The relativistic spin-orbit coupling of electrons results in intrinsic spin precessions and therefore spin-polarization-dependent transverse currents, leading to the observation of spin Hall effect (SHE) and the emerging field of spintronics. The coupling between charge&’s spin degree of freedom and its orbital movement is essentially identical to the coupling of the transverse electric and magnetic components of a propagating electromagnetic filed. To conserve total angular momentum, an inhomogeneity of material&’s index of refraction can cause momentum transfer between the orbital and the spin angular momentum of light along its propagation trajectory, resulting in a transverse splitting in polarizations. Such a photonic spin Hall effect (PSHE) was recently proposed theoretically to describe the spin-orbit interaction, the geometric phase, and the precession of polarization in weakly inhomogeneous media as well as the interfaces between homogenous media.
The experimental observation of spin Hall effect of light, however, is fundamentally challenging since the amount of momentum that a photon carries is exceedingly small. The exploration of such a weak process relies on the accumulation of the effect through many multiple reflections or ultra-sensitive quantum weak measurements with pre- and post-selections of spin states. Here we demonstrate experimentally the strong interactions between the spin and the orbital momentum of light in a thin metasurface of a two-dimensional electromagnetic nano-structure with designed in-plane phase retardation over the wavelength scale. In such an optically thin material, the resonance-induced anomalous “skew-scattering” of light destroys the axial symmetry of the system and we observed PSHE even at the normal incidence. In stark contrast, for conventional interfaces between two homogeneous media, the spin-orbit coupling does not exist at the normal incidence.

Strong chirality has brought yet another new twist to light. Chiral metamaterials have attracted considerable attentions due to their exotic properties, such as giant optical activity, circular dichroism, and negative refraction. Three-dimensional helical structures have been investigated to realize broadband circular polarizer, but their fabrication and metallization become challenging. Meta-foils were introduced as an all-metal self-supported free-standing terahertz metamaterial. Here we show that the basic building blocks of meta-foils are chiral and demonstrate numerically and experimentally how to build, from these elements, a broadband chiral meta-foil in the terahertz region. A chiral meta-foil is an all-metal self-supported free-standing chiral metamaterial assembled by an array of simple twisted metastructures with the same handedness. It is a unique approach to create a flexible electromagnetic metamaterials that is free of dielectrics, such as embedding photopolymer and supporting substrates. Its properties are thus solely determined by the geometric structure and the metal properties. Furthermore, the chiral meta-foil can be tailor-made into virtually any shape, bent, and wrapped around objects to hide and shield them from electromagnetic radiation, thus becoming true metamaterials on curved surfaces. Because of the strong coupling among these connected twisted metastructures, three-layer chiral meta-foils fabricated by conventional lithography can effectively operate as three-dimensional helical structures or twisted metamaterials with broadband optical response. This proposed concept can be applied to the configuration of other three-dimensional chiral metamaterials and bulk metamaterials. More broadly, chiral meta-foils can also be manufactured by imprinting and hot embossing in any frequencies, enabling cost-effective mass manufacture. Thus, it can be extended to any frequencies, for example, the higher infrared and optical frequencies, to be directly and realistically integrated into current devices and systems.

Chirality in the nanoscale systems is a rapidly developing field. Hybrid bio-assemblies are composed of plasmonic nanoparticles, quantum dots, and biomolecules. Plasmon-plasmon and exciton-plasmon interactions in such assemblies are strong and responsible for new mechanisms of chirality and circular dichroism (CD). If a system includes chiral elements (chiral molecules or nanocrystals), the exciton-plasmon interaction is able to alter and strongly enhance the circular dichroism of chiral components [1-5]. In particular, the exciton-plasmon interaction in a plasmonic hot spot (a nanoparticle dimer) with a chiral bio-molecule can create very intensive plasmonic lines in the CD spectra [2,4,5]. Strong CD signals may also appear in purely plasmonic systems with a chiral geometry and a strong plasmon-plasmon interaction between nanocrystals [6,7]. One recent demonstration of plasmonic CD has used a metal-nanoparticle helix fabricated with a help of the DNA-origami approach [7]. Potential applications of hybrid nanostructures include chiral sensors and new plasmonic materials.
[1] A.O. Govorov, Z. Fan, P. Hernandez, J.M. Slocik, R.R. Naik, Nano Letters 10, 1374 (2010).
[2] H. Zhang, A.O. Govorov, Phys. Rev. B, 87, 075410 (2013).
[3] B. Maoz, Y. Chaikin, A. Tesler, E. Bar, Z. Fan, A. Govorov, G. Markovich, Nano Lett. 13, 1203minus;1209 (2013).
[4] M. Layani, A. Ben Moshe, M. Varenik, O. Regev, H. Zhang, A. Govorov, G. Markovich, J. Phys. Chem. C (2013); DOI: 10.1021/jp400993j
[5] V.A. Gérard, Y. K. Gun'ko, E.Defrancq, A. O. Govorov, Chem. Commun. 47, 7383-7385 (2011).
[6] Z. Fan, A.O. Govorov, Nano Letters 10, 2580 (2010).
[7] A. Kuzyk, R. Schreiber, Z. Fan, G. Pardatscher, E.-M. Roller, A. Högele, F.C. Simmel, A. O. Govorov, T. Liedl, Nature, 483, 311 (2012).

