensity; this profile can be transferred into photoresist to create 3D nanostructure. To increase complexity of the 3D profile, larger grating period can be employed. This leads to high order diffraction from the grating, thereby generating highly complex 3D profile through the interference of multiple diffracted lights. For example, with a grating period of 2000 nm and the wavelength of incident light of 325 nm, we can generate up to 6th order diffraction and therefore, 7 different beams form a complex profile of light intensity. 2D slice of the intensity profile can be transferred into thin film of photoresist. By controlling a distance between the diffraction grating and the photoresist, a variety of 2D nanopatterns are created, of which structures are highly complex; from each 2D pattern, various unit structures such as doughnut, square, diamond, mesh, flower shapes are observed. One point that is worth stressing is that all the structures are prepared by single phase mask by simply controlling the distance between grating and photoresist film. Full 3D interference pattern and its 2D slice are in good agreement with calculations from finite-difference time-domain (FDTD) method. We use one of various 2D patterns, composed of the elliptical nanohole arrays in rectangular coordination, as template for creation of SERS-active substrate. Dense arrays of four elliptical gold dots have hot spots at their interstices with nanoscale gaps, which enable the highly efficient light confinement and therefore high degree of enhancement in SERS signal.

The study of interactions of molecules or molecular structures with plasmonic nanostructures is a rapidly growing research area having significant impact on several applications such as surface-enhanced Raman spectroscopy. The localized surface plasmons (LSPR) are excited on isolated nanostructures such as nanoparticles or lithographically prepared nanostructures [1]. The properties of surface plasmons depend on the thickness of metal film, type of the metal and roughness of the metal surfaces and dielectric constant of the adjacent medium. Incident electromagnetic field can be enhanced and localized in the slit region of a metal film [2]. This localized field couples surface plasmons on the surface of the metal.
In this study, circular plasmonic lens with different ring diameter and slit width were prepared by electron beam lithography (EBL) and the influence of plasmonic lens structure on SERS enhancement was investigated. We have demonstrated the influence of plasmonic lens properties (inner diameter and slit width) on SERS performance. The SERS intensity obtained from plasmonic lens having 3.0 µm inner diameter is 13.2 times higher compared to planar silver thin film which is consistent with the theoretical calculations [3]. Then, we present our preliminary results in designing plasmonic nano-patterned structures that can work as highly efficient SERS substrates. The proposed design gives more than two orders of magnitude larger signal intensity than plain gold film and nearly one order of magnitude larger than an optimally designed “etched-ring” plasmonic lens structure [4]. The relationship between the signal intensity and the physical sizes and parameters such as slit width, number of rings, thickness and material of separation layer and period should also be further studied. This study suggest that the strong relationship between the surface plasmons and SERS activity can be used to built unique structures to prepare well-defined arrays to use in several applications of SERS.
References
1. K. A. Willets, and R. P. Van Duyne, Annual Review of Physical Chemistry 58, 267-297 (2007).
2. Y. Xie, A. R. Zakharian, J. V. Moloney, and M. Mansuripur, Opt Express 12, 6106-6121 (2004).
3. M. Kahraman, S. Cakmakyapan, E. Ozbay, and M. Culha, Annalen Der Physik 524, 663-669 (2012).
4. Neval Cinel, Semih Cakmakyapan, Gulay Ertas, and Ekmel Ozbay, JSTQE-INV-NP-04491-2012.

Recent experimental observations of hot carrier generation and transport in metals require a quantum mechanical description to account for phenomena that contradict the classical Fowler theory for metal-semiconductor interface transport. In this context we present calculations for quantized plasmon fields indicating a correlation between plasmon polarization and the distribution of electron momenta resulting from the prompt decay of plasmons to excited ‘hot&’ carriers. Specifically, we report results of calculations for Au nanoantennas indicating that decay of localized plasmons to excited electrons and holes in an optical antenna results in strongly anisotropic momentum distributions for the excited carriers, in the regime prior to inelastic carrier relaxation. The hot carrier concentration and momentum anisotropy are strongly dependent on both the density of plasmonic modes excited by incident light and the way in which these modes align with the underlying metallic lattice. We discuss the role of virtual plasmons in nanoantenna-based devices, as well as the relative importance of interband and intraband transitions on the timescales of interest. The energy distribution of hot carriers after excitation, role of multi-photon excitation processes and correlation with plasmon polarization will be discussed using the model we have developed. Finally, we compare our theoretical predictions with experimental observations in solid-state nanoantenna-based energy conversion systems.

Surface plasmons in metal nanostructures have attracted considerable attentions for applications in nanophotonics and surface-enhanced Raman scattering (SERS) chemical sensors. Here we investigate the particle-film plasmon couplings of high-density gold nanostar (GNS) assemblies arranged on various metal and dielectric substrates (silver, gold, silicon, glass). In particular, we show how the GNS surface densities (or interparticle gap separations) affect the E-field enhancements from the particle-film plasmon couplings by analyzing the finite-difference time-domain (FDTD) calculation of E-fields and the experimental SERS intensities. We also present the interplay between the interparticle and particle-film plasmon couplings of high-density gold nanostars (GNSs) on metal and dielectric films as a function of interparticle separation. We show that the SERS enhancement factor (EF) of GNSs on a metal film as a function of interparticle separation follows a broken power law function, where the EF increases with the interparticle separation for the strong interparticle coupling range below an interparticle separation of ~0.8 times the GNS size, but decreases for weak interparticle coupling range (for an interparticle separation > 0.8 times the GNS size). Finally, we demonstrate optimally designed SERS substrates based on gold nanostar assemblies on a metal film which can detect attomole level of nitroaromatic explosives.

Basing on the sensitivity, speed and convenience, surface enhanced Raman scattering (SERS) is thought to be a powerful tool applying to bio-medical analysis. This technique may give high identification to organic samples without changing the original properties of them.
The hot-spot effect of SERS is concerned of the electric field distribution induced by surface plasma resonance (SPR). Usually, between the gap of noble metal nanoparticles or on the sharp tip area, where there is stronger electric field, there would be stronger enhancement. However, such gap area is so narrow and that restricts the application of hot-spot effects to macromolecules like proteins and viruses.
Previous study showed that with proper order, so-called deterministic aperiodic arrays of nanostructrures might give huge hot-spot area of hundreds nanometers. And the enhancement factor could reach ~10^7. We think the area is big enough to load macromolecules.
In our study, we prepared gold nano cylinders on silica wafer by electron beam lithography (EBL). The order of the array referred to the information from the previous article. After dropping the rhodamine 6G aqueous solution on our chip, we covered it on the polished site of the side-polished optic fiber (SPOF) connected to our home made Raman spectrometer with 532 nm laser source. The enhancement factor was calculated as ~10^5.
We are now trying to deposit indium tin oxide (ITO) on the SPOF to provide better conductivity that EBL needs, because we are also planning to copy the same array to SPOF. In this design, we can change the optic fiber after every single use to prevent the possibility of bio-contamination from dirt optic fiber.
Previous research showed ITO could be compatible for antibody conjugation after certain modifications. We wish the deposition of antibody could provide specificity for disease screening. Besides, with the advantage of optic fiber, this system could be capable of out-door or homecare usages.

Recently, the structural color has attracted great interest due to its potential as an alternative for colorant pigmentation. In contrast to conventional color pigments, the structural color is endurable to constant illumination with strong light intensities and does not require complicated processes of patterning multilayers for different colors on a single substrate [1]. For designing the structural color, a variety of methods such as a photonic crystal [2], a plasmonic nanostructure [3], and an optical microcavity [4] have been introduced so far. Among them, the optical microcavity-based approach is most promising because of the simple fabrication and the polarization-independent optical property. Moreover, compared to other kinds of the structural color, it is more versatile in that it can be fabricated either in a transmission or a reflection type [5]. However, it is difficult for the optical microcavity-based approach to realize three primary colors, red, green, and blue, on a single substrate since different microcavities are required for different colors.
In this study, we present a multilevel architecture of different optical cavities for different colors on a single substrate by microcontact printing. For fabricating the multilevel optical microcavities, patterns of a fluorous polymer (EGC-1700, 3M), used as a dielectric material, were printed on an aluminum reflector using an elastomeric stamp made of poly(dimethylsiloxane). A semi-transparent metallic film was then prepared on the multilevel dielectric layer by vacuum deposition. In this configuration, the structural color corresponding to the thickness of the optical microcavity was obtained under ambient light. This multilevel architecture provides a viable tool of fabricating color patterns for a variety of visual applications such as textile and decoration.
References
[1] T. Xu , H. Shi, Y. Wu , A. F. Kaplan , J. G. Ok , and L. J. Guo, Small 7 (2011) 3128.
[2] S. Kinoshita, S. Yoshioka, and J. Miyazaki, Rep. Prog. Phys. 71 (2008) 076401.
[3] S. Yokogawa, S. P. Burgos, and H. A. Atwater, Nano Lett. 12 (2012) 4349.
[4] Y. Yoon and S. Lee, Opt. Express 18 (2010) 5344.
[5] Y. Chena, and W. Liua, Optik 124 (2013) 13.
Acknowledgments
This work was supported by the National Research Foundation of Korea under the Ministry of Education, Science and Technology of Korea through the grant 2011-0028422.

Surface enhanced Raman spectroscopy (SERS) is a great analytical tool to obtain information on molecular composition. This technique has gained a reputation as one of the most sensitive spectroscopic methods available for the detection of a wide range of adsorbate molecules down to a single molecule detection limit. The most investigated metals for SERS substrates are gold (Au) and silver (Ag). Unfortunately, the fabrication of such devises poses a significant challenge due to an expensive deposition technology including, vapor deposition, electron-beam lithography, focused ion-beam lithography, and nano-transfer printing. Herein, we introduce a simple and low-cost approach to fabricate SERS substrates using roll-to-roll printing of matrix encapsulated gold nanoparticle arrays. The enhancement of Raman signals obtained using these materials was found to be comparable to commercially available SERS substrates. We expect that an on-going optimization of the film morphology should yield further enhancement of the demonstrated SERS architecture.

We present a convenient and low-cost method to fabricate large-area polycarbonate AR nanostructures to improve the luminous intensity and image clarity of a commercial 2.0-inch display panel in bright condition. The polycarbonate AR nanostructures were nanoimprinted by the graded-density nanoporous silicon template with nanoparticle-catalyzed etching. The average reflectivity of the AR film in visible wavelength was reduced from 10.2% to 4.8% in the optimized case. After attaching on the display panel to reduce the light reflection on the substrate, the brightness enhancement and decrease of ambient light reflection were observed. Due to the enhancement of contrast ratio, the quality index of the Lena image test was improved from 0.85 to 0.92 under strong ambient illumination.

There has been intensive research into the manufacturing of nitride based ultra violet light emitting diodes (UV LEDs) as the solution to the current bulky and high voltage UV light emitting Hg lamps, for its many useful applications including water purification. However these devices suffer from low efficiency (~2%) compared to the long established blue emitting InGaN LEDs (~70%). There are many different reasons for this performance issue, with high threading dislocation densities (TDDs) having one of the greatest impacts. Epitaxial Lateral Overgrowth (ELOG) has been a long established method of decreasing TDDs in nitrides. However patterning for ELOG is usually done by expensive lithographic processes often not reproducible on a large scale. In this study silica sphere lithography was applied to the initial growth layer in the multiple quantum well (MQW) structure by a self-assembly method. This simple process of producing a uniform patterned surface throughout the wafer, allowed for subsequential overgrowth in the Metal Organic Vapour Phase Epitaxy (MOVPE) reactor. The nano dimensions compared to usual micron scale patterns used for ELOG typically results in less time required for overgrowth. These layers fully coalesce by 500nm of growth much shorter compared to the reported ~7µm for micron scale patterns1 and even to the latest nano scale patterns at 2µm in AlN ELOG2. Transmission electron microscopy studies show the dislocation density has more than halved compared to the un-patterned AlN buffer layer under the same growth conditions, and almost completely eliminated the edge type. Another advantage of using this patterning visible by in situ MOVPE curvature measurements is the reduction in strain built up during growth resulting in the reduction of wafer bowing. The UV light emitting quantum wells can now be grown on a high crystal quality AlN buffer layer, leading to the decrease in non radiative recombination paths due to dislocations.
1. Zeimer, U., et al. Journal of Crystal Growth 377(2013)32-36
2. Dong, P., et al. Applied Physics Letters 102 (2013): 241113.

We demonstrate plasmonic enhancement of the luminescence of single upconversion nanophosphors (UCNPs) deterministically assembled in close proximity to the tips of gold nanorods (Au NRs). Individual UCNPs, composed of hexagonal phase NaYF4 doped with Yb3+ and Er3+, as well as single Au NRs are precisely positioned in lithographically defined templates via capillary assembly techniques. The Au NR aspect ratio is tailored to tune the localized surface plasmon resonance (LSPR) for longitudinally polarized excitation to match the 980 nm excitation wavelength of the UCNPs. The UCNP luminescence is characterized by scanning fluorescence microscopy in both the presence and absence of a Au NR. We observe a polarization-dependent luminescence enhancement: polarizing the incident laser beam along the axis of the Au NR resonantly excites its longitudinal LSPR, resulting in strong optical near-fields surrounding the NR tip where the UCNP is located. Longitudinal versus transverse excitation results in at least a 2x greater luminescence enhancement. Because the upconversion emission is a nonlinear process, the power dependence of the luminescence enhancement is also investigated. The template-based co-assembly scheme utilized here for plasmonic coupling offers a versatile platform for improving our understanding of optical interactions among individual chemically prepared nanocrystal components.

Reflective displays have attracted a lot of attention because of their low power consumption, simple manufacturing process, without the backlighting source. Many researchers have presented various types of reflective displays, such as cholesteric liquid crystal displays, bistable nematic liquid crystal displays, zenithal bistable displays with bistability, electrophorectic displays (EPD), and interferometric modulator displays. Among these, nowadays reflective liquid crystal displays (RLCDs) have been studied mainly for use in mobile devices, outdoor signboard applications, and e-books. Here, we developed novel reflective photonic device for low-voltage tunable full color display. The structural color matrix was made of virus (M-13 bacteriophage) through self-assembly process reported in our 2011 Nature paper. Recently, virus has been applied to various electronic devices due to its well known liquid crystal behavior property. Upon application of low bias voltage (<0.2V), the virus based nanostructure immediately exhibited the desired colors through structure modulation. We performed the various letters and numbers using the self-designed 7-digits display device with patterned micro-heater. We expect that virus based color display device will be used for next generation full-color displays.

Plasmonics is a rapidly emerging field of nanophotonics that focuses on the ability of noble metal nanostructures to manipulate light. For example, by using nanocorrugations etched in a metal film, light at optical frequencies can be coupled to surface plasmon polaritons (SPPs), electromagnetic waves that propagate along a metal-dielectric interface. SPPs are confined at the metal surface and are very sensitive to small changes in the refractive index of the dielectric (e.g. aqueous solutions with biochemical analytes).
We have recently reported on a compact, high-throughput plasmonic sensor based on plasmonic interferometry optimized for real-time monitoring of glucose in aqueous solutions [1]. The sensor consists of a spatially dense, planar array of plasmonic interferometers (with a density >1,000/mm²), where each interferometer is composed of two 200nm-wide, 10µm-long grooves flanking a 100nm-wide slit etched in a 300nm-thick silver film. The distances between each groove and slit were varied between 0.25 to 10µm in steps of 25nm. The detection limit of the plasmonic sensor for glucose in aqueous solutions is 5.5mu;M with a sensitivity of 105,000%/RIU (refractive index units) or 0.2 × 10⁵ % / M at 590nm.
In order to improve the sensor selectivity to glucose, we adopt a novel molecular recognition scheme that couples plasmonic interferometry with dye chemistry, specifically the Amplex Red enzyme assay. In this implementation, glucose oxidase is added in solution to rapidly convert D-glucose into D-gluconolactone and H₂O₂ in a 1:1 stoichiometry. The H₂O₂ reacts with horseradish peroxidase (HRP) to oxidize Amplex Red into Resorufin, a dye molecule which is characterized by a strong optical absorption coefficient at ~571nm. The reaction is monitored in real-time by simply measuring changes in the light intensity transmitted through the slit of each interferometer.
This device is both highly sensitive, with a measured intensity change of 1.7 × 10⁵ % / M (i.e. about one order of magnitude more sensitive than without assay) and selective for glucose in picoliter samples, across the physiological range of glucose found in human saliva (20 minus; 240 mu;M). Since plasmonic interferometry enables spectroscopic fingerprinting of a sample and the assay is selective for glucose, the detection of glucose is possible within a complex mixture of proteins, small molecules, and salts typically found in samples of saliva or serum.
In addition, to make the device feasible for real-time glucose monitoring in saliva, the underlying reactions of the assay are studied in detail. The effective rate constants are determined and used to tune the reaction time and expand the detection range of the assay. By varying the dye assay used, the plasmonic interferometry can provide a general, real-time approach to sensing low concentrations of a selective molecular target within a very small volume of biological fluid.
[1] J. Feng et al., Nano Lett. 2012, 12 (2), pp 602-609

Polymer nanocomposites are already technologically important today and have attracted attention for new applications in recent years [1-3]. Researchers use small objects such as nanoparticles (NPs) for tailoring the electrophysical, optical and magnetic properties of these composites. Here, we consider composite coatings containing semiconducting nanocrystalline quantum dots and mixed rare earth oxide nanoparticles for photonic devices. Similar coatings produce and harvest excitons in photovoltaic cells, electro-luminescent devices and scintillation detectors for ionizing radiation [4,5]. We analyze the distribution of the particles with respect to the polymeric phase to correlate it with the performance of the materials.
Mixtures of P3HT, PCBM and different nanoparticles were sprayed onto flat substrates to obtain hybrid films [1]. The materials were analyzed using a combination of focused ion beam preparation, scanning electron microscopy, and transmission electron microscopy to examine the distribution of particles in the composite. Our goal was to relate the colloidal properties of the particles and the microstructure of the resulting hybrid material. The FIB parameters were adopted to obtain cuts through the soft polymer matrix and the intersected hard QDs or nanoparticles in one smooth plane and to obtain suitable samples for SEM and TEM examinations. Electron microscopy revealed that depending on the colloidal state of the particles as well as details of the deposition and annealing process, composites with different morphologies form. We discuss the formation of interlayers due to partial de-mixing, agglomeration that causes strong and inhomogeneous distribution of particles, and discuss implications on the distribution of components of the polymer phase with respect to particle-polymer interfaces. Finally, first attempts to rationally influence the particle distribution in the composites will be presented.
[1] Rauch T., Böberl M., Tedde S. F., Fürst J., Kovalenko M. V., Hesser G., Lemmer U., Heiss W., Hayden O., Nature Photonics 3, (2009) 332.
[2] Wagner B. K., Kang Z., Nadler J., Rosson R., Kahn, B., Proc. of SPIE 8373, (2012).
[3] Saunders B. R., Turner M. L., Advances in Colloid and Interface Science 138, (2008) 1.
[4] Lawrence W. G., Thacker S., Palamakumbura S., Riley K. J., Nagarkar V. V., IEEE Transactions on Nuclear Science 59, (2012) 215.
[5] Mills C. A., Al-Otaibi H., Intaniwet A., Shkunov M., Pani S., Keddie J. L., Sellin P. J., J. Phys. D: Appl. Phys. 46 (2013) 1.