Metal nanoparticles assemblies and polymer/nanoparticle hybrids are promising building blocks for photonic applications, i.e., metamaterials. The efficient and rational design of such building blocks is an important prerequisite for the successful fabrication of plasmonic, functional materials as well as their integration in 2- and 3-dimensional particle frameworks. Different design strategies can be utilized to obtain well-defined nanoparticle assemblies with controlled structural parameters. Two efficient routes will be introduced and will be finally combined to obtain tailor-made plasmonic structural characteristics. The tunable bottom-up synthesis of poly(ethylene imine)-gold-nanoparticle clusters by the utilization of poly(ethylene imine) (PEI) as a reducing agent for tetrachloroauric acid will be introduced to obtain building blocks on the meta-atom level. Depending on the reaction conditions different structures ranging from single gold nanoparticles to gold-PEI clusters with a diameter of 200 nm can be obtained. By control of the reaction parameters as well as by application of seed-growth techniques, important properties of the PEI-gold-nanoparticle assemblies can be tuned to manipulate the plasmonic properties. These clusters are theoretically and experimentally investigated and show promising properties for plasmonic applications. In a second step the guided self-assembly of these building blocks is addressed by the utilization of a top-down fabrication scheme which provides chemically active templates which can locally bind individual building blocks on predefined positions on a substrate. This approach utilizes the chemical modification of a self-assembled monolayer by means of an electrochemical scanning force lithography approach, which provides surface templates with a lateral resolution down to a few nm. Chemical as well as electro-static, hydrophilic and hydrogen bonding interactions between the template and the meta-atoms can be utilized to attach nanomaterials on the inscribed patterns and, thus, in predefined positions on a substrate. The local modification of a self-assembled monolayer of n-octadecyltrichlorosilane (OTS) is achieved by the application of a negative tip bias voltage, which results in the formation of polar acid groups that can be utilized to bind even individual meta-atoms. Based on this strategy another degree of organization can be introduced in the fabrication of plasmonic nanostructures to implement the controlled fabrication of metamaterials in a combined “top-up” approach.

Interference between dark and bright modes in plasmonic metamolecules can lead to interesting behavior such as Fano resonances and plasmonically induced transparency (PIT), which can be used to realize ultra-sensitive optical sensing devices. A key structure which strongly exhibits this behavior is the “dolmen” or “pi”, consisting of a single bar with a bright dipolar resonance that is capacitively coupled to a dimer which supports a dark quadrupolar mode. This coupling has first been studied with far-field optical microscopy where it was shown that the interference between the two modes leads to a transparency window in transmission for certain wavelengths. However, the near-field interaction of these dolmens, which forms the basis of their special optical behavior, has not been measured so far due to the subwavelength dimensions.
Here we perform electron energy loss spectroscopy (EELS) with spatial resolution of ~1 nm in combination with cathodoluminescence (CL) imaging spectroscopy to elucidate the interplay between dark and radiative modes in individual Au dolmens. While CL is sensitive to the radiative modes alone, EELS probes both dark and radiative modes. In both cases the structures are driven with a tightly focused electron beam (300 keV electron energy for EELS, 30 keV for CL), which acts as a local broadband source of radiation. Combining the two techniques provides deep-subwavelength spectral information on the fundamental scattering properties of the dolmens and gives full information about the radiative and non-radiative part of the local density of optical states (LDOS).
30 nm thick gold dolmens were made on a 15 nm thick silicon nitride membrane using e-beam lithography, thermal evaporation, and liftoff. The size (120 - 220 nm long bars) and spacing between the dimer and single bar (30 - 60 nm) is varied to control the coupling between the elements. Compared to an isolated reference bar, clear additional spectral features are observed in the EELS spectra when the dipole mode of the single bar is driven with the e-beam, which we attribute to coupling with the dimer. The high spatial resolution of EELS allows direct visualization of the coupled modal field, directly probing the positions where efficient coupling between the single bar and the dimer occurs. For larger dolmens strong transverse dipole and quadrupolar resonances are observed in both the spectra and the modal field maps and play an important role in the hybrid dolmen modes. By comparing EELS and CL data on the same sample, the bright and dark modes can be separately identified.
The data are corroborated by FDTD simulations using the exact experimental geometry retrieved from transmission electron microscopy images. We simulate the dolmen response both for point dipole and plane wave excitation in order to relate the EELS and CL results to optical scattering spectra of the dolmen metamolecules.