Lanthanide-doped nanoparticles exhibit near-infrared-to-visible upconversion (UC) luminescence that could enable improved solar energy harvesting and more powerful bioimaging. However, these upconverters are generally limited to quantum efficiencies below 1%. To improve UC efficiencies, we consider a novel approach - strain engineering - that tunes the structure of the host material. Such strain engineering systematically modifies interionic separations and crystal field interactions, thereby influencing the UC efficiency of the material.
In this work, we consider as a model system trivalent Er and Yb ions doped into NaYF₄ nanoparticle matrices. We monitor the upconversion luminescence and excited-state lifetimes as the host is compressed hydrostatically in a continuous fashion via an applied pressure. To more thoroughly explore the relation between structure and UC properties, experiments were conducted using particles of both the hexagonal and cubic phases of NaYF₄. Nanoparticles with diameters of 90 and 200 nm, respectively, were colloidally synthesized and characterized by x-ray diffraction to verify phase purity. Nanoparticles were then compressed up to 25 GPa using an optically accessible diamond anvil cell, which enabled in situ analysis of upconverted emission and lifetimes upon excitation with a 980-nm diode laser. Finally, in situ high-resolution x-ray diffraction (XRD) was conducted using synchrotron radiation to quantify the structural changes induced in both hexagonal and cubic lattices upon an applied pressure.
The luminescence properties of both types of nanoparticles exhibited significant changes with pressure, highlighting the key role host structure plays in UC systems. Emission intensity from the hexagonal-phase NaYF₄:Er³⁺,Yb³⁺ particles decreased rapidly with compression and remained greatly reduced even upon complete pressure relaxation; additionally, spectral shifts as large 20 meV evinced changes to the local crystal fields surrounding the ions. XRD measurements confirmed the engineered strains in the hexagonal-phase particles to be entirely reversible while also demonstrating the introduction of inelastic, non-uniform strain, suggesting that the observed nonreversible intensity trends result from the generation of quenching defects at high pressure. In the cubic-phase particles, UC luminescence intensity increased up to 13 GPa before decreasing with further compression. The UC emission profile also changed markedly at this same pressure, hinting at the occurrence of a phase transformation which was subsequently confirmed with XRD. Finally, excited-state lifetimes were measured as a function of pressure as a means to decouple the effects caused by lattice compression. These results demonstrate the promise of using structural manipulation as a means to influence UC efficiency and provide crucial insight into methods to significantly improve this UC system.

Magnetically or electrically tunable colloidal crystals have been investigated as new display materials for a past decade. Their coloration mechanisms have been recognized as obvious light scattering. Incident lights with wavelength longer than the distance between each particle in the colloidal crystals come upon the colloidal crystals and diffract at Bragg&’s angles, resulting in the coloration of the colloidal crystals. The driving force of this coloration was thought as magnetic or electric field that can pull or push charged particles in the colloidal system such that the distance between charged particles changes. Here we report on a new mechanism that a reduction or oxidation reaction should be accompanied along with the magnetic or electric fields.
A series of experiments were carried out to reveal the coloration mechanism of the colloidal crystals. First, colloidal crystals (ETX-INK, Nanobrick) were inserted between ITO glasses. When the electric field was applied, the colloidal crystals showed clear blue color, which was dependent on the electric field intensity. Next, a dielectric substrate was inserted between ITO glasses, e.g., below top ITO glass. Then, the electric field was applied, however surprisingly the colloidal crystals did not show color change regardless of the electric field intensity. We postulated that the dielectric substrate might prevent some events happened without it. The event turned out to be an oxidation or a reduction reaction depending on the charge of the colloidal nanoparticles. We proved this fact using zeta potential. Based on this new finding, we designed a fiber display, which can be utilized as smart skins, and characterized its performance. The detailed experiments and results will be presented at the Conference.

We provide evidence for optical non-reciprocity in planar texture chiral nematic solid state films of nanocrystalline cellulose (NCC). To our knowledge, this is the first example of optical non-reciprocity without the simultaneous intervention of an applied field in a system of biological origin. NCC is derived by careful hydrolysis of wood fibers. The nanocrystallites consist of rather uniform rods of cellulose with typical dimensions of 10 nm width x 125 nm length. These high aspect ratio objects are known to form an anisotropic chiral nematic liquid crystal phase at rather low weight percent in water. The nanorods can be aligned with careful evaporation of water in the presence of a magnetic field to give very homogeneous planar texture films in which the NCC assembly helicoid axis is oriented orthogonal to the substrate plane. The films reflect left circularly polarized light. Highly ordered solid films express a metallic-like gold luster and reflectivity reminiscent of the chrysalis of the butterfly Euopea clore. Under certain conditions in which an asymmetric boundary is imposed, the gyrotropic NCC films exhibit optical non-reciprocity. We provide evidence for resonant and non-resonant non-reciprocity for left and right circularly polarized light in both transmission and reflectance modes. A simple quantitative model is offered to account for the effects. We discuss some possible implications of the findings for insect and plant biology optics.

Transparent conductive oxides (TCOs) are a broad class of organic and inorganic materials exhibiting both optical transparency and electrical conductivity simultaneously. TCOs are currently utilized as top-contact passive layers in a number of optoelectronic devices. Recently, they are also attracting considerable attention as an active platform in UV light emission and detection, and as alternative plasmonic materials in the near-IR telecom wavelength range. Among them, Zinc Oxide (ZnO) is the most promising candidate for optoelectronic applications due to its large band-gap, high refractive index, low cost, and Si compatibility. We propose ZnO as a novel material platform for telecom and bio-compatible optoelectronics, plasmonics and solar cell devices. We investigate its linear and nonlinear optical properties as we consider different dopant elements for the same material.
First, we demonstrate multiband near-IR emission of rare earth (RE) dopants, Er and Nd, in ZnO thin layers under optical and electrical injection. We have investigated the RE excitation mechanism, driven by energy transfer from the ZnO band-gap and its optical-active defect centers. Moreover, we introduce Si in the matrix, and we demonstrate and quantify efficient Si-mediated energy sensitization of the rare earth ions. Si influence on the RE emission lifetime is elucidated as well. Material optimization will be discussed in detail. The influence of the growth parameters as well as thermal annealing processes on the optical and electrical properties of the thin films grown by magnetron sputtering will be elucidated. A proof-of-concept electroluminescent device based on Er-doped Si-rich ZnO with very low injection current density and turn-on voltage will be shown. In addition, Aluminum-doped ZnO (AZO) has been demonstrated as a transparent and conductive plasmonic material in the near-IR wavelength range. Opportunities and advantages of AZO as a metal-free Si-compatible platform for novel metamaterial devices towards the engineering of enhanced light emitting devices will be discussed. Furthermore, the nonlinear optical properties of ZnO thin films with different dopants, doping concentrations and post-processing treatments have been experimentally investigated using ultrafast excitation conditions and will be discussed.

Organic semiconductor technology paves the way for low cost lightweight, flexible, printable electronics circuits and sensors. A novel lateral multilayer organic semiconductor photosensor is fabricated using small molecule organic semiconductor and Inorganic amorphous material layers. A specialized interface layer is introduced between the contact metal and organic semiconductor layer. The interface layer material has properties such as large band gap and low electronic conductivity. The addition of interface layer limits the charge injection from the electrodes to the organic semiconductor and further improves the photosensor dark current performance with an advantage to apply high voltage for improved collection. This design has low dark current with high photo-to-dark current ratio and can be set to high bias mode of operation.
Lateral interdigitated aluminum metal electrodes are pattered on a glass substrate using conventional photolithography techniques. The substrate is then coated with a 180nm thick layer of polyamide followed by vacuum thermal evaporation of organic semiconductors, 50nm thick 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) and 50nm thick Copper-Phthalocyanine (CuPc) layers respectively. Current through the sensor is measured in both dark and in light (wavelength 400nm). The dark current density in a 1mm2 photosensor area with 5mu;m lateral electrode spacing at 10V/mu;m measured equal to 5x10-5mA/cm2 and a photocurrent density of of 1x10-3 mA/cm2 under 5mW/cm2 incident optical power. The photo to dark current ratio is measured to be equal to 100.
This photosensor has an application in large area imaging for example portable lightweight detectors. Other applications of this sensor include indirect medial imaging and as a biosensor in UV Spectroscopy study of bacteria cultures.

Low coherence light sources are critical for a number of high-resolution imaging and microscopy techniques. We present a novel and highly sensitive method for measuring the spatial coherence of an optical field. We determine the diameter of the spatial coherence area of a broadband Xenon lamp in the range of 1-5µm, constituting the lowest spatial coherence measurements yet made in the far-field. By employing plasmonic interferometers, consisting of grooves and slits milled in a thin silver film, we take frequency domain interference spectra at hundreds of different spatial separations known to nanometer precision. From the visibility of interference fringes, we quantify the degree of coherence for multiple illumination conditions. It is known that previously demonstrated means of measuring spatial coherence using some variant of Young&’s double pinhole interferometer are susceptible to difficulties associated with surface plasmons. In contrast to previous methods, the use of plasmonic interferometry uniquely accounts for such effects. In addition, we provide evidence for a previously unobserved optical pathway which may be present in any interferometer which is susceptible to the effects of plasmonic modes. In this pathway, a component of a light field is shown to interfere with a temporally, but not spatially, separated component of the same field, causing interference patterns to arise from spatially incoherent fields which may be unexplainable without plasmon modes.

The energetic efficiency of current photovoltaic devices suffers from incomplete light trapping. A promising method for achieving enhanced light trapping is the incorporation of 3D metallo-dielectric photonic crystals into PV devices. We present the fabrication methodology and characterization of 3-dimensional silver coated polymer nano-truss lattices whose hierarchical dimensions span from tens of nanometers in individual strut diameter, to several microns in unit cell size, to hundreds of microns in overall structure extent. The tailored design of metallo-dielectric nanolattices allows these structures to act as tunable 3D waveguides, directing EM mode propagation over a wide range of wavelengths. Nano-trusses were fabricated by first designing the lattice as a 3 dimensional CAD model, then using two photon lithography to trace the design into IP-L 780, a negative acrylic-based photoresist, with a 780nm femtosecond pulsed laser focused down to a submicron elliptically shaped region. This submicron laser spot has sufficient energy to crosslink and form the polymeric truss skeleton. Fabrication is then followed by conformally sputtering silver over the periodic polymer architectures. Optical characterization, in the form of transmission and reflection spectra, was performed on the nano-trusses by collaborators at Stanford University in the Dionne Group. Nano-mechanical experiments were performed in house using an in-situ mechanical instrument known as the SEMentor. A custom-built optical fiber-based setup inside a nanoindenter then enabled the collection of mechanical and optical measurements simultaneously. Shifts in photonic band gap as a function of strain are discussed.

Periodic micro- and nano-structures are slowly gaining traction in diverse areas such as bio-sensing, surface enhanced Raman spectroscopy (SERS), solar cell concentration and antireflection strategies, solid state lighting, as well as in integrated micro-optics, lasers, optical filters, cloaking structures, and even in electrochemical devices. Unfortunately, obtaining a reliable, cost effective, yet flexible manufacturing source for such structures has often been a challenge. Many research groups and companies perform their own lithography and etching or use a foundry to generate such structures, but this typically limits the substrate size to a few square inches and can have large up-front costs for masks and other tooling. In addition, the periodicities of structures readily available from commercial sources are typically limited to several hundred nanometers and larger. Moxtek has addressed some of these challenges by leveraging existing capabilities in wafer-scale patterning of sub-wavelength wire grid polarizers into the fabrication of one- and two-dimensionally periodic plasmonic structures.
This presentation will discuss fabrication and characterization of 2D plasmonic arrays for potential applications in surface plasmon resonance (SPR) sensing, surface-enhanced Raman scattering (SERS), and surface-enhanced fluorescence spectroscopy (SEFS). Potential markets include micro-arrays for bio-related assays and trace level chemical detection. In addition, the presentation will review recently-commercialized work on narrow-band, cloaked wire grid polarizers composed of nano-stacked metal and dielectric layers patterned over 200 mm diameter wafers for projection display applications. The stacked nanowire grid approach results in a narrow-band reduction in reflectance by a factor of about 20, which can be tuned throughout the visible spectrum for stray light control.

We present new spectroscopic techniques that enable visualization of nanoparticle phase transitions in reactive environments and light-matter interactions with nanometer-scale resolution. First, we directly monitor hydrogen absorption and desorption in individual palladium nanocrystals. Our approach is based on in-situ electron energy-loss spectroscopy (EELS) in an environmental transmission electron microscope. By probing hydrogen-induced shifts of the palladium plasmon resonance, we find that hydrogen loading and unloading isotherms are characterized by abrupt phase transitions and macroscopic hysteresis gaps. These results suggest that alpha and beta phases do not coexist in single-crystalline nanoparticles, in striking contrast with conventional first-order phase transitions and ensemble measurements of Pd nanoparticles. Then, we then extend these techniques to monitor nanoparticle reactions in a liquid environment. By constructing a flow chamber, we directly monitor growth and assembly of colloidal plasmonic metamaterial constituents induced by chemical catalysts. Lastly, we introduce a novel tomographic technique, cathodoluminescence spectroscopic tomography, to probe optical properties in three dimensions with nanometer-scale spatial and spectral resolution. Particular attention is given to reconstructing a 3D metamaterial resonator supporting broadband electric and magnetic resonances at optical frequencies. Our tomograms allow us to locate regions of efficient cathodoluminescence across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. The experimental signal can further be correlated with the radiative local density of optical states in particular regions of the reconstruction. Our results provide a general framework for visualizing chemical reactions and light-matter interactions in plasmonic materials and metamaterials, with sub-nanometer-scale resolution, and in three-dimensions.

Colloidally synthesized gold nanorods are widely used for imaging applications because of their strong absorption and scattering which is tunable into the infrared, the optical window in biological tissue. While the scattering frequency of a gold nanorod is commonly tuned by changing the size or aspect ratio of the nanorod, the optical properties of nanorods can also be manipulated by bringing the rods in proximity in a controlled way. Toward this goal, our lab has developed a process by which short strands of DNA can be used to precisely spatially order gold nanoparticles. Recent research has focused on the incorporation of anisotropic nanoparticles to tailor more complex resonances in these DNA-assembled structures. We have demonstrated for the first time that gold nanorods can be assembled by this method.
We have also demonstrated control over the relative orientation of the nanorod pairs. To create orientation controlled DNA-assembled nanorods, oligonucleotides of 60 basepairs with a 10, 20, or 30 basepair overlap are conjugated to the gold surface of a nanorod with dimensions ~18nm by ~45nm via a 5&’ branched trithiol moiety. After surface protection by a short chain PEG ligand, singly conjugated nanorods are separated from other products using anion exchange HPLC. Complementary conjugates are reacted; dimers are separated using agarose gel electrophoresis. By tuning the conditions of electrophoretic separation and the DNA structure, a sample of gold nanorod dimers that are attached side-by-side can be isolated from those attached end-to-end. This separation is achieved due to the difference in radius of gyration of possible nanorod pair geometries. TEM characterization of these oriented dimers shows a statistically significant deviation from randomly oriented nanorod pairs. For example, TEM analysis of pairs of side-to-side dimers show an average angle between longitudinal axes of 9.7+/-7.1 degrees while randomly attached dimers show an average angle of 43.7+/-31.8 degrees. Controlling the orientation of nanorods in this way provides an opportunity for DNA-conjugated nanorods to be artificial atoms in metamaterials processed from solution.
Because of the high yield of oriented nanorod pairs, this colloidal sample can serve as an ensemble biosensor. A restriction site for the endonuclease EcoRV was programmed into the double stranded DNA. Monitoring the longitudinal and transverse plasmon bands of the nanorod dimer, the progress of the DNA-cutting enzyme can be ascertained. In the case of side-by-side dimers with a 20 basepair (~9nm) separation, plasmon resonance peaks at 521nm and 625nm shift to 513nm and 631nm after one hour of incubation with EcoRV. As a dipole coupling model predicts, these shifts depend on the length of DNA attaching the nanorods. Single particle darkfield spectroscopy measurements confirm that nanorod dimers show different scattering spectra depending on the relative orientation of the nanorods.

By controlling the shape, size, spacing and aspect ratio of metallic nanostructures, their corresponding optical properties can be tuned via coupling to surface plasmon waves [1]. Large enhancement of local electric field can be created by plasmon oscillations because these nanostructures can condense light into subwavelength volumes [2]. Hence, the ability to dictate the geometry of metallic nanostructures can lead to new plasmonic materials that have interesting optical applications.
Here, we report an inexpensive, facile and reproducible fabrication route that makes use of colloidal lithography and electrodeposition to create two-dimensional arrays of bi-pyramidal gold nanopillars embedded in a dielectric medium. This approach relies on the self-assembly of monodisperse polystyrene (PS) beads on a conductive substrate and subsequent heat treatment to modify the inter-particle spaces between the beads to create ordered and porous templates for electrodeposition. A sacrificial metal layer is first deposited at the bottom before gold is deposited in the middle of the pores. Once the electrodeposition of gold is complete, the polystyrene beads are removed. The voids are then filled with polydimethylsiloxane (PDMS) which fills up the pores (left after PS bead removal) and also covers the entire top surface of the substrate (to provide mechanical stability). The sacrificial metal layer is then dissolved in acid and the PDMS layer with the gold nanopillars embedded within can be simply ‘peeled&’ off the surface of the conductive substrate. Each gold nanopillar has bi-pyramidal shape with three sides that is narrower in the middle because they take the shape of the pores which are formed between three adjacent PS beads during the self-assembly process. Simulation of our device architecture shows the presence of localized modes that result from the arrangement of the gold pillars in the dielectric medium under illumination (400 - 1400 nm). Experimental results agree with the simulations and show similar trends. The influence of both the inter-nanopillar distance and the aspect ratio of the nanopillars on the optical properties are being investigated. In addition, the positioning of light emitters at the tips of the nanopillars allows us to study the plasmons - emission interaction.
[1] S. Habouti, M. Mátéfi-Tempfli, C.-H. Solterbeck, M. Es-Souni, S. Mátéfi-Tempfli, and M. Es-Souni, J. Mater. Chem., vol. 21, no. 17, p. 6269, 2011.
[2] D. Lis, Y. Caudano, M. Henry, S. Demoustier-Champagne, E. Ferain, and F. Cecchet, Adv. Opt. Mater., vol. 1, pp. 244-255, 2013.

This presentation will highlight recent progress of the chemical design of plasmonic building blocks with an emphasis on chemical control of both metallic and highly doped semiconductor plasmonic systems. There has been significant progress in the preparation of plasmonic Au nanorod dispersions in the past decade and several effective routes to their production have been established. We will share our strategy of using of unsaturated amphiphiles (aromatic and alkenes) co-surfactants with a wider range of quaternary ammonium based surfactants to improve the seeded growth of Au nanorods. This approach has enabled the production of nanorods with an improved control over the rod aspect ratio and the corresponding spectral properties, while retaining tight tolerances in the size and shape particles even when prepared in large batch sizes (2 liter +). As methods have been developed to push Au nanorod resonances from the visible into the NIR there has been simultaneously an intense interest in exploring doped semiconductor plasmonic systems as an IR tunable plasmonic option. Strategies to control size, shape and the doping of semiconductor nanocrystals are yielding new options for sharp tunable optical resonances in the NIR. Progress in the development of dope cadmium oxide based systems (specifically indium doped cadmium oxide ICO) will be shared as an example of progress in this area. In addition to a discussion of the properties of the dispersed metal and semiconductor base plasmonic nanocrystals we will detail methods to organize these into thin film structures to enable future integration with optical and optoelectronic systems.which exhibit long-range orientational and positional order. The design of shape complementary anisotropic building blocks offers the possibility to self-assemble binary superlattices with predictable and desirable structures. The potential for further development of the shape complimentary assembly in 2D and 3D will be discussed.

Nanoscale integration of phases with metallic and non-metallic character is critical for the
realization of optical metamaterials. However, in the near-infrared and visible wavelength regime, the creation
of useful metamaterials requires the ability to fabricate three-dimensional (3D) complex structures with high
precision using techniques that ultimately enable large-scale manufacturing. A new paradigm for the design
and fabrication of metamaterials is therefore needed. Our team is appling unique template-based and
post-synthetic materials transformations in conjunction with powerful computational design tools to develop
the scientific underpinnings of and to produce 3D metamaterials derived from directionally solidified eutectics.
Our approach involves close interactions among computational design, photonic theory, eutectic materials
development, template fabrication, materials chemistry, and optical characterization.

Optical and electrical properties of metals and semiconductors are profoundly related to their composition and geometry at the nanoscale. For example, optical excitation of a metal nanostructure can generate intense and highly localized electromagnetic fields because of the interaction of the light and the electrons in the metal, a phenomenon known as the localized surface plasmon resonance. The magnitude and frequency of this resonance is strongly dependent upon the shape and size of the nanoparticle. Nanoscale structure is also important for semiconductors as core-shell nanowires composed of optically active p- and n-type semiconductors have photovoltaic properties that are superior to their planar counterparts due to more efficient charge separation owing to the high interfacial surface area and relatively small distance required to separate electron-hole pairs. Coupling plasmonic and excitonic processes within individual hybrid nanostructures holds great promise for enhancing important effects such as photoluminescence, photoconductivity, and photocatalytic activity. Here, we present a novel approach which allows for the fabrication of functional multicomponent nanowires with unprecedented control over the composition and size of semiconducting active regions, with well-defined plasmonic materials integrated at precise locations. These nanowires were used to study plasmon/exciton interactions and as testbeds to explore emerging photonic applications such as optical nanorulers or plasmonic photoconductive devices.

The sacrificial templates used in galvanic replacement reactions dictate the properties of the hollow metal nanostructures formed. The work presented will demonstrate that substrate-based Au-Ag nanoshells with radically altered properties are obtained by merely coating silver templates with an ultrathin layer of gold prior to their insertion into the reaction vessel. The so-formed nanoshells exhibit much smoother surfaces, a higher degree of crystallinity and are far more robust. Dealloying the nanoshells results in the first demonstration of substrate-based nanocages. Such cages exhibit a well-defined pattern of geometric openings in directions corresponding to the {111}-facets of the starting template material. Of significance is the fact that the nanocage is a highly asymmetric structure due to both its hemispherical-like geometry and the presence of the geometric openings. This places the nanocage in a class of symmetry breaking structures (e.g. semishells, nanobowls, and nanocrescents) which offer polarization dependent plasmon modes with large local field enhancements and a high degree of tunability. Additional tunability is derived from the fact it is possible to engineer the cage geometry through adjustments to the orientational relationship between the crystal structure of the starting template and that of underlying substrate. These discoveries will be discussed and used to advance the understanding of the mechanisms governing substrate-based galvanic replacement reactions.

Advanced nanofabrication techniques have enabled rapid advancements in creating plasmonic nanoparticle arrays with complex optical resonance properties that are determined by their intrinsic geometry and extrinsic environment. These structures have been adopted for a wide range of applications that rely on either near-field enhancement or far-field diffractive coupling, including waveguides, sensors, solar cells, and metamaterials. The most common approach to fabricate nanoparticle arrays employs conventional top-down lithographic patterning followed by lift-off of an evaporated plasmonic metal. Although this method provides excellent control of nanoparticle size and placement, the lift-off process creates cylindrical particles with asymmetric tilted sidewalls, which can broaden the plasmonic peak width compared to an ideal isotropic spherical particle array. An alternative strategy has used localized laser induced heating of a planar evaporated Au film to create 2D spherical plasmonic particle arrays. However, the minimum spacing between adjacent particles has been limited to the micron scale, which prevents fabrication of arrays with strong interparticle coupling. This presentation will describe a new nanofabrication method that employs Au-enhanced oxidation to synthesize 2D spherical plasmonic nanoparticle arrays with well-controlled particle placement, diameter, and spacing down to the nanometer scale. The process begins by defining an array of cylindrical amorphous-Si/Au nanoparticles on a fused silica substrate using electron-beam lithography and evaporation. The amorphous-Si/Au particle array is then converted into a Au/SiO2 core-shell nanoparticle array by thermally oxidizing the entire structure in dry O2. During the thermal treatment, the Au core is transformed from a cylinder into a sphere to reduce the interfacial energy between the Au core and the SiO2 shell. In this process, the final spherical Au nanoparticle diameter and interparticle spacing is determined entirely by the starting lithographic pattern and the evaporated Au volume. To evaluate the optical properties of these structures, the angular response of a 2D periodic spherical nanoparticle array with 135 nm diameter Au particles and 360 nm spacing was compared to a conventional evaporated Au cylindrical nanoparticle array with the same geometry. The spherical nanoparticle array had a narrower resonance peak width and coincident transmission spectra for transverse electric (TE) and transverse magnetic (TM) polarizations at normal incidence. Additionally the isotropic particle geometry resulted in congruent reflectance in both the TE and TM modes at oblique angles of incidence. This approach is also being applied to fabricate 2D aperiodic nanoparticle arrays with Ammann-Beenker and Penrose tilings, which lack transitional symmetry and thus excite multiple plasmonic/photonic hybrid modes for a broader resonance response.

Nanowire enabled photovoltaics offer several advantages over thin film architectures. These include strong optical resonances, (Cao et al., Nature Materials, 2009), and the possibility to create a radial junction in a core-shell geometry. The presence of a metal core adds further advantages, such as the confinement of light in the semiconductor wrapped around the core, which has been predicted to lead to an extreme absorption in the shell (Mann et al., Nano Letters, 2013), and the efficient carrier collection by the metal core, that acts also as an electrode (Garnett et al., Annual Review of Materials Research, 2011).
Ternary I-III-VI2 semiconductors have been receiving increased attention as promising materials for photovoltaics because of their large absorption coefficient, high conversion efficiency and low toxicity (Yu et al., Advanced Energy Materials, 2013).
Lanaite (AgFeS2) has a band gap of 1.2 eV and it has been reported to be a promising absorbing material for solar cells (Han et al., Small 2013). However, to the best of our knowledge, only the synthesis of small lanaite nanocrystals (asymp;10 nm) has been reported thus far (Han et al., Small 2013). In this work we&’ll show a novel solution phase synthesis of AgFeS2 nanowires, and core-shell heterostructures, with a metal core and lanaite as the semiconducting shell. XRD, HRTEM, EDX, SAED characterization results will be presented to support our findings. Furthermore, we will show optical characterization (extinction and absorption) of single core-shell nanowires and compare the results to simulations. This hybrid core-shell architecture that leads to extreme light trapping is promising for the realization of cheap and efficient solar cells.

Abstract: We experimentally demonstrate the first optical nanoantenna link (lambda; = 785 nm) which enables low-loss communication across a distance of 38 lambda; and allows dynamic reconfiguration of the link using a phased array transmitter.
1. Introduction
In the radio-frequency regime, wireless signal transfer with antennas instead of cable connections revolutionized communication technology. In the optical regime, a similar scheme of free-space signal transfer using plasmonic antennas has recently been proposed theoretically [1]. It was predicted that a free-space nanoantenna link (power law signal decay) outperforms plasmonic waveguides (exponential signal decay) without sacrificing deep sub-wavelength field-confinement at the transmitting and receiving points. The efficiency to transmit power can be further boosted by orders of magnitude using antenna arrays due to improved directive emission and reception. Furthermore, the transmitted beam can be steered to different receiving points by controlling the driving phase of the elements in the antenna array [2].
Here, we report experimental realization of efficient signal transmission between two plasmonic nanoantennas at the near-infrared wavelength lambda; = 785 nm. We optimize the directivity of our transmitter and measure a power law decay sim;1/d². A receiving antenna at 38 lambda; distance enhances the power transmission by almost one order of magnitude. Finally, we use an optical phased array as transmitter in the antenna link and demonstrate dynamic beam steering in a range of 29 degrees, allowing us to dynamically reconfigure the transmission link.
2. Optical nanoantenna link
We first characterize the radiation pattern of single plasmonic dipole antennas and one-dimensional antenna arrays with varying number of dipoles. In order to map their radiation pattern, a layer of fluorescent molecules is spin-coated onto the structure and the sample is embedded in a homogeneous dielectric environment. We find that an array of five dipole antennas has optimal directive emission in our geometry.
We position a row of nanoscopic fluorescent disks as receivers along the emission direction of our optimized transmitter using electron beam lithography. A schematic of the experiment is shown in Fig. 1b (top panel). The furthest receiver, 38 lambda; away from the transmitter, contains a receiving plasmonic antenna. The luminescence image when we drive the transmitter, Fig. 1b (middle panel), shows an intensity decrease with increasing distance from the transmitter. A distinct signal increase is observed for the receiver with the antenna. The luminescence as a function of distance is presented in the lower panel of Fig. 1b together with a 1/d² decay (green curve). We observe excellent agreement between theory and experiment, demonstrating the predicted advantage of optical antennas in point-to-point links compared to plasmonic waveguides. Finally, a 7.7-fold intensity enhancement is observed for the fluorescent disk with receiving antenna.
3. Optical phased transmitter in nanoantenna link
We next use the five-element antenna array as an optical phased array transmitter allowing us to dynamically address different receivers. In our experiment we use a high-NA objective to focus the incident light on the sample, leading to steep curvature of the incident wave-fronts close to the focal plane. By moving the sample out of the focal plane and controlling the lateral position of the incident beam with respect to our antenna array we induce a phase gradient across the phased array elements. This steers the transmitted beam towards the left, as observed in the luminescence image. The signal is collected by a ring (10 mu;m radius) of fluorescent disk receivers where every second disk contains also a plasmonic antenna. We now observe a strong signal away from the ±y directions.
By shifting the incident beam laterally with respect to our antenna array we control the phase across the antenna array and achieve continuous beam steering of the transmitted beam over a range of 29 degrees. We compare the experimental data to a model where each antenna element in the array is represented by a dipole. The driving phase is taken from the calculated phase distribution of the tightly focused incident laser beam. We observe very good agreement of the steering angle between experiment and theory.
4. Conclusion
An experimental demonstration of an optical plasmonic nanoantenna link has been realized for the first time. We used directive plasmonic nanoantenna arrays as transmitters and measured the predicted power law 1/d² decay for the signal transmission. A 7.7-fold efficiency enhancement of our nanoantenna link due to a receiving plasmonic antenna was observed at a distance of 38 lambda; away from the transmitter. We further demonstrated high flexibility of our device by reconfiguring the antenna link using dynamic beam steering in a range of 29 degrees.
We acknowledge support from ERC, DFG, BMBF, BW-Stiftung, Zeiss-Stiftung, and GIF.
[1] A. Alugrave; and N. Engheta, “Wireless at the nanoscale: Optical interconnects using nanoantennas,” Phys. Rev. Lett. 104, 213902 (2010).
[2] D. Dregely et al., “3D optical Yagi-Uda nanoantenna array,” Nat. Commun. 2, 267 (2011).

Resonant absorbers based on plasmonic nanostructrures and optical metamaterials have gained extensive attraction in recent years. Plasmonic absorbers have the advantage of wide range spectral tunability by either dynamically or passively changing the structural parameters in addition to being extremely thinner than conventional coatings. The possibility of utilizing these materials as for example nanoscale heat concentrators has opened new ways in photothermal cancer therapy and thermo-photovoltaic applications. There have been several proposals for effectively scattering sunlight into ultrathin film semiconductors using plasmonic and photonic light trapping mechanisms. However, there has been little thought that went into investigation of direction-dependent absorption, reflection and transmission characteristics of optical materials. Here, we will describe a unique optical material that exhibits asymmetric optical absorption and reflection behavior that utilizes free standing nanostructured ultrathin-films.
Our structure consists of a bilayer membrane of a Si3N4/Ag stack (150/50 nm). A square lattice of nanoholes is etched via ion-beam milling on this free standing film. Finite Difference Time Domain (FDTD) simulations were performed to investigate the mechanism of the enhanced absorption and to optimize the design parameters for the best performance. Our calculations predicted more than 40 times difference in absorption depending on the illumination direction with finite transmission. We measured the total reflection and transmission spectra of the fabricated structures in the range 400 800 nm for illumination from top and the bottom sides. We experimentally demonstrated 4 times more absorption when the structure is illuminated from the top side compared to illumination from the bottom side. The rather weak performance in experiments can be attributed to the imperfections in fabrication and surface roughness. We also performed FDTD simulations using various semiconductors like aSi and Ge. Results are encouraging for realization of bidirectional multicolor narrow band photodetectors.
There is a great interest in forming photonic equivalent of electrical circuit elements like diodes. While it is rather challenging to realize asymmetric transmission -a true diode behavior- unless nonlinear processes are involved, here we show that it is possible to design optical materials with a “diode like” asymmetric absorption profile, i.e. material absorbs light in one direction and reflects in the other direction. Using asymmetric absorbers and reflectors, one could actually achieve ultra-high efficiency solar cells by overcoming the well-known Shockley-Quisser limit. In particular, materials that absorb/emit light in one direction and reflect in the reverse direction could find applications not only in photovoltaics, but also in several energy conversion and conservation area including thermophotovoltaics, light emitting devices and thermal management.

Electromagnetic absorbers are of significant interest for a variety of infrared (IR) applications such as spectroscopy, emissivity control, solar energy harvesting, and thermal imaging. Metamaterials composed of nanoscale metallic resonator arrays have emerged as compelling candidates for IR absorbers, with recent efforts aimed at demonstrating mid-IR devices with customized single-band, multi-band, and wide field-of-view (FOV) responses. However, achieving nearly ideal absorption over a broad and well-defined range of IR wavelengths has remained an open challenge due to the narrow bandwidth of each electric and/or magnetic resonator element as well as the poor free-space impedance match across the entire band. This presentation will describe a metamaterial absorber (MMA) based on an electromagnetic band-gap (EBG) structure that provides nearly ideal spectral absorptivity across a broad mid-IR band with specified band-edge wavelengths. The EBG MMA is composed of a doubly periodic array of metal nanostructures patterned on a thin dielectric layer that is backed by a solid metal ground plane, which together create a lossy resonant electromagnetic cavity. In this work, a genetic algorithm was employed to optimize the design of the nanostructured metallic screen geometry that supports multiple overlapping electric resonances to produce high absorption over greater than an octave bandwidth. An additional dielectric superstrate layer was also introduced to improve the impedance match to free space, thereby minimizing reflectivity and maximizing absorptivity across the band. The traditional Au metal used in most metallodielectric MMA structures was replaced by Pd to broaden the MMA bandwidth and to improve fabrication reproducibility. Measurements of the optimized EBG-MMA show strong agreement with simulations, demonstrating a 98% average spectral absorptivity over the 1.77 µm to 4.81 µm band and ±45 degree angular range. This work represents a significant step toward realizing manufacturable optical MAA&’s that provide near-unity broadband absorption over a specified wavelength range.

Phased antenna arrays patterned on optical waveguides can strongly affect mode coupling and propagation in the waveguides. The effect can be used to realize small-footprint, broadband optical devices for integrated photonic circuits.
The phased array optical antennas have subwavelength spacing and spatially varying geometries so that they provide an effective wavevector that facilitate phase matching between waveguide modes of different orders. Conversion between TE and TM waveguide modes can also be achieved because nano-antennas are able to mediate strong interactions between the modes. Furthermore, the directional wavevector provided by the phased antenna array allows us to break the symmetry of light propagation in waveguides, which makes it possible to create integrated optical diodes.
We used full-wave simulations to demonstrate broadband, small-footprint optical waveguide mode converters, polarization converters, and optical power diodes in the telecom and mid-infrared spectral ranges. The efficiency of mode conversion and polarization conversion, defined as the percentage of optical power coupled from one waveguide mode to the other, ranges from 10% to 50%. The purity of converted modes approaches unity. The on-off ratio of optical diodes, defined as the ratio between optical power transmittance spectra in two opposite directions, can be larger than 20dB over 30% of the central operating wavelength. We developed a coupled mode theory that can precisely explain the operation of our devices. The theory is unlike conventional coupled mode theory and it considers interaction between different waveguide modes and interaction between waveguide modes and surface waves propagating along antenna arrays.

Plasmonic gaps offer the possibility to develop a great variety of applications in nanooptics that range from optimization of optical signal in field-enhanced spectroscopy and microscopy, to testing quantum transport processes within the cavity gap at optical frequencies. With use of a plasmonic gap, doubly resonant with the vibronic transition of a sample molecule, it is possible for example, to obtain single-molecule chemical identification using tip-enhanced Raman scattering (TERS) with subnanometric resolution. Furthermore, when the separation distance in plasmonic gaps becomes subnanometric, new quantum phenomena emerge as a result of the quantum tunneling of electrons between the metallic surfaces forming the gap. This regime has been identified experimentally very recently. Additionally, when an emitter is located in a plasmonic gap, the dynamical processes involving the electronic states of the emitter and those of the metal become richer and more complicated. Generally, the strong coupling regime between the quantum emitter and the gap plasmon gives rise to plexcitonic splitting of the optical response, however, for distances of the emitter to the surface below 1 nm, direct electron transfer from the emitter's localized state into the continuum of states of the plasmonic particle can further modify the optical response, even making the fingerprint of the emitter to disappear from the spectrum. We study these quantum effects in the optical response of subnanometric plasmonic gaps with use of time-dependent density functional theory (TDDFT) and compare fully quantum and classical approaches for each case. Resonant electron transfer (RET) can be relevant in many situations of nanooptics dealing with quantum information, field-enhanced spectroscopy, catalysis, and photochemistry in general.

Nanoscale confinement of optical energy in metallic gaps can lead to many applications in plasmonics such as surface-enhanced spectroscopies, optical sensing, and nonlinear optics, among others. Here, we present a simple yet powerful new patterning method - atomic layer lithography - to produce 1-nm-wide gaps on the wafer scale using standard photolithography, atomic layer deposition, and adhesive tape. Using this technique, we make vertically aligned nanogaps in metal films along contours of arbitrary patterns. The gaps we have made are as narrow as 9.9 Å in width, and they can extend along centimeter-scale loops and densely pack on an entire 4-inch wafer. In our structures, optical transmission occurs only through the nanogap, thus enabling background-free measurements over a broad spectral range. Extreme light confinement has been demonstrated in visible range with an effective refractive 17.8. Near-infrared radiation can pass through 1-nm-wide gaps that are ~1,500 times smaller than the wavelength. Atomic layer lithography can provide powerful new methods to study light-matter interactions inside 1-nm and sub-nanometer metallic gaps in an unprecedented manner. We will describe potential applications of these structures in studying optical emitters inside nanogaps, surface-enhanced sensing and spectroscopy, as well as novel device applications.

The plasmonic resonances in multiparticle systems demonstrate high tunability based on specific nanoparticle geometry, spacing, and arrangement, enabling applications as nanoantennas, metamaterials, and surface-enhanced spectroscopy substrates. To better understand the nature of these hybridized plasmonic modes, scanning transmission electron microscopy and electron energy-loss spectroscopy (STEM-EELS) can be employed. This technique allows direct visualization and spatially-selective excitation of the targeted nanostructure, facilitating the observation of dark and higher-order plasmonic modes not detectable with optical microscopy. Additionally, the electron beam can interact with the nanoparticles to modify the interparticle distance and induce the particles to converge. We have recently used these techniques to study individual nanoparticles and dimers in the classical and quantum realm.
In this presentation, we extend our analysis to a plasmonic metamolecule supporting both electric and magnetic modes. Specifically, we focus on nanoparticle trimers composed of three colloidally synthesized silver nanospheres with 25 nm diameters. These particles are dispersed on silicon dioxide TEM membranes and self-assembled with interparticle gaps of ~1 nm due to steric hindrance from their citrate ligands.
The STEM-EELS analysis involves focusing the electron beam on three positions relative to the nanostructure: the trimer&’s center, edges, and vertices. By selecting the plasmonic excitation location, various symmetry adapted linear combinations of modes can be observed, including super-radiant, sub-radiant, and dark trimer modes. For example, at 1 nm particle gap distances, we can identify a trimer super-radiant bonding dipolar resonance at 2.8 eV and a dark mode resonance at 3.5 eV. The electron beam is further used to reduce particle separation and induce coalescence, allowing an investigation of the evolution of the plasmonic resonances in the sub-nanometer regime. This dynamic imaging, manipulation, and spectroscopy technique, combined with three-dimensional boundary element method simulations and molecular orbital theory analysis, enables a deep understanding of timer electric and magnetic modes. These results open new avenues for in-situ nanoassembly and analysis of complex plasmonic metamolecule geometries in the quantum size regime.

The last decade has seen a flourishing field devoted to the study of the quantum fluidic behaviour of light. One branch of this field has focused on exploiting the properties of cavity polaritons: hybrid light-matter quasiparticles formed in semiconductor microcavities. These demonstrations have, for the most part, been limited to inorganic semiconductors and cryogenic temperatures. The large exciton binding energy characteristic of organic Frenkel excitons, however, make these an attractive candidate for the realisation of room-temperature quantum fluids.
In this talk, we will review the physics of exciton-polaritons and previous demonstrations of polariton lasing using organic semiconductors. We will then describe a room-temperature organic polariton condensate using a novel oligofluorene enclosed within an optical microcavity. In contrast to previous demonstrations of organic polariton lasing, we find that interactions between polaritons in this material system lead to a power-dependent blueshift of the condensate energy above the condensation threshold. We will show how these interactions allow the rich physics of the Gross-Pitaevskii equation to be studied at room temperature. In particular, we will explicitly demonstrate the onset of macroscopic spatial coherence beyond the condensation threshold and the presence of pinned vortices.