We have developed active metamaterials capable of quickly tuning their electrical and magnetic responses over a wide frequency range. These metamaterials are based on superconducting elements to form extremely low insertion loss, physically and electrically small, highly tunable structures for the next generation RF electronics. The meta-atoms are RF superconducting quantum interference devices (SQUIDs) that incorporate the Josephson effect. RF SQUIDs are essentially a quantum version of the split-ring resonator in which the inductance now includes a contribution from the Josephson inductance of the junction. This inductance is strongly tunable with DC and RF magnetic fields and currents. We will present experimental and numerical results from the first generation samples, and discuss the degree of tunability of these new meta-atoms. This work is supported by the NSF-GOALI program through grant # ECCS-1158644, and CNAM.

The conventional approach of transformation optics is mainly enabled by metamaterials with varying indices and anisotropies in both the dielectric permittivity and magnetic permeability tensors. Being inspired by the parallel development for homogeneous metamaterials that the artificial atoms with lower symmetries or with magneto-electric coupling can provide exotic optical properties such as huge optical activity, electromagnetic-induced transparency and asymmetric transmission, etc., the ability of transformation optics is far from being fully utilized by only covering a limited palette of optical responses. Some of the recent theoretical approaches are already going into this direction. Here, we further investigate two dimensional transformation optical devices enabled by generally bianisotropic metamaterials with field transformation. In particular, we explore the additional degrees of freedom in controlling polarization and impedance, comparing to the conventional approach. We will discuss illustrative examples in designing transformation optical devices with the generally bianisotropic metamaterials. For example, we can design a device to achieve complete cross-polarization conversion. We can also obtain additional impedance control when a transformation optical device is matched to an object.

Most metamaterials (MMs) to date consist of small metallic structures that are effectively replacing atoms as the basic unit of interaction with EM waves. Unfortunately, the use of metals results in substantial dissipation due to ohmic heating at optical frequencies. We combine experimental methods with computer simulations to develop a set of novel MMs exhibiting resonances that store energy mainly in a dielectric material. This avoids resonant loss in the metals and we indeed demonstrate electric and magnetic metamaterial resonators with very large quality factors. The resulting structures can be straightforwardly scaled at optical frequencies to create low-loss MMs with a wide range of properties.
Our study shows that bound states in dielectric structures can be used as constituents of resonant metamaterials and allow for much larger quality factors than metallic meta-atoms. Here we have used a bound state on a dielectric slab, but surface states on an array of rods or a 2D/3D photonic crystal could equally be used. The dielectric meta-atoms can also be straightforwardly scaled to terahertz and optical frequencies, where high-performance dielectrics such as fused silica can be taken advantage of and even higher quality factors can be expected. Also, in contrast to Mie resonant dielectric particles, we do not need a particularly large permittivity for the dielectric slab. The main limitation of the metamaterial demonstrated here is that it is highly anisotropic and only works for normal incidence, since the slab can be made thin only in the propagation direction. This is acceptable, though, in many metamaterial applications involving a single normally incident beam, like delay lines, polarization rotation or optical isolation. Here we have demonstrated a slow light metamaterial and a negative-permeability metamaterial, but one could likewise design, e.g., a metamaterial with strong dichroism or optical activity by using a chiral scatterer. In this way, the dielectric meta-atom offers a universal design framework in which we can modify the response of the metamaterial by a well-chosen no resonant secondary scatterer, while at the same time the response is dramatically enhanced by the strong, low-loss resonance of the dielectric meta-atom and the large energy storage in the dielectric material.