Metamaterials, artificially structured nanomaterials, have enabled unprecedented phenomena such as invisibility cloaking and negative refraction. Especially, hyperbolic metamaterials also known as indefinite metamaterials have unique dispersion relation where the principal components of its permittivity tensors are not all with the same signs and magnitudes. Such extraordinary dispersion relation results in hyperbolic dispersion relations which lead to a number of interesting phenomena, such as super-resolution effect which transfers evanescent waves to propagating waves at its interface with normal materials and, the propagation of electromagnetic waves with very large wavevectors comparing they are evanescent waves and thus decay quickly in natural materials. In this abstract, I will discuss our efforts in achieving the unique optical property overcoming diffraction limit to achieve several extraordinary metamaterials and metadevices demonstration. First, I will present super-resolution imaging device called “hyperlens”, which is the first experimental demonstration of near- to far-field imaging at visible light with resolution beyond the diffraction limit in two lateral dimensions. [1] Second, I will show another unique application of metamaterials for miniaturizing optical cavity, a key component to make lasers, into the nanoscale for the first time. It shows the cavity array which successfully captured light in 20nm dimension and show very high figure of merit experimentally. [2] Finally, if time allows, I will show the recent achievements of photo-induced switching of reconfigurable negative index metamaterial device and large scale negative index metasurface with anomalous spin-hall effect. [3, 4] I believe our efforts in sub-wavelength metamaterials having such extraordinary optical properties will lead to further advanced nanophotonics and nanooptics research.
References:
[1] J. Rho et al., “Spherical hyperlens for two-dimensional sub-diffractional imaging at visible frequencies,” Nature Commun., vol. 1, pp. 143, 2010
[2] J. Rho* et al., “Experimental realization of three-dimensional indefinite cavities at the nanoscale with anomalous scaling laws,” Nature Photon., vol. 6, pp. 450-454, 2012
[3] S. Zhang* and J. Rho et al., “Photoinduced handedness switching in terahertz metamolecules,” Nature Commun., vol. 3, pp. 946, 2012
[4] X. Yin and J. Rho et al., “Photonics spin hall effect at metasurfaces,” Science, vol. 339, pp. 1405-1407, 2013

A photonic crystal slab is composed of an optical waveguide with a periodic nanostructure. Such a photonic crystal slab supports guided mode resonances, i.e., optical modes in the waveguide that couple to far-field radiation. Flexible photonic crystal slabs are interesting as strain sensors. A deformation of the photonic crystal slab causes a change of the wavelength position of the guided mode resonances. This change may be evaluated remotely by measuring a reflection or transmission spectrum. Here, we present and compare two methods for fabricating flexible photonic crystal slabs. The first method is based on oblique-angle evaporation of a high-refractive-index material on a periodically nanostructured membrane. The second method employs spin coating of high-refractive-index nanoparticles on a nanostructured membrane.
Nanostructured polydimethylsiloxane (PDMS) membranes are obtained by pressing a linearly nanostructured master stamp into uncured PDMS on a poly(methyl methacrylate) (PMMA) surface. When cured and pulled off a thin nanostructured PDMS membrane is realized. In the first approach we deposit a thin layer of SiO by vapor deposition. In order to obtain a flexible and functional photonic crystal slab, the continuous SiO layer needs to be broken periodically. Periodic breaking is supported by oblique-angle vapor deposition with an angle of 45° to the surface and 90° compared to the grooves of the nanostructure. This produces shading effects and creates weak spots in the SiO layer at the same position in every period of the nanostructure. The continuous SiO layer is broken by bending and stretching the membrane to more than 200% of its original surface area. Cracks along the grooves of the nanostructure formed at the weak spots in almost every period. In the second approach TiO2 nanoparticles are deposited on the nanostructure. Here, the membrane can be stretched and serves as a photonic crystal slab without the need of any further treatment.
For both types of flexible photonic crystal slabs a shift of the guided mode resonances to longer wavelengths is observed upon stretching. For a 20% strain a resonance shift of more than 50 nm is obtained. Using an imaging technique local strain values may be extracted remotely.

A cholesteric liquid crystal (CLC) with a periodic helical structure of rodlike molecules is a chiral photonic crystal that exhibits a photonic band gap (PBG) for circularly polarized light with the same handedness as the CLC helix. In the CLC, the spectral position of the PBG is equal to an optical pitch of the CLC and can, in practice, be varied by adding chiral dopants or by adjusting temperature. The PBG width of a conventional CLC is proportional to the anisotropy of refractive indices and is in the range of 50 - 100 nm. For obliquely incident light, the PBG from this CLC shows spectral blue shift with increasing of incident angle. Recently, we have demonstrated new CLC systems going beyond the optical limits of the conventional CLCs such as multiple PBGs in a single-pitched CLC systems [1], polarization-independent PBGs [2], and color-temperature tunable white reflections [3].
An amorphous perfluoropolymer poly[perfluoro(4-viny;ox-l-butene)](PPFVB), used in this study, has been used for antireflective coatings and optical fibers due to its low refractive index 1.34 and high transmittance over a wide spectral region (200 nm to 2 mu;m). Also, the PPFVB film has been investigated as a vertical alignment layer because of its strongly hydrophobic surface with low polarity and low polarizability [4].
Here, we control molecular arrangements of CLCs by introducing the PPFVB film as an alignment layer and investigate their optical characteristics such as reflectance (or transmittance), spectral position, width, and incident angle dependences of the PBGs. The PPFVB film was chosen because of its strong hydrophobicity and high transmittance. The optical pitch of the CLC on the PPFVB film depended on thickness of the CLC cell, and we could tune the pitches of CLCs from ~560 nm to ~640 nm by adjusting the thicknesses from ~3 mu;m to ~13 mu;m. The CLC on the PPFVB film showed a wide PBG with a full wave half maximum (FWHM) of ~ 120 nm at the thickness of ~13 mu;m, whereas the FWHM of the CLC on polyimide (PI) films was ~ 80 nm at the same thickness. In addition, PBGs of CLCs on PPFVB films did not show spectral shift but spectral broadening for obliquely incident light. The present system can extend practical applications of CLCs to new photonic devices.
[1] N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, Nat. Mater. 7, 43 (2008).
[2] N. Y. Ha, S. M. Jeong, S. Nishimura, and H. Takezoe, Appl. Phy. Lett 96, 153301 (2010).
[3] N. Y. Ha, S. M. Jeong, S. Nishimura, and H. Takezoe, Opt. Express 18, 26339 (2010).
[4] S. M. Jeong, J. K. Kim, Y. Shimbo, F. Araoka, S. Dhara, N. Y. Ha, K. Ishikawa, and H. Takezoe, Adv. Mater. 22, 34 (2010).

Plasmonic metal nanostructures have unique optical extinction and emission properties in the visible and near infrared regions. Plasmonic oscillation frequency varies with the dimension and morphology of metal nanostructures, which make them useful platforms for environmental, biological, and medical applications. Beyond the tremendous studies on surface-enhanced Raman scattering, recently, metal-induced fluorescence enhancement is an emerging research subject. In principle, radiative rate of fluorophores locating nearby plasmonic metal are highly affected by the plasmon lifetime of the adjacent metal. Fluorescence lifetime typically decreases as a result of cooperative interactions between the metal and the adsorbed fluorophore thereon, accompanying with the increment of emission intensity.
In this study, we observed fluorescence enhancement for dye molecules in 3-dimensionally ordered macroporous (3-DOM) silver skeletons, which was made by electrochemical templating method using polystyrene colloidal opal templates. The fabricated 3-DOM silver skeletons are composed of large void part and surrounding interconnecting windows. Fluorescent probe molecules, Rhodamine123, are adsorbed onto the 3-DOM skeletons and subjected to measure fluorescence intensity and lifetime modulation depending on the void size change, hence, simultaneously accompanied with interconnecting window size change. A fluorescence lifetime imaging (FLIM) microscope was used to measure fluorescence lifetime and intensity variation in 2-dimensional space for the dye adsorbed 3-DOM silver skeletons.
Briefly, bulk fluorescence lifetime of Rhodamine123 in water, 4.0 ns, is a little decreased to 3.4 ns when adsorbed on bare silver plates. The emission lifetime is further decreased to 2.7 ns for the dye adsorbed on the 3-DOM silver skeletons with ~1 um void size. Fluorescence intensity is much brighter than that of the dyes adsorbed on bare plates. By changing void size, the observed fluorescence intensity and lifetime is interestingly modified. These experimental results were comparatively investigated with theoretically modeling study for the dye adsorbed 3-DOM skeletons. The results of this study would be very useful for the development of nanophotonic materials and devices based on plasmonic metal nano- and microstructures.

Nobel metal nanoparticles (NPs) assembled into “plasmonic molecules” with different interparticle separation, size and geometry possess various optical properties. Due to their high symmetry, one-dimensional (1D) NP clusters are important model systems for elucidating the short- and long- range coupling mechanisms in NP clusters.
We investigated the near- and far- field response of 1D gold NP clusters fabricated with high structural control through template guided self-assembly. We showed manipulation of interparticle gap (g=0.5-1.5nm) through control of ligand density on NP surface and buffer conditions (ionic strength). The overall size of the cluster (n=1-7) was controlled by tuning size of the template. The ability to independently vary n and g realized a rational tuning of the optical response in individual NP clusters over a broad spectral range (over 250nm) with an enhanced e-field concentrated in the gap area.
We are currently working on utilizing these NP chains to achieve enhanced and directed emission of fluorescence.

The epsilon-near-zero (ENZ) response of matter enables unique optical functions such as tunneling of electromagnetic waves and directive emission. Metamaterials composed of multilayer stacks of metals and dielectrics can constitute effective media with ENZ for a small range of wavelengths that depends on the geometry and materials [1,2]. However, in previous reports, the ENZ wavelength is fixed at the time of metamaterial fabrication, and frequency-tunable ENZ metamaterials have not yet been explored in the visible and near IR. In this paper, we demonstrate the design of such a tunable ENZ metamaterial based on metal and transparent conductive oxide (TCO) multilayers, using the field effect to tune the permittivity of the TCO across thin gate dielectric layers [3]. Tunability of the ENZ wavelength by as much as 80 nm is possible in the 400-700 nm wavelength regime.
Our design consists of a multilayer geometry of two layers of 20 nm of Au, separated by a 15 nm active layer of indium tin oxide (ITO) with carrier concentration 5#8729;1020/cm3. The two materials are isolated from each other by 5 nm Al2O3 dielectric layers. A Drude-Lorentz model fitted by experimental data is used for the permittivity of Au and the Drude model is used for the permittivity of ITO. We obtain the complex reflection and transmission coefficients of the stack using an analytical transfer matrix model. This enables the retrieval of the effective permittivity of the metamaterial, by directly relating the metamaterial response to analytical expressions for the transmission and reflection coefficients of the single-homogeneous layer problem [4].
Under applied field between the Au and the ITO, an accumulation layer is formed at the Al2O3-ITO interface. To model this active layer in our calculations, we introduce a 5 nm layer of ITO with modified carrier concentration [3]. We consider carrier concentration changes of the active layer up to one order of magnitude, comparable to those obtained experimentally. The structure is excited with a TE-polarized plane wave. Our calculations show that the ENZ wavelength shifts from 468 nm for carrier concentration of 5#8729;1020/cm3 to 388 nm for carrier concentration of 5#8729;1021/cm3.
Currently, we are fabricating field-effect tunable ENZ metamaterials with e-beam and sputtering deposition. Ellipsometric and transmission/reflection spectra measurements will be used for comparison to our metamaterial effective index calculations, and phase interferometry will be used to experimentally measure the phase advance of the field through the metamaterial, which approaches zero at the ENZ wavelength.
J. Gao, et al., Appl. Phys. Lett. 103, 051111 (2013).
T. Xu et al., Nature 497, 470-474 (2013).
E. Feigenbaum, K. Diest and H. A. Atwater, Nano Lett. 10, 2111-2116 (2010).
C. Menzel, et al., Phys. Rev. B 77, 195328 (2008).

The discovery of topological insulators has started considerable interest in the study of topological phases of matter [1]. Topological phases consist of various band insulators or superconductors that have gaps in their spectrum. In these insulators, spin-orbit effects take the role of an external magnetic field, with spins of opposite sign counter-propagating along the edge [2-3]. The main characteristic of these novel phases is the emergence of topologically protected boundary phenomena, e.g., quantum pumping, surface states related to exotic models from particle physics, and quasiparticles with non- Abelian statistics [1]. On the other hand, recently Kraus et. al. [4] has, for the first time, demonstrated the connection between quasicrystals and the topological phases of matter. The question is interesting, because quasicrystals may be constructed by taking a cut through a standard crystal in a higher dimension, thus motivating the prospect that they could provide a way of probing higher-dimensional topological phases in lower-dimensional systems.
It is the aim of this work to investigate theoretically the transmission spectra in one-dimensional photonic crystal, made up from dielectric materials, organized in accordance to a discrete varying electric permittivity profile that obeys an analogous of the quasiperiodic potential in the so-called Audry-André model [5], to modulate the index of refraction. It describes electrons hopping on a one-dimensional chain, with a spatially modulated on-site potential, in such a way that the period of the potential is incommensurate with the lattice period. Our results show that due to the incommensurate dielectric distribution, the spectrum splits into a fractal set of pass- and forbidden-band structure [6]. Intercalated in these states, there are special ones that are not standard surface state. By studying the transmission spectra as a function of Phi;, the modulation phase, we find states that lie within the gaps that are boundary states, localized either on the left or on the right boundary of the system, where the Dirac cone will appear, characterizing the topological states.
[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] B. A. Bernevig, T. L. Hughes, and S.-C. Zhang, Science 314, 1757 (2006).
[3] M. Konig et al, Science 318, 766 (2007).
[4] Y.E. Kraus et al, Phys. Rev. Lett. 109, 106402 (2012).
[5] S. Aubry and G. André, Ann. Isr. Phys. Soc. 3, 133 (1980).
[6] M.S. Vasconcelos, E.L. Albuquerque and A.M. Mariz, J. Phys.: Condens. Matter 10, 5839 (1998).

Biological and biomimetic systems can be used to develop novel materials currently difficult to fabricate by conventional methods. Of particular interest is the biological photonic crystal structure found in weevil and butterfly species, which features the diamond and gyroid symmetries. The diamond symmetry offers one of the widest complete band gap structures available. Current synthetic approaches to 3-D photonic crystals with this symmetry rely on a top-down approach, requiring specialized equipment such as holographic or multiple beam interference lithography. Biological photonic crystals are often superior to current synthetic materials and produced under ambient conditions. Biological photonic crystals can only be used as a destructive template for the fabrication of titania photonic crystals and are therefore unsuitable for long-term use. As an alternative, current work is being done using biomimetic systems to develop a fabrication method of these complex photonic crystals. This involves investigating self-assembly of lipids into cubic lipid structures, organelle membranes which naturally fold into cubic symmetries and possible ways to manipulate existing lamellar structures into a cubic symmetry. This has been done using C. carolinesis which form cubic membranes in vivo as well as designing methods for in vitro cubic membranes using long-chain fatty acids and phospholipids. Characterization is done by electron microscopy and small-angle x-ray scattering. A cell-free, bottom-up fabrication method would allow for low-cost 3-D photonic crystals with complete band gaps thus increasing the utility of photonics.

Plasmonics describes how the behavior of light can be influenced by sub-micron optical structures. Specifically, the interaction of light with objects such as nano-apertures or nano-scatterers can result in surface plasmon polaritons (SPPs) that propagate along metal-dielectric interfaces. In solar cells, nanoholes on the back contact can excite these SPPs, increasing the path length of light in the direction perpendicular to the direction of incidence, thus improving light-matter interaction and enabling bulk-like absorption in optically thin films. Therefore, thin-film solar cells in particular have much to gain from this type of technology, since their optically thin layers can severely limit the absorption of sunlight over a wide range of wavelengths, especially near the absorption bandgap of the dielectric medium.
We have recently shown absorption enhancements of up to 700% in a 24 nm layer of P3HT:PCBM over a wide range of wavelengths compared to unpatterned metal back contacts¹, by using a silver film substrate patterned with quasiperiodic nanohole arrays.^2,3 However, this improved performance has not yet been shown in a complete working device. Furthermore, real-world applications of plasmonic technology are limited by the costly and time-consuming fabrication methods, often featuring focused ion beam or electron beam lithography, needed to produce subwavelength hole structures.
Here we report an increase in short circuit current density and open circuit voltage in ultra-thin film P3HT:PCBM bulk heterojunction organic solar cells that have been patterned with quasiperiodic nanohole arrays using nanoimprint lithography, a relatively cheap and manufacturer-friendly method. This effect is attributed to the ability of the nanoholes to function as plasmonic concentrators on the back contact of the cell thus enabling a higher photogeneration rate in the active layer. These results suggest not only the ability of plasmonic concentrators to improve the performance of organic photovoltaics, but also the potential of nanoimprinting to make plasmonic technology cost-effective and easily scalable.
¹ A. Ostfeld and D. Pacifici, Plasmonic concentrators for enhanced light absorption in ultra-thin film organic photovoltaics, Appl. Phys. Letters 98, 113112 (2011).
² P.W. Flanigan, A.E. Ostfeld, N.G. Serrino, Z. Ye, and D. Pacifici, A generalized “cut-and-projection” method for the generation of quasiperiodic plasmonic concentrators for ultra-thin film photovoltaics, Optics Express 21, 2757-2776 (2013).
³ P. W. Flanigan, A. E. Ostfeld, Z. Ye, N. G. Serrino, and D. Pacifici, "Quasiperiodic plasmonic concentrators for ultra-thin film solar cells", Book Chapter in Optics of Aperiodic Structures: Fundamentals and Device Applications, edited by Luca Dal Negro, Pan Stanford Publishing Pte. Ltd. (2014).

High efficiency-optical absorbers have been widely studied for diverse applications such as solar cells and light emitting diodes [1]. In particular, the thinner structures can lead to reduction in cost as well as electron-hole recombination. As a result, there has been considerable research to enhance light absorption with ultrathin layers [2]. A recent study shows that one can harvest near-unity absorption by using an ultrathin semiconductor layer on top of a metallic substrate [3]. Its working principle is the impedance matching between the incident region and the semiconductor-metal configuration. Although it exhibits near-unity absorption with angle-robust and polarization-insensitive properties, there is a still limitation that the impedance can only be matched at a certain wavelength, which is determined by the material property of the semiconductor and metal that are used in the configuration. In this presentation, we show that we can arbitrarily tune the resonance wavelength with near-unity absorption by invoking metal grooves and tailoring the reflection phase pickup.
At first we examine the impedance matching condition for ultrathin semiconductor absorbing layer on metallic substrate from the view point of transmission line. We show it is guaranteed that there can be a certain geometrical design that results in the unity absorption for given material properties at a specific wavelength. Based on this approach, the dependence of the impedance matching condition on the reflection phase is elucidated. By making subwavelength gratings in the metal substrate, it is possible to tailor the reflection phase, which allows for tuning of the impedance-matched critical coupling at any wavelength in the visible regime. The full-field numerical investigation showing good agreement with the theoretical prediction is also provided. We believe that the presented configuration can shine light to further studies for extreme light absorption by using ultrathin semiconductor layers.
[1] N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, “Infrared perfect absorber and its application as plasmonic sensor,” Nano Lett. 10, 2342 (2010).
[2] M. A. Kats, R. Blanchard, P. Genevet, and F. Capasso, “Nanometre optical coatings based on strong interference effects in highly absorbing media,” Nat. Mater. 12, 20 (2013).
[3] J. Park, J.-H. Kang, A. Vasudev, D. Schoen, H. Kim, E. Hasman, and M. L. Brongersma, “Omni-directional near-unity absorption in an ultrathin planar semiconductor layer on a metal substrate,” (in preparation).

Plasmonic metal-insulator-metal (MIM) structures have shown promise for applications as color filters in the visible and near IR parts of the spectrum. Due to their subwavelength mode volumes, these plasmonic filters are well matched to the small size of state-of-the-art active pixels (~ 1 mu;m²) for dense integration in CMOS image sensor arrays. Typically MIM plasmonic filters exhibit rather broad (~ 100 nm or greater) transmission bandwidths, but if a dramatic reduction in the peak width of filter transmission spectra were possible, this would open the door to the realization of CMOS hyperspectral (imaging + spectroscopy) imaging arrays. Filter design for hyperspectral imaging requires the FWHM of filter transmission spectra to be less than 30 nm. We have designed and optimized 5 layer metal-insulator-metal-insulator-metal (MIMIM) structures that possess transmission FWHM as small as 17 nm. Using both finite difference time domain calculations and a boundary value solution method for repeated MIM structures, we have calculated MIMIM dispersion relations to identify the key nanophotonic parameters responsible for narrowband filter transmittance.
Further, the transmission peaks of these filters can be controllably shifted throughout the visible and near infrared spectrum. The transmission element of the filter is an array of parallel sub-wavelength slits etched through the MIMIM stack which acts as a narrowband transmission resonator. The in-plane periodicity of the slit array is a readily controllable parameter that dictates the filter transmission wavelength, so the same MIMIM architecture can enable dense integration of adjacent filter elements for multiple wavelength narrowband wavelength intervals by simply adjusting the spacing between the slits. Additionally, the MIMIM structure itself is highly tunable. Along with changing the periodicity of the slits in the filter, the active plasmon mode can be manipulated by controlling the thickness of one or both of the insulating layers, by changing the indices of the constituent materials or by adjusting the thickness of the middle metal layer, a feature which serves to couple the two individual MIM structures that compose the device together.
We have fabricated prototype MIMIM filter arrays on SiO₂ substrates via deposition of alternating metal and insulator layers within the same deposition system, followed by focused ion beam nanofabrication of arrays of slits into MIMIM planar multilayers to create our final filter structures. Results of MIMIM filter transmission measurements and prospective for integration with CMOS image sensors will be discussed.

Thermal radiation plays an important role in energy conversion and thermal management applications. However, any macroscopic thermal emitter is limited in its emitted power within any given frequency range to that of a blackbody with the same surface area. In the field of imaging, hyperlenses have been demonstrated to allow evanescent field to couple into the far-field, enhancing resolution beyond the diffraction limit. At the same time, recent work has demonstrated near-field heat transfer enhancement between two bodies beyond the black body limit between polar dielectrics due to contributions from evanescent fields. In this work, we theoretically investigate whether hyperbolic metamaterials can be used to extract the energy carried by evanescent waves in the near-field to the far-field, in a concept similar to a recent demonstration of thermal extraction. Our result demonstrates that hyperbolic metamaterials could potentially offer enhancement in thermal radiation in addition to imaging applications.

Solar-blind ultraviolet (UV) photodetectors have raised extensive interest in both scientific community and high-technology industry for the unique device properties and wide applications in weak signal detection, such as corona inspection, combustion flame monitor, free-space communication, and so on. As the most important component of a photodiode for solar-blind UV detection, many wide bandgap semiconductor materials including AlxGaN1-x, SiC, and GaN have been characterized to achieve low dark current, while their declining tails in response curve are rarely discussed. In fact, since the solar spectral irradiance exists fundamentally, the declining tails of the materials will bring the significant noise, which has a great impact on the performance of the photodetectors. In this work, we evaluate the materials of the state-of-the-art UV photodetectors by quantitatively comparing the noise from the dark current and the declining tails, in both terrestrial and atmosphere background. It proves that the detectivity of common solar-blind UV detectors is practically limited by the background noise, which attributes to the inevitable declining tails of the materials and the objective background spectral irradiance. We therefore introduce the concept of background limited ultraviolet photodetector (BLUP) in solar-blind UV detection. For the wavelength below 285 nm of terrestrial background, the photon arrival rate is less than 1 ph-cm-2-nm-1-s-1, and it increases exponentially to about 1013 ph-cm-2-nm-1-s-1 at 300 nm, and stays at 1013 ~ 1014 ph-cm-2-nm-1-s-1 above 300 nm. Accordingly, the BLUP detectivity is calculated as 9.5×1017 cm-Hz1/2-W-1 at 285 nm, which is very close to the ideal signal fluctuation limit (SFL) detectivity of 1×1018 cm-Hz1/2-W-1, and it decreases to about 1012 cm-Hz1/2-W-1 at 300 nm, and approaches to a flat above the wavelength of 300 nm. It reveals that the declining tails above 285 nm in the response curves should be one of the most important characteristics when the materials are chosen for practical solar-blind UV photodetectors. The reported AlxGaN1-x photodetector shows the potential in detectivity improvement for its lower response above 285 nm. To approach to the ideally ultimate SFL detectivity, further optimizations of the materials are indispensable for practical solar-blind UV photodetectors. These results advance our understanding of the detectivity improvements in practical solar-blind UV detection, and then give a guideline for evaluation and optimization of the materials for the solar-blind UV photodetectors.

Abstract
A graphene/silicon-heterostructure waveguide formed by integrating graphene onto a silicon optical waveguide with near to mid-infrared spectral ranges is fabricated. Saturable in-plane absorption due to interband transition in the graphene is observed, when the photon energy hnu;>2Ef ( Ef is the Fermi energy of graphene ). More interestingly, it is also found that an extremely high in-plane absorption along the waveguide occurs, when the mid-infrared light with photon energy Ef

Graphene has emerged as one of the leading materials in condensed matter physics due to its superior electrical, optical, magnetic, thermal, and mechanical properties. In particular, graphene possesses unique optical and plasmonic properties that could enable the next generation of photonic and optoelectronic devices. Graphene has been recently shown to support localized and propagating surface plasmons over a wide range of wavelengths including infrared (IR) and terahertz (THz) wavelengths.
Here, we will describe nanostructured graphene films for applications as mid-infrared optical transmission, reflection and absorption filters. The interaction of graphene with light is relatively inefficient due to its single-atom thickness, which is well below the diffraction limit. It has been shown that by introducing holes into the graphene layer, one can enable the excitation of plasmonic and photonic resonances around the plasmon resonance wavelength of graphene. Most of the prior graphene plasmonics research has focused on the optical properties of graphene at far-infrared and terahertz wavelengths.
By controlling the Fermi energy level of graphene films, it is possible to control the resonances wavelength of graphene layers. We used full-field electromagnetic simulations to study the optical properties of single layer and multi-layer graphene nanostructures at mid-infrared wavelengths (6-16 mu;m). Our numerical calculations show that graphene nanostructures can behave as an efficient transmission filter by blocking the light transmission at the localized plasmonic resonances of graphene nanostripe arrays. In particular, we have observed significant reduction of transmission at 10 mu;m using a single layer, one atom thick graphene nanostructured film with a Fermi energy level, E_f=0.4 eV. Moreover, the resonance wavelength and the transmission intensity can be controlled and tuned by manipulating the critical parameters of graphene such as the width, the period, the Fermi level and the mobility. Additionally, we numerically studied the coupling between multilayer graphene structures and observe strong plasmon hybridization to the asymmetric substrates that we are utilizing in our calculations. In addition to the simulations, we will also describe our preliminary experimental results on graphene infrared filters. We will describe the resonant behavior in graphene nanostructured films using a systematic analysis and highlight opportunities of graphene for applications such as IR filters, photodetectors, and surface-enhanced and refractive index based biochemical sensors.

In nanophotonics, monolayer graphene has shown promising potential as a plasmonic material both because the optical properties of the graphene are controllable with an electrostatic bias, and the plasmons in graphene exhibit extremely high confinement factors. In particular, experimental studies have proven that nano-fabricated graphene structures, such as graphene ribbons, can modulate optical signal in a mid-infrared (IR) range, and the plasmonic modes supported in these structures have been shown to have wavelengths more than 100 times shorter than freespace.
In this presentation, we report observations of resonant interference between freely propagating and graphene plasmon modes across a subwavelength plasmonic slit. Further we demonstrate that the graphene plasmonic resonant effect enables an integrated plasmonic modulator. Specifically, we designed a plasmonic modulator consisting of metal-insulator-metal (MIM) plasmonic waveguide and a subwavelength graphene slit. In this structure, the modulation effect is accomplished by the interference between the freely propagating MIM modes and graphene plasmon modes confined in the subwavelength graphene slit. Since the resonant graphene plasmon frequency depends on the carrier density of the graphene and the MIM modes are independent of the graphene, we can tune the interference phase by adjusting the electrostatic gating.
Theoretical analysis indicates that resonant interference of over 70dB could be achieved in principle, and that an applied voltage difference of only 4V is needed to induce such an extreme modulation effect. Moreover, the width of the graphene slit required to modulate 7.5um light is only 140 nm owing to the extremely high mode confinement factor in graphene. We have fabricated MIM Au waveguides with integrated 100 nm graphene-coated slits by a multilayer electron beam lithography and graphene deposition process on Au coated 200 nm silicon nitride membranes. Experimental measurements indicating 5.6% modulation of the MIM plasmonic mode transmission by resonant interference across the graphene slit at 1560 cm-1 with Fermi level of 0.43eV. Our experimental results further show the effect that graphene slit position, width and doping have on the modulation intensity of our nanofabricated structures.

First-generation dyes - coumarin, perylenes, and rhodamines - used in luminescent solar concentrators (LSCs) suffer from both concentration quenching in the solid-state and small Stokes shifts which limit the current LSC efficiencies to below theoretical limits. Aggregation-induced emission (AIE) from twisted aromatics exhibits atypical properties, e.g. a large Stokes shift and fluorescence enhancement that can arise from aggregation. While AIE fluorophores have been proposed as biological sensors and light-emitting diodes, we have explored the potential of these materials for LSC applications. Both experimental and computational models show that these materials have at least comparable efficiencies and performance to first-generation dyes in LSCs. To achieve panchromatic absorption in the UV-Visible region, we have also utilized solid-state energy transfer in mixed AIE fluorophores. Our results demonstrate that AIE fluorophores show distinct advantages for materials in LSCs.

The need to interface optical signals with increasingly small electronic components has led to an interest in subwavelength waveguides. Surface plasmon polaritons (SPPs) are longitudinal surface charge density oscillations localized to a metal/dielectric interface, and as such are capable of confining energy in a structure which is not diffraction limited. Waveguides based on the excitation of SPPs are promising for short-range applications, but in these structures Ohmic damping significantly limits propagation length due to the bulk of the electric field propagating along a metal interface. Here we demonstrate that through selection of materials with specific optical properties, plasmonic modes exist for which Ohmic damping can potentially be drastically reduced. Specifically, high index dielectric plasmon polariton modes (HID-PPMs) exist in structures having a core dielectric layer with a higher refractive index than the substrate supporting them. Modes in this region exhibit oscillatory electric fields with the bulk of their electric field confined in the dielectric layer, similar to a total internal reflection waveguide. Damping and insertion losses may therefore be drastically reduced in such structures. Here we report the demonstration of HID-PPMs in Au/TiO2/Au MIM devices using attenuated total reflectance measurements in the visible wavelength region. Characterization of these modes was performed as a function of core dielectric thickness. Results are in good agreement with theory. We will discuss the application of these waveguides to several technologies related to solar energy conversion.

The conventional semiconductor based LED emits the narrow band light because of the limitation of the band gap energy. Recently, a new type LED, which emitted the broad band white light from a single chip, has been reported by the authors (1-3). This new type of LED has the MOS structure that includes a very thin metal oxide gate dielectric film, such as Zr-doped HfO2 (ZrHfO) or HfOx. The light emission mechanism is the thermal excitation of the nano size conductive paths formed during the dielectric breakdown. Since the post deposition annealing (PDA) condition affects the material properties and stability of the high-k stack (4), it is important to understand how the light emitting characteristics change with the variation of the material and electrical properties of this kind of device. Devices with three different PDA temperatures, i.e., no annealed, 800oC, and 1,000oC, were prepared and studied. Independent of the PDA temperature, all samples emit lights in the same visible wavelength range. According to the CIE 1931 chart, they all fall in the white light region. The intensity of the emitted light is affected by the PDA temperature and the magnitude of the applied gate voltage (Vg). Under the same Vg = -50 V stress condition, the non annealed sample emits the light with the highest intensity and the 800oC annealed sample emits the light with the lowest intensity. The light intensity is consistent with the magnitude of the leakage current of the device. For example, the non annealed sample has the largest leakage current while the 800oC annealed sample has the lowest leakage current. In addition, the light intensity increases with the increase of the magnitude of the applied Vg, which caused the increase of the leakage current. For the thermal excitation process, the light emission intensity is related to the resistance of the conductive path. Since the leakage current is related to the physical thickness and film strength of the bulk and interface layers, the light emission phenomenon can be explained with the change of these material properties. Moreover, the high color rendering index (CRI) of ~98 was obtained in both the non-annealed and 800oC annealed samples. The 1,000oC annealed sample has a lower CRI, of ~95. The CRI and correlated color temperature changes can be explained by the change of the light in the short wavelength range. The lifetime of this type of LED is long, e.g., > 1,000 hours under the atmosphere without a passivation layer.
Reference:
1.Y. Kuo and C.-C. Lin, App. Phys. Lett. 102, 031117 (2013).
2.Y. Kuo and C.-C. Lin, ECS Solid State Letters 2, Q59 (2013).
3. Y. Kuo and C. -C. Lin, Solid-State Electron. 89, 120 (2013).
4. J. Yan, Y. Kuo, and J. Lu, ECS Solid State Letters 10 , H199 (2007).

Terahertz (THz) technology has been developing rapidly in recent years for a wide range of applications. Powerful THz source such as solid-state Quantum Cascade Lasers (QCL) and efficient measurement systems such as THz Time Domain Spectroscopy (THz-TDS) have been reported at the same time. Active components for modulating THz waves are essential for many applications but they are still to be demonstrated. Here, we report an electrically switchable THz Fresnel lens based on high birefringence nematic liquid crystals (LCs) mixture BL037. The 550 µm LC cell was assembled on 700 µm thick fused silica substrate and consists of subwavelength gold gratings, which act as both efficient transparent electrode and polarizer for THz wave. By photolithography and lift-off process, the gratings were fabricated on the inner side of each substrate with a pitch size 6 µm and 4 µm grating width. The thickness of deposited gold was set to 25 nm to optimize conductivity, transmittance and bonding strength. Moreover, eight Fresnel zone were designed and adjusted in dimension in order to minimize the fringing effect in thick LC cell. Field intensity of 2.0 THz beam generated by a high power QCL was measured by scanning the position of Golay detector. Switching of the binary Fresnel lens (f=5cm) is realized by applying a 280 V square wave signal to adjacent zones. The experimental data shows good consistency with simulation result, and the measured diffraction efficiency is around 9.2%, which can be increased significantly by using LC materials with larger birefringence and lower loss. The in-line nature of this device allows easier alignment in comparison with widely used parabolic mirror, and the electrical tunability of liquid crystals provides a convenient way to turn “on” and “off” the focusing. Such device may be useful in imaging and other narrow-band THz optics applications.

Semiconductors are promising materials for THz plasmonics. They acquire metallic behavior when sufficient free carriers are present. Plasmonic structures fabricated out of these semiconducting materials can sustain localized surface plasmon polaritons (LSPPs). In this work we investigate the excitation of LSPPs in doped Silicon bow-tie antenna structures at THz frequencies. We demonstrate that such structures can resonantly enhance the far field scattering and absorption of THz radiation. Moreover, in this contribution we will present for the first time near field characterization of semiconducting plasmonic resonators, demonstrating near-field enhancements in volumes significantly smaller than the wavelength. This far- and near-field investigation of plasmonic structures may be a platform for future sensing applications.

Abstract - Metamaterial is an artificial material in which the electromagnetic properties, such as permittivity and permeability, can be controlled. Common methods to fabricate terahertz metamaterials have been photolithography for fine patterning, which have limitations in large-scale fabrication. We present a direct fabrication method for metamaterial using the electrohydrodynamic jet printing. Pulse DC voltage was controlled to make drop-on-demand (DoD) operation, through which flexible high refractive-index metamaterial could be fabricated in the form of I-shaped silver electrodes with 10-µm widths and 5-µm gaps on polyimide substrate. The peak refractive index was 11.12 at a frequency of around 0.5 THz for single layer metamaterial.
Keywords: Electrohydrodynamics jet printing, drop-on-demand, metamaterial, high refractive index.

The major challenge in practical application of surface enhanced Raman scattering (SERS)-based sensor design lies in the poor sample reliability, poor reproducibility, and limited chemical selectivity. Recently graphene has been proposed as a promising SERS substrate. Thus, integrating graphene with plasmonic nanostructures such as gold could lead to significant enhancement in the SERS signals. Here, we demonstrate a facile cost-effective approach to fabricate large area Si substrates with uniformly dispersed graphene shell encapsulated gold nanoparticles (GNPs). The gold nanoparticles with controlled size and inter-particle spacing were uniformly coated on Si substrate through a wet chemical and thermal processing route. Graphene shells with a pristine thickness of ~3 nm were observed to encapsulate the gold nanoparticles in a high throughput and scalable chemical vapor deposition approach. The influence of GNP size (~20 nm to ~100 nm) and density on the SERS sensitivity was demonstrated. SERS signal enhancement of as high as ~105 was observed for GNPs with size of ~50 nm The SERS detection limit of ~10-10M was observed for Raman dyes. In addition, this SERS substrate also demonstrated chemical selectivity towards Raman dye. The plasmonic behavior of GNPs was further mathematically modeled and experimental results were correlated with the model. Such versatile SERS substrate with chemical and electromagnetic enhancement is promising for future photonics-based sensors.

Inverted quantum dots light-emitting diode device formed by all solution processing is simply fabricated by using low-work function material polyethylenimine ethoxylated. From transmission electron microscopy study, the mono-layered CdSe/ZnS QDs are uniformly distributed on the polyethylenimine ethoxylated surfaces, resulting in Inverted LEDs utilizing the two kinds of hybrid polymer [poly (N-vinylcarbazole) + poly(N,N&’-bis(4-butylphenyl)-N,N&’-bis(phenyl)benzidine and poly (9,9-di-n-octyl-fluorene-alt-benzothiadiazolo) + poly(N,N&’-bis(4-butylphenyl)-N,N&’-bis(phenyl)benzidine] with hole transport layer for improving hole transport ability. At a low-operating voltage of 5 V and 3 V, the device emits spectrally orange and red radiation with high brightness up to 1151 and 2900 cd/m2, respectively.

The phenomenon of light passing through a subwavelength aperture in a metal has long been a subject of scientific inquiry. In spite of the significant amount of theoretical and experimental work done in this field, most papers use light or holes with very specific parameters (hole size, hole shape, incident wavelength, incident polarization, etc.), which can make it difficult to fully comprehend the underlying physics. This is particularly true for systems that consist of an array of subwavelength holes (e.g., for studies of extraordinary optical transmission), where the output is a convolution of interference between scattered waves, incident light, and coupling to surface plasmon polaritons.
This report presents a systematic study of light transmission through individual nanoscale apertures with varying parameters, such as hole shape and size, as well as incident wavelength. Both rectangular and circular holes were studied across a wide range of characteristic sizes (100 to 2000 nm, increased in steps of 50 nm). The incident radiation came from a white light source, measured from 400 to 750 nm. The incident polarization was varied between two orthogonal states (0 to 90°, in steps of 2°) with respect to the long axis of the slit. Within the parameter space, the system was studied in both the subwavelength (aperture smaller than the wavelength) and geometric transmission regimes. It is known that the scattered light will behave differently depending on which regime the system is in. For example, in the geometric regime, two things are expected: (1) the incident polarization state will not affect the magnitude of the transmission, and (2) the ratio between the transmission and the cross-sectional area remains constant.
The experiments were performed on 300nm-thick layers of silver that had been milled with a Focused Ion Beam (FIB). To confirm the validity of the results, two different methods were used. In the first, the system was recreated using Finite Difference Time Domain (FDTD) simulations. In the second, the transmission data was compared to predictive models based on analytic equations. The agreement between these three methods was very strong, thus giving future simulated and theoretical studies a benchmark for accuracy.

Optics and opto-electronics of graphene is one of most vibrant, rapidly developing and exciting areas which has already led to some commercial applications. Rather than being just another new photonic material, it combines a wide palette of unique aspects which promise breakthroughs in several outstanding problems of nanophotonics and optoelectronics, including broadband photodetection and sensing, on-chip manipulation of nanoscale optical fields and lasing.
In this talk, the most recent developments of graphene nano-photonics, plasmonics and photoconversion for near-infrared and infrared frequencies are being reviewed. Strong interactions between graphene and nanoscale light-emitters are actively controlled and detected by tuning graphene from an absorbing to plasmonic material. Graphene plasmons, which allow for strong confinement of optical fields are visualized in real space and gate-tunability and long propagation lengths are demonstrated by employing high mobility graphene.