Light-matter interactions are inherently slow as the wavelengths of optical and electronic states generally differ greatly. Surface plasmon polaritons have generated significant interest because their spatial scale is decoupled from the vacuum wavelength, promising accelerated light-matter interactions; especially spontaneous emission, Raman scattering and non-linear frequency mixing. However, the possibility of accelerated dynamics in recently demonstrated surface plasmon lasers remains unclear due to the inherent interplay of both spontaneous and stimulated emission. In this talk, we report the observation of picosecond scale pulsing from hybrid plasmonic zinc oxide (ZnO) nanowire lasers. Operating at room temperature, ZnO emission and gain occurs near the SPP frequency in such silver-based plasmonic lasers, leading to accelerated spontaneous emission and gain dynamics compared to conventional ZnO nanowire lasers. To quantify these characteristics, we use a novel double optical pump technique that exploits a laser&’s inherent non-linearity to expose its temporal dynamics. With the capability to combine surface plasmon localization with ultrafast amplification we can potentially generate extremely intense optical fields with applications in sensing, non-linear optical switching, as well as in the physics of strong field phenomena. While there is a great deal of research to do on plasmonic laser systems, this talk highlights the feasibility of nano-scale light sources and the potential to do laser science at the nanoscale.

Many conventional optoelectronic devices consist of thin, stacked films of metals and semiconductors. In this presentation, I will demonstrate how one can improve the performance of such devices by nano-patterning the constituent layers at length scales well below the wavelength of light. The resulting metafilms and metasurfaces offer opportunities to dramatically modify the optical transmission, absorption, and reflection properties of devices. To illustrate these points, I will show how nanopatterned metals and semiconductor layers can be used to enhance the performance of photoelectrodes for water splitting, photodetectors, and solar cells.

Thermophotovoltaic (TPV) devices, or photovoltaic cells that are made to convert infrared wavelengths into electricity, can enhance the efficiency of existing energy generation infrastructure by reclaiming the heat lost during production processes. In order to maximize the efficiency of these devices, the conversion efficiency of the TPV system needs to be optimized. Most TPV systems consist of three discrete stages: a thermal emitter, a filter, and a TPV diode. This research focused on the emitter as an avenue to increase TPV efficiency, by replacing the wide band emitters presently in use with a selective thermal emitter. The selective emitter would absorb wide-band radiation from a heat source and convert it into narrow bandwidth peaks of radiation tailored to the rest of the system. This would dramatically reduce energy loss due to reflection, diode heating, and carrier relaxation. Using metamaterial perfect absorbers, we have created selective emitters that tailor the incident light spectrum to the band gap of GaSb.
Most research into metamaterials has utilized gold as the conducting metal. With a melting point of 1064 °C, gold patterns will completely degrade at, if not before, the operating temperature of most TPV cells. The most common TPV cells in use today are bulk GaSb diodes. The band gap of this material is 0.7eV, which corresponds to a light wavelength of 1.7mu;m. According to Wien&’s Law, to achieve a blackbody radiation curve with this wavelength at the peak intensity the radiating body would have to be at a temperature of 1400 °C, well above the melting point of gold. Exploring alternate materials and their viability as emitters would lead to great advances in current achievable efficiencies.
Sample emitters were designed using simulations from CST Microwave Studio and created by the deposition of a metallic grounding plane onto a silicon or sapphire substrate. A dielectric film was grown on top, followed by the application of a nanostructured pattern via electron beam lithography in order to create the metallic patterned layer. We created samples using platinum or molybdenum as the conducting metal and alumina as the dielectric. A control sample consisting of only the grounding plane and dielectric was made for each metal to determine the transmission effects due to the metamaterial. Testing of these samples include ellipsometer measurements and emission testing using a high temperature blackbody source. Preliminary results have shown that simulated data matches well for both emission and absorption data from actual samples fabricated using platinum as the conducting metal in a repeating circular metamaterial.