The 2D nature of graphene, along with its novel electronic structure, makes it an intriguing platform to study propagating surface plasmons. Behaving as an extraordinarily thin waveguide, graphene has been recently shown to support electronically tunable ,Mid-IR plasmons with optical mode volumes that are 107 times smaller than freespace, and plasmon wavelengths more than 100 times shorter. In addition to these unique effects, the extreme thinness of a monolayer graphene sheet make the plasmonic properties strongly dependent on the dielectric properties of the surrounding media. In this talk we show that the large optical confinement properties of monolayer graphene allow for the plasmonic dispersion relations to be strongly altered by the nearby optically active excitations, such as phonons or excitons. We will demonstrate, experimentally, that the phonons of just a single, underlying boron nitride (BN) sheet can be used to create new surface phonon plasmon polariton modes (SPPPs) in graphene. By studying the wavelength and doping dependence of these modes, we map out their dispersion relation and observe anti-crossing behavior between the graphene plasmon and this new SPPP mode. We further show that the high quality factor of the BN optical phonon at 170meV leads to epsilon near zero (ENZ) behavior in the SPPP mode as the wavelength varies from 160 to 600nm. These experimental observations are compared to a theoretical model that has been developed to explain optically active graphene devices, and we find good agreement.

We report here results of simulations and experiments demonstrating a resonant enhancement of optical absorption in graphene nanoresonators. Theoretical predictions for ‘perfect&’ two-dimensional absorbers have shown that, in principle, by carefully tuning of the geometry of graphene ribbons on a lambda;/4n thickness dielectic membrane with a metallic back reflector, the transmission and reflection coefficients may be nulled, resulting in 100% absorption of light in the graphene at its optical resonance^[1]. The plasmonic resonance of a graphene nanoribbon is determined by the ribbon width and its charge carrier density, wherein incident light is strongly localized in half wavelength resonant modes of the ribbon. At this resonant wavelength, we can tune the reflection coefficient to zero by selecting a membrane of thickness lambda;/4n so that infrared radiation reflected from the top surface is perfectly out of phase with that reflected from the back metal surface. Transmission is blocked by using an opaque metal layer on the back of the structure. The combination of these three factors results in resonant infrared absorption enhancement in graphene nanoribbon resonators.
Using finite element electromagnetic calculations we optimized our design to achieve a theoretical 100% absorption. We also fabricated a resonant interference absorber structure with graphene nanoresonators of varying width on a 1µm thick Si₃N₄ membrane with a 100nm thick gold back-reflector. Broadband reflectivity measurements were obtained using Fourier transform infrared spectroscopy. Our current results have demonstrated 17.6% light absorption at 1340cm^-1.
We extend this mechanism of enhanced light absorption to a broadband absorption by layering multiple lambda;/4n dielectrics beneath the graphene resonators, analogous to a traditional antireflection coating. By selecting materials and layer thicknesses to tune the accumulated phase upon reflection and path length traveled by the light, we can achieve resonant interference at the multilayer stack top surface for a broad range of wavelengths^[2]. As previously, an opaque metal layer on the back surface blocks transmission. Finally, patterned graphene nanoribbons of different widths tuned to their resonant frequency form the basis of achieving broadband perfect absorption. We will also discuss extensions of total optical absorption to light-trapping techniques in other 2D materials such as direct bandgap semiconductors MoSe₂ and MoS₂, which have potential as ultra-thin and lightweight optoelectronic devices and solar cells.
[1] R. Alaee et al, Optics Express 2012, 28017
[2] K. Rabinovitch and A. Pagis, Applied Optics 1975, Vol.14, No.6

Losses in metal-based plasmonic systems currently trigger the search for novel materials supporting low-loss surface polaritons. Surface phonon polaritons (SPhPs) on SiC and plasmons in graphene are two promising candidates.
In this talk we will describe near-field microscopy studies of phonon-resonant near-field interactions [1] and real-space imaging of propagating SPhPs [2,3] and graphene plasmons [4,5]. We visualize and measure the most fundamental properties such as the polariton wavelength and propagation length, and most intriguingly, how near fields can be electrically controlled by gate-tuning of graphene plasmons.
We also apply near-field microscopy to study the reflection of graphene plasmons at grain boundaries in CVD graphene [6] and at nanoscale gaps in graphene [7], and how structuring of the sample surface can be used for excitation and wavefront engineering of SPhPs [3], graphene plasmons [8] and their hybrids.
[1] R. Hillenbrand, T. Taubner, F. Keilmann, Nature 418, 159 (2002)
[2] A. Huber, et al., Appl. Phys. Lett. 87, 081103 (2005)
[3] A. Huber, et al., Appl. Phys. Lett. 92, 203104 (2008)
[4] J. Chen, et al., Nature 487, 77 (2012)
[5] Z. Fei, et al., Nature 487, 82 (2012)
[6] Z. Fei, et al. Nature Nanotech. (2013)
[7] J. Chen, et al., submitted
[8] P. Alonso Gonzales, et al., submitted

Metamaterials and their two-dimensional cousins, meta-surfaces, represent a remarkably versatile platform for light manipulation, biological and chemical sensing, and nonlinear optics. Mid-infrared part of the spectrum is particularly ripe for metamaterial applications because of its importance to molecular spectroscopy/fingerprinting, thermal imaging, and novel optical tools development. They can be easily functionalized with atomic and molecular monolayers, such as graphene or proteins. The resulting hybrid meta-surfaces can then be used to either develop agile optical structures (lenses, modulators, absorbers), or to perform ultra-sensitive spectroscopic characterization of the minute amounts of matter.
In the first half of my talk, I will provide a brief introduction to metamaterials and describe their applications to the development of active optical elements in the infrared based on graphene-functionalized metasurfaces and graphene spectroscopy. I will describe how electrically gated single-layer graphene can be used to inductively tune the infrared optical response of Fano-resonant meta-surfaces. Several implementations will be introduced: graphene of the meta-surface, graphene directly under the meta-surface, and graphene separated by a thin spacer from the meta-surface. Experimental realizations of infrared amplitude and phase modulators based on hybrid-functionalized meta-surfaces will be presented. We will demonstrate how the spectral shifts of metamaterials resonances introduced by the graphene can be utilized to extract graphene&’s electronic properties such as the complex-valued resistivity.
In the second half of the talk, I will describe how the most severe limitation of plasmonic metamaterials, their high Ohmic loss, can be overcome by using all-dielectric metamaterials. Experimentally-realized examples of Fano-resonant optical meta-surfaces supporting optical resonances with quality factors that are almost an order of magnitude sharper than those supported by their plasmonic counterparts will be presented. These silicon-based structures are shown to be planar chiral, opening exciting possibilities for efficient ultra-thin circular polarizers and narrow-band thermal emitters of circularly polarized radiation.

Graphene has been an attractive 2D material in recent years owing to its interesting electrical and mechanical properties. Strong light-matter interactions make graphene favorable for new plasmonic applications due to the field enhancement and confinement. Numerous applications based on metamaterials [1], photodetectors [2], photovoltaics [3], and nanoantennas [4], where electronics and plasmonics are combined in nanocircuitry, have been shown. The optical conductivity of graphene depends on the sheet carrier concentration of the graphene layer. Since high frequency interband transitions in graphene can be exploited through electrical gating [5], the plasmon resonance of graphene-hybrid structures such as nanoantennas can be modulated by applying a gate voltage [1, 4].
We report the modulation of the optical response for split ring resonator (SRR) arrays and bowtie nanoantennas fabricated on graphene. Strong localized fields on nanoantenna based structures couple to the incident field through a resonant process, causing a strong localization of electromagnetic waves. These antennas enhance the incident field, and therefore increase the interaction between light and graphene. We have shown the design, fabrication, and measurement of devices comprising a split ring resonator (SRR) arrays and bowtie nanoantennas on graphene. We obtained gate voltage dependent resonance broadening and tuning of these structures by utilizing a transistor-like graphene device. We obtained a frequency shift and electrical damping owing to the tunable carrier concentration of graphene, which consequently modulates its optical conductivity.
References
1. S. Cakmakyapan, L. Sahin, F. Pierini, and E. Ozbay, accepted by APL (in production process).
2. T. Mueller, F. N. A. Xia, and P. Avouris, Nat Photonics 4, 297-301 (2010).
3. M. Freitag, T. Low, F. N. Xia, and P. Avouris, Nat Photonics 7, 53-59 (2013).
4 Y. Yao, M. A. Kats, P. Genevet, N. Yu, Y. Song, J. Kong, and F. Capasso, Nano Lett 13, 1257-1264 (2013).
5. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, Science 320, 1308-1308 (2008).

Graphene as a monolayer of carbon atoms in a honeycomb lattice has attracted significant interest for its unique optical, electrical, mechanical properties for a range of applications. Metal based plasmonic devices and sensors can capitalize on graphene for unprecedented new functionalities if synergistically integrated. One such intriguing property of graphene that plasmonics can benefit from is its impermeability to gas molecules even as small as a single He atom. Capitalizing on this we demonstrated that nanoantennas made of silver, the ideal plasmonic material that tends to oxidize due to sulfur containing ambient gases, can be effectively passivated with a monolayer graphene. Due to its atomic thickness, graphene also does not perturb nanoantenna near-fields significantly maintaining the full potential of silver nanoantennas in sensing applications.
Graphene is also a very promising material as a bioactive layer due to its ability to effectively adsorb biomolecules through pi-pi stacking interactions. If graphene is used a monolayer functionalization layer, lengthy sensor surface modifications steps will not be necessary. We studied the binding affinities between several different proteins and graphene and found that adsorption can be as strong as that of a specifically binding antigen-antibody pair.
We will also discuss a new multimodal opto-electro-mechanical device that synergistically combines a graphene field effect transistor based nanoelectronic sensor and a nanoantenna based photonic sensor on a mechanical resonator sensor. This hybrid approach combining electrochemical, refractive index, and mass sensing functions on the same device footprint opens up new directions in nano-bio-sensors with unprecedented features. This proof-of-concept nanosensor experimentally achieves sub-picomolar label-free detection limits across all three independent sensing modes, and possesses a dynamic range ~2-3 orders of magnitude larger than that of any single mode nanosensor.

Nanoscale antennas sandwiched between two graphene monolayers yield a photodetector that efficiently converts visible and near-infrared photons into electrons with an 800% enhancement of the photocurrent relative to the antennaless graphene device [1]. The antenna contributes to the photocurrent in two ways: by the transfer of hot electrons generated in the antenna structure upon plasmon decay [2], as well as by direct plasmon-enhanced excitation of intrinsic graphene electrons due to the antenna near field. This results in a graphene-based photodetector achieving up to 20% internal quantum efficiency in the visible and near-infrared regions of the spectrum. This device can serve as a model for merging the light-harvesting characteristics of optical frequency antennas with the highly attractive transport properties of graphene in new optoelectronic devices [3].
References
(1) Z. Fang*, Z. Liu, Y. Wang, P. M. Ajayan, et. al. Nano Lett. 12, 3808, 2012.
(2) Z. Fang*, Y. Wang, Z. Liu, A. Schlather, P. M. Ajayan, et. al. ACS Nano 6, 10222, 2012.
(3) Z. Fang, S. Thongrattanasiri, A. Schlather, et. al. ACS Nano 7, 2388, 2013.

Metal nanostructures can concentrate optical fields into highly confined, nanoscale volumes, which is important for plasmonic nanolasers, white-light generation, and enhanced non-linear optical effects. This talk will describe how arrays of strongly coupled nanoparticles and three-dimensional bowtie nanoantennas provide new routes not only to achieve these extraordinary properties but also to scale them for wide-spread applications. First, we will describe a new type of nanocavity based on arrays of metal nanoparticles. These structures support lattice plasmon modes that can be amplified and that can result in room-temperature lasing with directional beam emission. Second, we will focus on NP dimers—3D bowties—that can function as optical nanoantennas. The extremely high electric field enhancements within the gap originate from the near-field interactions between the NPs. These localized fields can provide feedback for a new type of plasmonic laser based on 3D bowtie nanoparticle arrays.

Nanostructured metals received significant amount of attention in recent years due to exciting plasmonic and photonic properties such as strong-field localization, light concentration, and strong absorption and scattering at the localized or delocalized surface plasmon resonance frequencies. In particular, resonant absorption phenomenon in plasmonic nanostructures is widely studied for applications in photothermal therapy, thermophotovoltaics, heat assisted magnetic recording, hot-electron collection, and biosensing. However, it has proven to be challenging to realize very narrow absorption bands using plasmonic elements due to radiative and dissipative losses. Here, we theoretically and experimentally demonstrate an ultra-narrow band absorber (NBA) based on the surface lattice resonances in periodic nanowire and nanoring arrays fabricated on a reflecting metallic substrate. In experiments, we observe very sharp absorption resonance peaks with 12 nm bandwidth (Quality factor is 63) with over 90% total absorbance in visible frequencies. Simulated absorption bandwidths are as narrow as 3 nm. The resonance absorption wavelength, amplitude of the absorption peak and the bandwidth can be controlled by tuning the period of NBA arrays, as well as the metal thickness and grating width. Unlike conventional plasmonic absorbers that utilize metal-insulator-metal (MIM) multilayers, our NBA design consist of only metals, which enable strong interaction between the surface lattice resonance (SLR) of periodic nanostructures and underneath highly reflective film. The radiative losses are significantly reduced in our design enabling an ultra-sharp resonance peak. Furthermore, we studied the coupling between nanoring and nanowire gratings by controlling the position of the nanowire with respect to the nanoring. We observe SLR mode splitting facilitated by the coupling between two individual SLRs from nanoring and nanowire arrays. We calculate the figure of merit (FoM=(dlambda;/dn)/Δlambda;)for plasmonic sensor as high as 226, which is due to narrow resonance bandwidth and highly localized electric field surrounding nanoring/nanowire system. Designing such all-metallic nanostructure arrays is a promising route for achieving ultra-narrow band absorbers which can find use in absorption filters, narrow-band thermal emitters in thermophotovoltaics, and ultra-sensitive plasmonic biosensors.

Optical scatterers such as nano-holes, grooves or slits etched in a metal film are efficient sources of propagating surface plasmon polaritons (SPPs). Interference between SPP waves excited by an array of light scatterers can lead to unprecedented control of light at the nanoscale, such as higher-efficiency solar cells, surface enhanced Raman scattering, and compact biochemical sensors. Plasmonic interferometry has the potential to bring the advantages of conventional optical interferometry to the micro- and nano-scale regimes, bearing the merits of high throughput and real time techniques.
Here we propose the detection of light transmission through a dense array of plasmonic interferometers as a novel optical technique to measure the dispersion of the optical constants of liquid and solid materials over a broad wavelength range for any dielectric material on the metal surface, over small area (< 10 µm²) and volume (as low as femtoliters).
A typical plasmonic interferometer consists of a groove-slit pair etched in a metal film. A broadband beam incident on the groove excites SPPs propagating toward the slit, where interference with the incident beam occurs. Light intensity transmitted through the slit of each interferometer carries information about the effective SPP excitation efficiency β due to diffractive scattering by the groove, and the propagating phase dependent on refractive index of the dielectric.
We fabricated several plasmonic interferometers with varying arm lengths and on different supporting metal films. Measuring and stacking transmitted light intensity spectra by increasing the interferometer arm length in the range between 0.25 - 10 µm, a color map can be generated which captures the unique optical fingerprints of the particular metal/dielectric combination. Color maps for different interfaces, such as Ag/air, Ag/water, Ag/ethanol and Au/Air were obtained experimentally.
A “cut” in the color map at a fixed wavelength can reveal a sinusoidally varying intensity profile, which contains information about the scattering efficiency of the SPP source as well as the refractive index of the dielectric material. The difference between intensity maxima and minima is proportional to the scattering efficiency of the groove, which can be determined over a broad wavelength range, and for various metal/dielectric combinations, as well as varying groove width and depth.
The dispersion of the refractive index (both real and imaginary parts) for Ag, Au, water, ethanol and methanol, as well as other materials, can also be derived from a detail analysis of the color maps. A database for the dispersion of different materials can be established in a similar fashion.
Our findings show that plasmonic interferometry can be employed as an alternative, powerful optical tool for accurately determining the optical constants of dielectric materials at the nanoscale, over a broad wavelength range, using extremely small areas and sample volumes.

Nanoscale active devices, such as all-optical modulators and electro-optical transducers, can be enabled by integrating plasmonic nanostructures with functional materials. Vanadium dioxide (VO₂), a strongly-correlated electron material, is appealing for active plasmonic applications because its reversible insulator-to-metal transition (IMT) is accompanied by large changes in its electronic and optical properties. Here we exhibit all-optical control of the localized surface plasmon resonance (LSPR) in Au nanoparticles lithographically patterned in an array on a thin VO₂ film. In our experiment, periodic arrays of Au nanodisks of varying diameters were fabricated lithographically on a 50 nm thick VO₂ thin-film. The Au nanoparticles were arranged in a square lattice with a nominal period much shorter than the wavelength of interest in order to avoid diffraction effects in transmission measurements. Extinction spectra were acquired using an inverted optical microscope integrated with a Fourier-transform infrared spectrometer. By heating or cooling the VO₂ film, the Au plasmon response to ultraviolet light pulses during the IMT was effectively pinned to a strongly correlated state of the VO₂. Persistent, non-volatile blue-shifting and optical tuning of the LSPR was observed when the array was illuminated by successive ultraviolet-light pulses throughout the temperature range corresponding to the hysteresis of the film. Control experiments at temperatures above and below the region of strong correlation in the VO₂, on the other hand, showed no plasmonic tuning effect. The blue-shifted, persistent response of the LSPR appears to be an all-optical analog of the memory capacitance and the memristive response of VO₂ in electronic circuits. This demonstration of a persistent optical resonance controlled in nanometer-scale plasmonic structures opens the door to the studies of other hybrid material systems in which plasmons are coupled to strongly correlated electron materials.

Metallic nanostructures support localized surface plasmons that can be excited resonantly by suitably matching the excitation wavelength to the size of the plasmonic resonator. On resonance, plasmonic nanoantennas can localize the freely propagating radiation and produce very high optical currents in the metal. Due to the free electron absorption in metals, electromagnetic energy is efficiently converted to heat similar to generation of heat in a resistor at radio frequencies. Various types of resonant plasmonic absorbers utilizing this effect have been demonstrated for radiation absorption across the electromagnetic spectrum. The electrical tunability of a plasmonic absorber that enables electro-optical switching is highly desirable, however has not been achieved previously. Recently, it has been reported that significant voltage-tunable changes in the refractive index of indium tin oxide (ITO) can be achieved through accumulation of carriers through the electric field effect when utilizing ITO in a metal-oxide-semiconductor capacitor configuration. We apply this concept to localized plasmonic resonators and experimentally demonstrate an electrically tunable plasmonic mid-infrared absorber with near unity absorption efficiency at 4 mu;m wavelength.
Specifically, the perfect infrared absorption is realized by an array of gold nanostrip antennas separated from a back reflector by a thin dielectric layer. Periodic gold nanostrips are fabricated on top of a free standing silicon nitride membrane with a silicon frame using standard electron beam lithography and lift-off processes. The backside of the nitride membrane spacer is coated with a 100nm thick gold layer, acting simultaneously as the back electrode and the back reflector allowing for near unity absorption in the plasmonic absorber design. The gold nanostrips and back plate also act as the top and bottom contact electrodes for carrier injection and electrostatic gating, respectively. For electrical tunability, a thin layer of ITO is then deposited directly on the nanostrip antennas, where the near field intensity is maximum. The carriers injected into the ITO create an accumulation layer and modulate the optical refractive index in the near field of the plasmonic absorber, which in turn leads to the tuning of the plasmonic absorption resonance.

The strongly confined fields associated with surface plasmons cause their response to be strongly affected by nanoscale displacements. Conversely, the strong field enhancements can give rise to pronounced optical forces. We demonstrate the sensitive transduction of the motion of nanomechanical oscillators integrated with plasmonic nanocavities. Such plasmon-based optomechanical devices exhibit ultrastrong frequency shifts in the THz/nm range. The efficient coupling of plasmonic resonators with free-space light allows straightforward readout of the thermal motion of Si3N4 and metallic nanobeams and membranes. We demonstrate parallel sensing of arrays of resonators and explore the limits to displacement sensitivity in such systems.
Moreover, we study optical interactions in hybrid plasmonic-dielectric systems, consisting of nanoantennas evanescently coupled to whispering gallery mode cavities. Scattering experiments reveal how the nanoantenna polarizability is affected in proximity to the microcavity. We elucidate the role of the cavity Purcell factor in this effect. Simultaneously, the antenna-cavity coupling causes the cavity mode spectrum to be strongly altered by the presence of the nanoantennas. Interestingly, both antenna-induced blue and red shifts are observed, as well as both decreased and increased cavity quality factors, depending on the nanoantenna properties. Such tunable hybrid systems could find application for sensing and as interfaces with quantum emitters. Moreover, strong optical forces between cavity and antenna of both attractive and repulsive nature are associated with the alterations of the microcavity modes. By placing the nanoantennas on nanomechanical oscillators, we aim to directly observe these nanophotonic forces and realize a new class of nanomechanical transducers and actuators.