Development of efficient light trapping techniques is essential for a variety of applications, including photovoltaic and thermophotovoltaic energy conversion, optical sensing and spectroscopy. We use a recently introduced theoretical concept of light trapping in nanoscale volumes via pinning optical vortices to photonic and plasmonic nanostructures [Boriskina & Reinhard, Nanoscale 4(1), 76, 2012] to design new light harvesting platforms. We show that the ability to control, both spectrally and spatially, the circulating optical powerflow through the optimally-designed nanostructures enables efficient light trapping and routing with low parasitic dissipative losses (e.g. losses due to absorption in metals rather than in semiconductor materials).
We reveal that formation of optical vortices is the underlying mechanism behind many efficient light-trapping schemes including whispering gallery modes excitation in microresonators, light nanofocusing in plasmonic nanolenses and emergence of Fano resonances in nanostructures. Our calculations also provide insight into nucleation, spatial positioning and annihilation of optical vortices in multilayered metal-dielectric metamaterials that underline the transition of the metamaterial from the conventional elliptical to the hyperbolic dispersion regime. Finally, we use the developed understanding of the internal structure of the optical powerflow in nanostructured devices and metamaterials to optimize in- and out-coupling of light generated by external and embedded light sources.
This work has been funded by the ‘Solid State Solar-Thermal Energy Conversion Center (S3TEC)&’, funded by the US Department of Energy, Office of Science, and Office of Basic Energy, Award No. DE-SC0001299/DE-FG02-09ER46577 and by DOE SunShot grant No. 6924527. A.M. acknowledges the support via the MIT MPC-CMSE 2013 Summer Scholars Program.

Graphene and conducting oxides have intriguing prospects as a tunable metamaterials. Although negative permittivity materials have been characterized in the 2minus; 6 THz range, there is also great interest in understanding the resonant plasmonic and polaritonic properties of graphene across the infrared spectrum, especially at energies exceeding the graphene optical phonon energy. We have used infrared spectroscopy to observe the modes of tunable plasmonic graphene nanoresonator arrays with features as small as 15 nm. We have mapped the wavevector-dependent dispersion relations for graphene plasmons at mid-infrared energies from measurements of resonant frequency changes with nanoresonator width. By tuning resonator width and charge density, we have probed graphene plasmons with wavelengths 1/100th of the free space wavelength, with resonances as high as 310 meV (2500 cm-1), yielding the smallest attributable mode volumes reported to date for a resonator. In this talk, we also demonstrate how the resonant absorption of graphene ‘perfect absorber&’ structures is enhanced by controlling the thickness and permittivity of the supporting substrate; recent experiments indicate that 20% absorption is achieved by carefully selecting the properties of underlying boron nitride and silicon nitride substrates. We also describe the enhancement of coherent quantum electrodynamic phenomena in epsilon-near-zero media, such as those formed from tunable conducting oxide metallodielectric multilayers and optical waveguides operating near cutoff. An enhanced local density of states in these media leads to an increased Purcell factor and an anomalously large Lamb shift. We also show how an epsilon-near-zero medium can expand the spatial and length scales over which coherent emitter-emitter phenomena (e.g., concurrence, superradiance) can be observed.

Plasmon nanolasers attract much attention in recently years due to its potential applications such as coherent light sources and amplifiers in subwavelength scale [1]. The conventional end-facet Fabry-Pérot (FP) cavity for plasmon laser has large radiation loss and thus can only provide weak feedback. The improvement of feedback can be achieved by increasing the length of the FP cavity, using distributed Bragg reflectors [2] or total internal reflection [3], while at the cost of size increment. We studied a plasmon-induced transparency (PIT) nanocavity for achieving nanoscopic coherent light source.[4, 5] The compact cavity is constructed by a pair of detuned nano-stubs incorporated with four-level gain medium. The PIT response enables the reduction of the coupling loss from cavity to waveguide while keeping the cavity size unchanged, different from the former proposed plasmon laser cavities in which the radiation loss decreases at the cost of size increment. In order to study the lasing behavior of surface plasmon wave in the PIT cavity, the self-consistent finite element method [6] is extended to model the interactions between gain and propagating surface plasmons. The dynamics of the whole lasing process is demonstrated, and the linear output-input relation is obtained for the single mode plasmon lasing. We find that smaller stub-pair detuning provides stronger feedback inside the cavity. Consequently, the lasing threshold of pumping rate decreases quadratically with the decreasing of detuning. In addition, the output-input extraction efficiency can be improved when the detuning is not so small. One of the advantages for the proposal is that the lasing output power from the cavity can directly couple towards the metal-dielectric-metal waveguide platform, unlike the end-facet FP cavity which wastes large part of power into free space. It is promising to realize on-chip molecular scale coherent light sources and repeaters in the integrated plasmonic circuits.
Reference
1.R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, "Plasmon lasers at deep subwavelength scale," Nature, 461, 629, (2009).
2.M. J. H. Marell, B. Smalbrugge, E. J. Geluk, P. J. van Veldhoven, B. Barcones, B. Koopmans, R. Nötzel, M. K. Smit, and M. T. Hill, "Plasmonic distributed feedback lasers at telecommunications wavelengths," Opt. Express, 19, 15109, (2011).
3.R.-M. Ma, R. F. Oulton, V. J. Sorger, G. Bartal, and X. Zhang, "Room-temperature sub-diffraction-limited plasmon laser by total internal reflection," Nat Mater, 10, 110, (2011).
4.Zi-Lan Deng, Jian-Wen Dong, and Jensen Li, "Power transmission and group delay in gain-assisted plasmon-induced transparency," AIP Advances, 3, 032138, (2013).
5.Zi-Lan Deng and Jian-Wen Dong, "Lasing in Plasmon-Induced Transparency Nanocavity," submitted to Opt. Express
6.C. Fietz, and C. M. Soukoulis, "Finite element simulation of microphotonic lasing system," Opt. Express, 20, 11548, (2012).