The ability of a sensor to efficiently detect or absorb an external signal is granted through strong interactions with the applied electromagnetic field. Intuitively, this implies that the presence of the sensor should also be strongly felt in its surrounding, and the more "visible" a receiver is, the better it may detect or absorb. However, this property is undesirable in many situations, such as in optical near-field imaging (e.g. using an NSOM tip), biomedical sensing and closely spaced energy harvesters. In all these examples the level of perturbation imparted on the primary field, intrinsically limits the performance of the system and directly affects the validity and efficiency of its overall response.
In this presentation, we propose the concept of minimum-scattering superabsorbing nanostructures and demonstrate the possibility of realizing highly efficient sensors, which absorb as much as desired, but at the same time keeping their presence imperceptible for an external observer. We begin by discussing the principles of operation of conventional sensors and antennas and show that the limitations associated with undesired scattering from these structures is due to the fundamental trade-off between maximum achievable absorption and minimum total scattering in a receiving dipole. In this context, we briefly explain the possibility of adjusting typical receivers - e.g. through proper loading - to scatter much less than they absorb, but at the price of having very low absorption levels. Next, we propose a solution to overcome these inherent limitations, discussing the possibility of maintaining absorption at any desired high level, while arbitrarily reducing the total scattering from the sensor, by staggering few absorption channels in a single receiver. Our study reveals the "optimal" combination of these channels that guarantees the least perceivable detector design. We show that, unlike traditional single-channel receivers, all contributing harmonics must be excited at the same, largely mismatched level. Consequently, each scattering harmonic supplies only a small portion of the total absorption, proportional to its order, and the overall scattering can decrease to any arbitrarily small level by proper interference among more harmonics.
Even more importantly, we show that, due to the large level of mismatch, the system shows a moderate overall Q-factor even in the presence of higher-order scattering harmonics, making the proposed optimal minimum-scattering absorbers practically realizable over reasonable bandwidths. We present several examples of realistic structures based on core-shell plasmonic nanoparticles with absorption levels comparable to current sensor designs but scattering 7 to 13 times less, each suitably designed to include different absorption channels. Our findings may have a broad range of applications in sensor design, opening up exciting possibilities in biomedical technology, energy harvesting, security and imaging.

The resonant plasmonic properties of metallic nanostructures depend strongly on charge carrier density. While researchers have reported shifts of the resonant absorption frequency of plasmonic nanostructures due to electrostatically induced changes of charge density, the converse —the dependence of charge density and electrostatic potential on optical absorption— has been largely overlooked. Recently, we have reported a theoretical framework and provided experimental evidence for a ‘plasmoelectric effect&’, a newly described mechanism for generating electrochemical potentials in plasmonic nanostructures via narrowband absorption. Our initial work with Au colloid nanoparticles has shown that, unlike the more familiar thermoelectric or photovoltaic effects, the magnitude and sign of the plasmoelectric potential depends on the frequency difference between the plasmon resonance and incident radiation [1]. Radiation at shorter wavelength induces an increase of electron density that blue-shifts the plasmon resonance. This response is driven by the increased heat that accompanies increased absorption. Similarly, radiation at longer wavelengths decreases electron density to induce a red-shift of the absorption maximum.
Here, we report measurements on lithographically patterned samples with a design that has been optimized via iterative full wave simulations (FDTD method) to maximize plasmoelectric potentials, and to further distinguish the phenomenon from the thermoelectric effect. The structure consists of an electrically grounded 20 nm Au thin film on glass with lithographically defined square-lattice arrays of 100 nm diameter holes. The spectral position of the plasmonic resonance of the hole arrays is very sensitive to the inter-hole spacing, allowing sharp and highly tunable absorption peaks to be defined across the visible spectrum, between 550 nm and 700 nm, by varying the pitch between 150 to 300 nm in fabricated devices.
Scanning Kelvin probe force microscopy (KPFM) determined the surface potential of device structures while varying the wavelength of incident radiation near the plasmon resonance. Under 30 mW cm-2 monochromatic illumination, we measure induced potentials of ± 150 mV from hole arrays, with a characteristic sign change for illumination blue or red of the absorption maximum. This is a 2-order of magnitude increase of measured plasmoelectric potentials compared with the response of Au colloid particles. By comparing devices with plasmonic resonances spanning the optical spectrum, we observe clear evidence for the pitch-dependent and wavelength-dependent trends consistent with our theoretical framework. Our findings guide the development of solid-state power conversion devices based on the plasmoelectric effect, as our devices generate driving electrochemical potentials 1000x larger than comparable thermocouples under equivalent optical power.
[1] Sheldon, et al. (2012). Plasmoelectric Potentials in Metal Nanostructures, in review

Nanophotonics and nanoscale optics, which are aimed at coherent control and manipulation of single photons emitted by individual quantum emitters in a nanostructured photonic environment offer a revolutionary new approach to computation and information technology: bits can be carried in the state of light and processed by nanoscopic amount of matter. Research in information processing at the nanoscale has been limited to date by the weak interaction between light and matter which poses constrain to scalability, requires low temperature operation and careful on-resonance tuning of the devices. However, advances in nano-optics, plasmonics, complex dielectrics and nanofabrication are stimulating new research directions with the potential to revolutionize the way we control light and transfer information at the nanoscale.
In my talk I will discuss one of the fundamental properties that rules the coupling of nano-scale light emitters to complex photonic modes of their local environment. Light spontaneous emission, absorption and scattering are all related to the local density of optical states (LDOS) that can be engineered in dielectric and metallic nano structures. I will show how a nano-sized light probe can illuminate complex materials and shed light on their optical properties and modes.
I will report nanoscale mapping of the local density of states by cathode-luminescence microscopy, a combination of electron-beam scanning and optical spectroscopy. With unprecedented resolution (~10 nm) we image localised photonic crystal cavity modes in a nanostructured silicon nitride membrane, over the visible spectrum into the near-IR. We identify individual cavity modes and extended Bloch modes that are spatially different and we map their LDOS. Moreover, by momentum spectroscopy, we resolve the angular emission pattern of the radiation emitted, which exhibits rich diffraction patterns.
I will also discuss studies of fluorescence from a nano-sized emitter embedded in complex photonic media, such as photonic crystals and random powders. By fluorescence dynamics we measure LDOS distributions in 3D disordered systems. We observe a surprisingly long-tailed distribution of the LDOS with Purcell factors up to ~10. I will discuss how our experimental results get the ongoing discussion on the dependence of the C0 correlation function on macroscopic transport parameters.

Ten years ago this year, the Zero-Mode Waveguide was documented in Science Magazine. Today, it is a crucial part of the only commercial, single-molecule, real-time technology in existence, and one of just a handful of nanophotonics-based technologies that have made it to the market. In this talk, I&’ll review the concept, and then show the pathway this technology took to being part of a DNA sequencing system that is paving new roads into scientifically and clinically important research fields.

Plasmonic concentrators (PCs) can be constructed by etching long-range arrays of shallow sub-wavelength holes on the surface of a noble metal, such as silver. Each hole in the array can couple incident light to surface plasmon polariton (SPP) modes that propagate along the dielectric / metal interface. If the holes are close enough, SPP modes generated by neighboring holes can constructively interfere, thus enhancing the electromagnetic fields at the metal / dielectric interface by several orders of magnitude. Due to the field enhancement, absorption in a thin film resting above a PC can therefore exhibit broadband, polarization-independent enhancement (compared to a flat surface) [1]. This is not solely due to the increased effective path length of the incident light. Indeed, the average hole-hole spacing and the order of rotational symmetry of the pattern used can play a significant role, which can largely be understood in terms of increased constructive interference between SPPs.
In addition to the simple periodic 2D (e.g., honeycomb, square, hexagonal) and random arrays, here we include quasiperiodic (QP) arrays (e.g., Penrose) featuring higher levels of local and long-range order (specifically, rotational symmetry), and a higher surface density of point scatterers, despite lacking long-range translational symmetry. By generalizing a Penrose array generation algorithm [2], an entire spectrum of QP patterns can be created and studied. We demonstrate that QP plasmonic concentrators can provide higher field enhancements, and as a result better absorption enhancement, compared to periodic or random arrays [3-4].
Aside from the immediate application to thin film photovoltaics and biochemical sensing, the results presented here help provide a deeper understanding of SPP generation, propagation, and interference, as well as shedding light on higher order scattering effects contributing to the overall broadband field enhancements.
1. Ostfeld, A E, D Pacifici. Appl. Phys. Lett. 98, no. 113112 (2011)
2. de Bruijn N G. Kon. Nederl. Akad. Wetensch. Proc. Ser. A 84 (1981): 39-66
3. Flanigan P W, A E Ostfeld, N G Serrino, Z Ye, D Pacifici. Opt. Express. 21, no. 3 (2013): 2757-2776
4. Flanigan P W, A E Ostfeld, Z Ye, N G Serrino, D Pacifici. "Quasiperiodic plasmonic concentrators for ultra-thin film solar cells", Optics of Aperiodic Structures: Fundamentals and Device Applications, ed. Luca Dal Negro (2013).

Single-molecule fluorescence (SMF) imaging is a powerful technique that provides high sensitivity and nanometer-scale resolution for bio-imaging. By fitting the diffraction-limited emission profile of an isolated dye molecule, the position of the emitter is determined to a precision that improves with the number of photons detected—far better than the standard diffraction limit of light. While in vitro implementations of SMF have achieved 1.5-nm localization precisions, the resolution of SMF imaging in live cells has been generally limited to 10 - 40 nm. This limitation is due, in part, to the poor emissive properties of fluorescent proteins (FPs), the genetically encodable labels widely used for biological imaging. Increasing FP brightness and photostability will therefore improve the precision with which these fluorescent probes are localized in vivo and will thus push live-cell super-resolution imaging down to the 1-2 nm size of proteins.
In this work, we demonstrate that particle plasmonics can improve both the brightness and photostability of FPs. Upon coupling to the near field of gold nanorods, we observe that the emission rate of the red FP mCherry is more than doubled, and that immobilized molecules of the photo-activatable FP PAmCherry emit three times more photons prior to photobleaching. We also explore the interaction between the yellow FP mCitrine and the same nanorods, in which case we do not observe large emission rate enhancements. We attribute these differences to relative nanorod mode strengths: the red FPs mCherry and PAmCherry couple to the stronger, longitudinal mode, while mCitrine couples to the weaker, transverse mode. Additionally, through mapping single-molecule adsorption events on to gold nanorod images, enhanced emission is observed to correlate with the nanorod position at the nanometer scale. These in vitro experiments demonstrate the power of single-molecule super-resolution imaging for studying plasmon-enhanced fluorescence in FPs. We then extend our methods to in vivo experiments, where we prepare gold nanorods as extracellular imaging substrates for enhancing emission from membrane-bound proteins in live bacteria cells. In particular, we track plasmon-coupled FP-labeled proteins on the surface of single cells of Vibrio cholerae, agent of the human cholera disease. Ultimately, our results indicate that plasmonic substrates are advantageous for super-resolution imaging, and we anticipate that plasmon-enhanced imaging will find important applications in live-cell single-molecule microscopy.

Binding of analytes in solution to surface-immobilized receptors is of great importance for rapid and precise detection of biomolecules [1]. Research shows that the binding event is not simply controlled by the chemical reaction between analytes and receptors, but also depends on the availability of the analytes in the sensing area. The latter factor is directly related to the analyte delivery mechanism. In conventional biosensing systems, the sensor performance is limited by inefficient analyte transport especially when detecting analytes with larger sizes such as viruses and cells due to their low diffusivities. In this invited talk, we will first describe an actively controlled optofluidic system that merges actively controlled fluidic and optical nanosensors on the same platform to address this fundamental analyte transport problem [2,3]. We will next present our recent results demonstrating use of this novel platform with virus like analytes. We will show that the sensing response time is reduced by an order of magnitude from 4 hours to 30 minutes as compared to the conventional fluidic scheme [4]. A dynamic range spanning 7 orders of magnitude from 103 to 109 particles/mL is quantified, corresponding to a concentration window relevant to clinical diagnostic and drug screening. In addition, the virus like analytes are captured and detected intact without being damaged, so that the samples could be further studied. We will also present our numerical and analytical calculations demonstrating improved time response of surface sensors using active analyte transportation. Our lab-on-a-chip fluidic detection system, by offering superior analyte delivery efficiency, significant reduced sensing response time and minimized sample volume, could have profound implications in wide range of biological and clinical applications.
References:
[1] A. A. Yanik et al, Nano Letters Vol. 10, pp. 4962-4969 (2010).
[2] M. Huang et al, Optics Express Vol. 17, pp.24224-24233 (2009).
[3] A. A. Yanik et al, Applied Physics Letters Vol. 96, 021101 (2010).
[4] M. Huang et al, Lab on a Chip DOI: 10.1039/C3LC50814E (October 2013).

Nanoplasmonic surfaces, the surfaces with metallic nanostructures, have been studied for light enhancements (absorption or emission) for decades. However, most of them are metallic nanostructures with random size, spacing and arrangement, limiting high enhancements to a very few random hot spots. Others use a single isolated layer of nanostructures of regular size and spacing, which gives uniform but moderate enhancements.
Here, we present a drastically different approach in designing a plasmonic surface, termed “nanoplasmonic coupled-cavity antenna-array” (NCA2), that gives uniform and high enhancements. We discuss two NCA2 structures and their applications in significantly enhancing solar cell, LEDs, sensors (both SERS and fluorescence) and cathodes (photoelectron emitters).
A NCA2 comprises two layers of metallic structures coupled together, with at least one layer having metallic nanostructues of regular size and spacing. The coupling means the separation between the two layers is smaller than the wavelength of light. Two NCA2 structures will be discussed: PlaCSH (Plasmonic Cavity with Subwavelengh Hole-Array) [1] and D2PA (Disk-Coupled Dots on Pillar Antenna-Array) [2].
PlaCSH has a plasmonic cavity structure consisting of a tip metal (gold) electrode with subwavelength hole-array, a metal back electrode, and in between an active material layer (either for solar cell or LED). D2PA has an array of plasmonic nanocavity, consisting of a periodic non-metallic pillar array with a metal disk and a metal back-plane on top and base of the pillars respectively, dense metallic nanodots on the pillar wall, and nanogaps between the metal components.
The solar cells and LEDs built PlaCSH have shown significant increase of external quantum efficiency (over 60%) compared with the conventional using ITO top electrode [1, 3], plus rather unique angle dependence that further significantly improve the device performances.

The sensors built on D2PA have demonstrated uniform SERS enhancement of 10^9 (9 orders of magnitude) and fluorescence immunoassay sensitivity enhancement of over 10^6 (6 orders of magnitude). The cathodes efficiencies are also enhanced by several orders of magnitude [4].

The NCA2&’ss were fabricated on 4” wafers or thin PET films by planar or roller nanoimprint. PlaCSH, D2PA, and other NCA2 structure are opening up many new applications in solar cells, LEDs, sensors, photo cathodes, and other fields.

[1] S. Y. Chou and W. Ding, Optics Express 21, S1, A60-A76 (2013), published 28 Nov (2012).
[2] W. Li, F. Ding, J. Hu, and S. Y. Chou, Optics Express, 19 (5) 3925-3936 (2011).
[3] W. Ding and S.Y. Chou, EIPBN, 2013.
[4] Y.X. Liang, Y.X. Wang, and S.Y. Chou, EIPBN, 2013.

As solar cells approach the radiative limit, they can achieve significantly higher efficiencies and exhibit new effects owing to the significant number of radiatively emitted photons. One such effect is an increase in voltage by optically limiting the angles at which light is emitted from a cell to a narrow range around normal incidence. In materials such as GaAs where radiative recombination is dominant, a significant portion of the dark current results from radiatively emitted photons escaping the solar cell. Optically limiting the angles of emitted light recycles many of these emitted photons back to the cell, where they are re-absorbed. Thus, as fewer photons ultimately escape the cell, radiative dark current is reduced, and the cell voltage and efficiency are increased, with efficiencies >36% possible for Auger limited GaAs cells.[1] Despite clear theoretical predictions, only recently, with the introduction of cells lifted off the growth substrate, have GaAs cells begun to closely approach the radiative limit so that enhanced photon recycling by limiting emission angle may be observed.
As proof of concept, we have demonstrated enhanced photon recycling and open-circuit voltage (Voc) experimentally using a nanophotonic element that limits the emission angle only over the narrow wavelength range of emitted light in GaAs, so that diffuse light may still enter the cell. This angle restrictor consists of an optimized dielectric multilayer with 19 nanoscale alternating high and low index layers and less than 3 um total thickness. Integrating sphere measurements on the fabricated structure showed excellent normal incidence transmission and near unity reflectivity at oblique angles (>20°) for radiatively emitted wavelengths. By simply placing this narrow band angle restrictor in optical contact with a high quality GaAs cell, we observed a clear Voc increase at constant current. Measuring the Voc increase with angle restriction across four cells with external radiative efficiencies of 3-16%, we found that cells closest to the radiative limit showed the largest voltage increase (3.7mV) with angle restriction, as more radiatively emitted photons are available to be recycled. Furthermore, we found excellent agreement between the measured Voc increases and a realistic detailed balance and optical model. Consistent with theory, current-voltage measurements in the dark showed a reduction in dark current with angle restriction. In particular, the high-voltage (J_01) dark current component associated with radiative recombination showed a marked (>10%) decrease, consistent with the observed Voc increase under illumination, while other fitted parameters showed no change with angle restriction. Finally, using spacers to vary the height of the angle restrictor, we found that more closely coupling the angle restrictor to the cell leads to higher Voc, emphasizing the optical nature of the enhancement.
[1] Kosten, Light: Science & Appl., 2013

As the size of magnetic bits on hard-drives is miniaturized through advancements in write head and media fabrication technology, the bit density is limited by the superparamagnetic limit, past which the bits become thermally unstable, causing for thermal fluctuations to spontaneously co-align their magnetic states - translating to a loss of data in the hard-drive. Part of the solution to this problem is to create smaller-grain-size recording media composed of magnetically more stable (or coercive) materials; however, this means that higher applied magnetic fields are required to magnetize the media. This poses a problem, as not only does the recording field strength of the write head material saturate below what is required for these higher coercivity materials, but it also suffers a field drop-off that is inversely proportional to the size of its magnetic write pole. One solution to this bit miniaturization problem is to temporarily lower the coercivity of the media by applying to it an external form of energy (in the form of heat). In addition to being able to write with a saturated magnetic field, this approach also has the added advantage that the bit size can then be defined by the size of the thermal hotspot. One approach to achieving this is to use, what is a called, a near-field transducer, whose function is to transfer optical energy from a write-head-mounted diode laser onto a sub-diffraction-size spot on the recording media. The need to locally heat a sub-wavelength region using optical frequencies naturally calls for the field of plasmonics, where transferring optical energy into heat, seen as inefficient and detrimental for optical applications, seems very adequate for heat-assisted magnetic recording. In the field of plasmonics, we can use either apertures or antennas (or some combination of the two), to convert optical power into thermal energy on the disk, temporarily lowering its coercivity, ultimately allowing us to reach the fundamental limits of magnetic recording.

In this work, we have investigated properties of gold thin films and nanostructures relevant to their use in Heat Assisted Magnetic Recording (HAMR). HAMR is an exciting plasmonic application, however, reliability issues require better understanding of the connection between gold morphology and deformation behavior in Near Field Transducers (NFTs). Atomic Force Microscopy was used to study the roughness and grain size of gold thin films and additional power spectral density calculations allowed this analysis to focus on length scales shorter than 100 nm which have a larger impact on plasmonic properties. For a 100 nm thick electrodeposited gold thin film, grain diameter was found to be 24.3 nm with a roughness Ra value of 2.38 nm. Roughness power spectral density was found to be greatest at a length scale of 41.88 nm. Grain size and roughness were found to be tunable by varying deposition properties. Individual grain orientation plays an important role in gold nanoparticle deformation. Our simulations and initial results have shown that Transmission Electron Backscatter Diffraction (tEBSD) is capable of imaging individual grain orientation in a gold thin film with 20 nm grains. Simulations showed that a transmission configuration reduces the spread of backscattered electrons in our films to 17.5 nm compared with approximately 100 nm using conventional EBSD. Our tEBSD measurements verified this greatly improved resolution and we have been able to observe the orientation of individual gold grains as small as 20 nm. This relatively new characterization technique offers a way of directly observing morphology and failure mechanisms in NFTs. Gold thin films were also patterned using e-beam lithography and ion milling into NFT-like features to apply this measurement technique on nanostructured devices. Ellipsometry was used to understand the impact of deposition conditions on the optical properties, including complex permittivity, of gold thin films. These films were annealed to observe the effect of temperature on optical properties and better understand how materials behave at the high temperatures encountered during HAMR operation.

Metallic nanoparticles have been intensively studied over the last years due to the possibility of producing high levels of electromagnetic field confinement and enhancement, particularly desirable for lighting and sensing applications. While single metallic nanoparticles can sustain localized surface plasmon resonances (LSPRs), a periodic array of these nanostructures may exhibit collective resonances resulting from the radiative coupling of LSPRs [1]. Two distinct mechanisms enhancing the radiative coupling of LSPRs have been reported in literature. The first one consists of the hybridization of guided modes in thin dielectric layers with LSPRs of nanoparticles in the proximity of the guiding layer. For these hybridized modes, which are referred to as quasi-guided modes, a waveguiding structure is necessary. The second mechanism relies on Rayleigh anomalies (RAs) or diffracted orders in the plane of the array, which lead to the hybridized resonances known as surface lattice resonances (SLRs) [2]. Although both quasi-guided modes and SLRs have been studied, they have never been simultaneously reported in a single sample because of the different conditions to support them: quasi-guided modes need a waveguide that has a higher refractive index than the surrounding, while SLRs are favored when the medium surrounding the array is homogeneous [3]. We study the conditions that allow the excitation of quasi-guided modes and SLRs to achieve the coexistence of both modes in single structures [4]. In order to identify and compare both modes, we fabricate plasmonic arrays on top of a thin layer of emitting material acting as a light-emitting waveguide, and examine them through light extinction and emission spectroscopy. A strong modification of the luminescence as a result of the coupling is noticed. These results pave the way for the integration of plasmonics in plasmonic-based LEDs where hybrid modes allow tailoring the intensity, polarization and directionality of very efficient emitters employed in nanostructured lighting devices [5].
[1] V. Giannini, G. Vecchi and J. Goacute;mez Rivas, Lighting up multipolar surface plasmon polaritons by collective resonances in arrays of nanoantennas, Phys. Rev. Lett. 105, 266801 1-4 (2010).
[2] S.R.K. Rodriguez, G. Lozano, M.A. Verschuuren, R. Gomes, K. Lambert, B. de Geyter, A. Hassinen, D. van Thourhout, Z. Hens and J. Goacute;mez Rivas, Quantum rod emission coupled to plasmonic lattice resonances: A collective directional source of polarized light, Appl. Phys. Lett. 100, 111103 1-3 (2012).
[3] S.R.K. Rodriguez, S. Murai, M.A. Verschuuren and J. Goacute;mez Rivas, Light-emitting waveguide plasmon-polariton, Phys. Rev. Lett. 109, 166803 1-5 (2012).
[4] S. Murai, M. A. Verschuuren, G. Lozano, G. Pirruccio, S. R. K. Rodriguez, and J. Gomez Rivas, Hybrid plasmonic-photonic modes in diffractive arrays of nanoparticles coupled to light-emitting optical waveguides, Optics Express, 21, 4250 (2013).
[5] G. Lozano, D. J. Louwers, S. R. K. Rodríguez, S. Murai, O. T. A. Jansen, M. A. Verschuuren and J. Goacute;mez Rivas, Plasmonics for solid-state lighting: enhanced excitation and directional emission of highly efficient light sources, Light: Science & Applications 2, e66 (2013).

Plasmonic metamaterials allow confinement of light to deep subwavelength dimensions, while allowing for the tailoring of dispersion and electromagnetic mode density to enhance specific photonic properties. Here, we construct a plasmonic meta-surface through coupling of diatomic plasmonic molecules which contain a heavy and light meta-atom. Presence and coupling of two distinct types of localized modes in the plasmonic molecule allow formation and engineering of a rich band structure in a seemingly simple and common geometry, resulting in a broadband and quasi-omni-directional meta-surface. Surface-enhanced Raman scattering benefits from the simultaneous presence of plasmonic resonances at the excitation and scattering frequencies, and by proper design of the band structure to satisfy this condition, highly repeatable and spatially uniform Raman enhancement is demonstrated. It is shown that high spatial uniformity plasmonic enhancement can be used for single-molecule level SERS sensing and super-resolution Raman imaging. Complementary to Raman scattering, infrared absorption can also be enhanced by plasmonic surfaces, a method typically referred to as SEIRS. Such improvement in IR absorption based sensing can be achieved using metasurfaces with resonances in the infrared.We demonstrate that optical and chemical properties of Aluminum can be advantageous in fabricating metasurfaces that exhibit multiple resonances spanning the visible and the infrared. Particularly, we use all-aluminum metal-insulator-metal (MIM) resonators using the native oxide of the bottom aluminum layer as the dielectric layer and demonstrate tunable structures featuring two simultaneous resonances in the infrared and one plasmonic resonance in the visible region. Hierarchical metasurfaces are also fabricated by depositing silver nanoislands on top of the aluminum layers, where a MIM structure is obtained due to the presence of the native oxide layer. Monolayer sensitivity is demonstrated using infrared plasmonic metasurfaces based on aluminum nanostructures.

We have investigated a metal-insulator-metal (MIM) waveguide with a horizontal slot (HS) embedded in the intermediate insulator for stronger nanoscale field enhancement. The HS has a lower refractive index than the surrounding insulator and creates a high contrast in the dielectric-constant profile of the intermediate insulator. To examine the effect of the contrast in the dielectric constant on the field enhancement, we have applied the HS-MIM approach to our previously reported highly efficient three-dimensional (3D) nanofocusing photon compressor (3D NPC). The compressor consists of a MIM waveguide that tapers three-dimensionally into a deep sub-wavelength-scale tip. We targeted the telecom C-band (~1.55mu;m), and simulated a 3D-tapered SiO2-slot-embedded Ag-Si-Ag waveguide on a fused silica substrate. The dimensions of the Si layer (index n_Si = 3.5) varied linearly from h=200 and w=300 nm (body) to h=30 and w=45 nm (tip), while the thickness of the silica (index n_SiO2 = 1.45) slot remained constant at a given value below 10 nm. As light propagates through an HS-MIM-based 3D NPC, the electric field intensity in the embedded slot increases due to the presence of a contrast in the dielectric constant profile. And, the enhancement in the HS becomes significantly more noticeable as the dimensions of the waveguide decrease, especially inside the nanofocusing tip. When compared to the previous MIM counterpart, the HS-MIM-based 3D NPC shows more than a ten-fold and twenty-fold improvement in field intensity enhancement for a 3-nm-thick and 1-nm-thick slots, respectively. And, a thinner HS yielded an enhancement-improvement factor closer to the upper limit given by the material properties [n_Si/n_SiO2]⁴. Interestingly, the field enhancement is highly dependent on the slot thickness yet almost independent of its position inside the insulator. The focusing efficiency (the percentage of the energy delivered into the nanofocusing tip) remained almost unchanged from the value for the MIM counterpart (~85%). As we explored the fundamental mode of the HS-MIM waveguide, we have discovered that the performance improvement of the HS-MIM-based 3D NPC originates from more desirable mode properties influenced by the presence of the HS. We observed that a thinner HS produced a shorter characteristic propagation length. In addition, compared to the MIM counterpart, the optimized taper angle for the HS-MIM-based 3D NPC is smaller because it needs to compensate relatively larger mode mismatches that occur in the taper region. We also simulated different index configurations for the intermediate insulator, including the cases in which the layer was completely made of either low or high index material. Among the examined configurations, the HS-MIM approach shows the most significant improvement in field enhancement with almost no loss in focusing efficiency. In our presentation, we will discuss detailed simulation techniques, results, and potential plasmonic applications.

Infrared absorption spectroscopy is an important tool for chemistry, biology and medicine, because it delivers vibrational fingerprints of the molecular structure. However, the intrinsic cross-section of a molecular vibration for infrared absorption spectroscopy is rather small, which limits the sensitivity of the detection. Recently, e-beam lithographed nanostructures with plasmonic resonances have been introduced for surface enhanced infrared absorption (SEIRA) spectroscopy [1-2]. By tuning the plasmonic resonances to the vibrational bands of the molecules, a SEIRA enhancement 10^4-10^5 is achieved.
Here, we investigate SEIRA spectroscopy with gold strip gratings made by standard optical lithography. By exciting surface plasmon polaritons on both air-gold and gold-substrate interfaces, the resonance of the 1D gratings is linearly tunable with the grating period. With the field enhancement at the edge of the gold strips, a SEIRA enhancement factor more than 6000 for PMMA molecules is achieved. The strong SEIRA enhancement together with the easy fabrication makes the gold strip grating a promising candidate for SEIRA experiments [3].
[1] R. Adato, A. A. Yanik, J. J. Amsden, D. L. Kaplan, F. G. Omenetto, M. K. Hong, S. Erramilli, and H. Altug, Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays, Proc. Natl. Acad. Sci. U.S.A. 106(46), 19227-19232 (2009).
[2] L.V. Brown, K. Zhao, N. King, H. Sobhani, P. Nordlander, N. J. Halas, Sufrace-enhanced infrared absorption using individual cross antenna tailored to chemical moieties. J. Am. Chem. Soc. 135, 3688-3695 (2013).
[3] T. Wang, V. H. Nguyen, A. Buchenauer, U. Schnakenberg, T. Taubner. Surface enhanced infrared spectroscopy with gold strip gratings. Opt. Express. 21, 9005- 9010 (2013).

We report the development of resonant antenna probes for tip-enhanced infrared microscopy [1]. We employ focused-ion-beam (FIB) machining to fabricate high aspect ratio gold cones, which replace the standard tip of a commercial Si based AFM cantilever. Calculations show large field enhancements at the tip apex due to geometrical antenna resonances in the cones, which can be precisely tuned throughout a broad spectral range from visible to THz frequencies by adjusting the cone length. Spectroscopic analysis of these probes by FTIR and nano-FTIR [2] corroborates their functionality as resonant antennas and verifies the broad tunability. By employing the novel probes in a scattering-type near-field microscope and imaging a single tobacco mosaic virus (TMV), we experimentally demonstrate high performance mid-infrared nano-imaging of molecular absorption. Our probes offer excellent perspectives for unprecedented optical nano-imaging and nano-spectroscopy, pushing the detection and resolution limits in many applications, including nanoscale infrared mapping of organic, molecular, and biological materials, nano-composites, or nano-devices [3-4]. Furthermore, due to their well-defined geometry the antenna probes will significantly ease the qualitative description of the tip-sample near-field interaction, which will be essential for quantitative measurements of the local sample properties such as dielectric function and molecular absorption.
References
[1] F. Huth et al., Nano Lett. 13, 1065 (2013)
[2] F. Huth et al., Nature Materials 10, 352 (2011)
[3] J. Stiegler et al., Nature Comm. 3, 1131 (2012)
[4] A. Huber et al., Nature Nanotech. 4, 153 (2009)

Surface Plasmons (SP) enable light to be concentrated below the diffraction limit on metal-dielectric interfaces. In constrained geometries such as nanogaps, electromagnetic fields can be drastically enhanced resulting in single-molecule sensitivity[1] for optical sensing and studies of molecule-SP interactions.[2]
We study systematically the interaction of SPs in ultimately constrained gaps from 10 nm to 0 nm and use these antennas for spectroscopy on a few- to single-molecule level. We have developed a material-independent fabrication process using e-beam lithography in combination with ion-milling to create arbitrary and freestanding[3] antenna structures with gap sizes below 3 nm. As the substrate is flexible, a fine-tuning of the gap size can be achieved by application of mechanical strain (gap modulation: ± 3 nm). Compressive strain enables sub-3 nm gaps to be closed and tensile strain atomic-sized tips to be created by breaking, a procedure similar to break-junctions.[4]
In the first part, we will report on the optical characterization of individual antennas with rod, bowtie and dolmen geometries performed in a mu;-Raman microscope under ultra-silent conditions.[5] We will discuss effects of material composition (polycrystalline vs. monocrystalline), longitudinal gap dimensions and transversal apex. As the metal atoms are subject to diffusion, in particular upon illumination, sub-2nm gaps are inherently unstable. We reduce the surface diffusion by lowering the temperature down to 4 K in temperature-dependent measurements under ultra-high vacuum conditions. Under these optimal conditions, the transition from weak to strong interacting SP regime can be studied continuously.
In the second part, we will demonstrate spectroscopic operation and sensitivity of these antennas on the few-molecule level. In a first step, the entire antennas were functionalized with conjugated and thiol-terminated molecules from a 1 mMol/L solution in THF. Oligophenylene-based molecules with a sulphur-sulphur distance of up to 5.7 nm were synthesized to directly bridge the antenna&’s feed gap. Raman modes are detected on bowtie antennas only, whereas test structures consisting of isolated triangles do not reveal any peaks. Moreover, due to strongly modified optical selection rules at high electro-magnetic fields in these bowtie gaps, we demonstrate Raman emission from multiple infrared (IR) modes that are detected simultaneously with conventional Raman modes. Those modes are energetically separated from the normal Raman modes. This simultaneous detection of Raman and IR modes is an important step towards more sophisticated and comprehensive optical spectroscopy below the diffraction limit that reaches single-molecule sensitivity.
[1] Phys. Rev. Lett. 83, 4357-4360 (1999)
[2] Nano Lett. 12, 5989-5994 (2012)
[3] Nano Lett. 10, 4952-4955 (2010)
[4] Phys. Rev. Lett. 98, 176807 (2007)
[5] Nanoscale, 5, 10542-10549 (2013)

Here we report on Mie assisted Raman scattering amplification (MARS) signal of molecular species attached to silicon nanoparticles (SiNPs) resulting from the evanescent field of their Mie resonant modes [1]. In contrast with common plasmon-assisted surface-enhanced Raman scattering (SERS) spectroscopy, here the field enhancement does not come from the collective excitation of electrons [2], but from the resonances of the whispering gallery modes (WGMs) of high refractive index nanoparticles [3]. The very large scattering cross-sections of Mie modes induce enormous scattered fields around the particles that enhance the Raman scattering cross section. Although the enhancement factor provided by this process is slightly smaller than the normal SERS amplification by gold [4], the interest on Si nanoparticles is manifold. A) the wide range of experimental configurations that can be implemented including photonic crystals [5], B) the sharp spectral resonances easily tunable with the particle size [6], C) the biocom-patibility and biodegradability of silicon [7], and D) the possibility of direct analysis of mole-cules that do not contain functional groups with high affinity for gold and silver. Additionally, silicon nanoparticles are non-metallic and they present Raman enhancement effects at larger sizes than for gold.
REFERENCES
[1] Rodriguez, I. ; Shi, L. ; Lu, X.; Korgel, B. A. ; Alvarez-Puebla, R. A.; Meseguer, F.; Submitted for publication
[2] Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. P. ; An. Rev. Anal. Chem. 2008, 1, 601.
[3] Bohren, C. F.; Huffman, D. R.; Absorption and scattering of light by small particles; Wiley, 1983.
[4] Rodriguez-Lorenzo, L.; Alvarez-Puebla, R. A.; Pastoriza-Santos, I.; Mazzucco, S.; Stephan, O.; Kociak, M.; Liz-Marzan, L. M.; Garcia de Abajo, F. J. ; J. Am. Chem. Soc. 2009, 131, 4616.
[5] Shi, L.; Harris, J. T.; Fenollosa, R.; Rodriguez, I.; Lu, X.; Korgel, B. A.; Meseguer, F.; Nat Comm 2013, 4, 1904.
[6] Shi, L.; Tuzer, T. U.; Fenollosa, R.; Meseguer, F.; Advanced Materials 2012, 24, 5934-5938.
[7] Shabir, Q.; et al. ; Silicon 2011, 3, 173-176.

Plasmonics provides an unparalleled method for manipulating light beyond the diffraction limit, making it a promising technology for the development of ultra-small, ultra-fast, power-efficient optical devices. To date, the majority of plasmonic devices are in the solid state and have limited tunability or configurability. Moreover, individual solid-state plasmonic devices lack the ability to deliver multiple functionalities. By utilizing laser-induced surface bubbles on a metal film, we demonstrate, for the first time, a plasmonic lens in a microfluidic environment. Named the “plasmofluidic lens,” our device is dynamically tunable and reconfigurable. We have been able to record divergence, collimation, and focusing of surface plasmon polaritons using this device. The experimental results agree very well with numerical simulations. The plasmofluidic lens requires no sophisticated nanofabrication and utilizes only a single low-cost diode laser. Plasmonics exhibit unique properties of sub-diffraction-limited light confinement and strong local field enhancement, while optofluidics features unparalleled tunability due to the soft, fluidic nature of liquid. Our plasmofluidic lenses pave a new way for designing reconfigurable and multifunctional plasmonic devices in a microfluidic environment. The interplay between plasmonics and optofluidics, such as in the plasmofluidic lens demonstrated here, offers exciting opportunities to advance both fields. It will also likely stimulate new cross-disciplinary technologies for novel applications such as super-resolution imaging, ultra-sensitive biomedical detection, and optical information processing.

A novel method for the fabrication of plasmonic metal nanoparticles coatings on organic and inorganic functional substrates is demonstrated. A UV pulsed laser is utilized to fabricate Au and Ag nanoparticles on the surface of polymer materials such as PEDOT:PSS, PET and inorganic materials such as TiOx, ZnO, SiO2, graphene which are used as active substrates for plasmonic nano-photonic devices. Precise control of the particles&’ size and their density on various substrates is demonstrated. The advantages of this method are rapid processing times, compatibility with large scale production capable to meet industrial demands, fabrication of structures with tuneable plasmon resonance and ambient processing conditions. To study their real effect on optoelectronic devices, Au and Ag plasmonic metal nanoparticles were fabricated with this method and directly integrated without further processing into organic phototovoltaic (OPV) devices. The optical and electrical effects of these embedded particles on the power conversion efficiency of OPV are examined rigorously. Based on our findings, we propose design considerations for utilizing the entire AM1.5 spectrum using plasmonic structures, a step towards enhancing the efficiency of polymer solar cells using broad spectrum plasmonics.

In this work we apply a new replication technique called Degassing Assisted Patterning to the direct nano-embossing of TiO₂ sol-gel derived films [1]. This technique utilizes the mesoporosity of hard-PDMS/PDMS stamps [2] to improve the resolution and rapidity of the embossing process. The stamp, once degassed in a dessicator and brought back to atmospheric pressure, behaves like a sponge: after molding onto a spin-coated hybrid film, it creates a pressure difference that assists the capillary forces during the filling of the stamp protrusions by the hybrid film, quickly removing the solvents. The stamp is peeled off after stabilizing the hybrid layer at 110°C for 1 min, and the anatase crystalline phase of the replicated TiO₂ nanostructures is recovered by a post annealing at 450 °C. This technique does not use any embossing machine, is bubble defects-free and has no long-range deformation since no pressure is applied to the stamp.
Either dense or mesoporous hierarchical nanopatterns can be obtained by tuning the composition of the initial solution. In particular the formation of the mesoporosity is achieved by adding two kinds of amphiphilic block copolymers (Pluronic F127 and PB-PEO), which act as structuring agents. We demonstrate the replication of TiO₂ nanostructures as small as 20 nm over a large surface area of up to 10 cm². By direct embossing of TiO₂ onto a glass substrate, we fabricate 2D photonic crystal in the blue region of the visible spectrum over a surface area of 0,25 cm² with a quality factor of 150. The embossed structures are characterized by electronic microscopy while the mesoporosity is investigated by GI-SAXS and ellipsometry.
We also used the TiO₂ sol-gel as a nanoimprint lithography resist for the fabrication of plasmonic structures. Conventional polymeric resists used as sacrificial layers for metal lift-off have a relatively low surface tension. For this reason, metallic films grown on this sacrificial layer will have randomly oriented crystal grains with a typical diameter of about 30-40 nm. This surface roughness is transferred onto the nanostructures at the surface of the substrate, making it challenging to fabricate gaps or features smaller than 20 nm in a reproducible way. Unlike other resists, TiO₂ has a relatively higher surface energy, resulting in a smoother metallic film with lower lateral roughness. With this approach, we are able to fabricate gold antenna arrays with reproducible gaps as small as 10 nm.
[1] Galo J. A. A. Soler-Illia Nanoscale 4, 2549-2566 (2012)
[2] Cattoni et al., Microelectron. Eng., 87, 1015 (2010)