As a product of the interaction between photons and free electrons at the metal-dielectric interface, surface plasmons (SPs) can miniaturize the system significantly, and are promising in manipulating photons at nanoscale. Indeed plasmonic structures have shown the potential applications, such as nano waveguides and circuits, nanoantennas, routers and logic gates, and so on. However, the intrinsic damping mechanism limits the scope of SPs for the practical applications. For example, it remains challenges to achieve SPs waveguide with low loss. Besides, due to the high loss at the metal-semiconductor interface, it is also difficult to make the plasmonic devices compatible with current silicon technology. Therefore, trying to find loss compensation and amplification approach in the SPs devices is one of the most important tasks in this field. Here, we demonstrate 1D photonic crystal (PC) structures embedded with silver nanowires based on ebeam lithography technique. The 1D photonic crystal (PC) structures are constructed by gain material on the silicon wafer. In the structure, the propagation frequencies of the SPs can be controlled by the photonic band gap of the 1D PC, and simultaneously the signal intensities can be amplified by the gain material. Therefore, this configuration shows both high transmission efficiency and extremely good confinement, which can play the role of controllable deep-subwavelength SPs amplifier. We believe this device may have potential applications in designing silicon-based nanoscale photonic integrated circuits and chips.

Recent progress in the development of optical metamaterials allows unprecedented control over the flow of light at both the nano- and macroscopic scales. Metamaterials (MMs) are rationally designed artificial materials with versatile properties that can be tailored to fit almost any practical need and thus go well beyond what can be obtained with “natural” materials. We review the exciting field of optical metamaterials and discuss the recent progress in developing tunable and active MMs, nanolasers, artificial optical magnetism, semiconductor-based and loss-free negative-index MMs, and a new means for engineering the photonic density of states with MMs. A powerful paradigm of shaping space for light with transformation optics, which can enable a family of new applications ranging from a flat magnifying hyperlens to an invisibility cloak, will be also discussed. Finally, we review a new approach for controlling light by using meta-surfaces. Similar to the surface science that in the past revolutionized the physics and opened up up a family of new phenomena and applications unattainable with 3D systems, we envision that metasurfaces can make a difference for the fields of metamaterials and transformation optics as well as for the science of light in general.

For optical metamaterials and for individual optical antennas, knowledge of the scattering cross-section spectra and the absorption cross-section spectra of individual metallic nano-objects is highly desirable. However, until recently, these two quantities were usually only available experimentally in uncalibrated form. In 2012, we have introduced a measurement approach based on a spatial modulation technique combined with a common-path interferometer. We briefly review this approach and its usage for individual split-ring resonators and the transition towards wire antennas. Here, we emphasize more recent experiments in which we have also studied rectangular and square-shaped antennas as well as bow-tie and slot antennas. The extinction cross-section of the square-shaped antenna aproaches hat of an ideal point dipole. The ratio of scattering cross-section to geometrical area is largest for the thin wire antenna. Finally, we will also compare these quantitative optical spectra with electron-energy-loss spetroscopy (EELS) spectra taken on the identical individual antennas.

In optics we generally describe a material by its electrical permittivity. Sometimes the permittivity is dispersive and depends strongly on frequency, as is the case for metals, but usually it is assumed to be independent of wave vector. This assumption works well on the scale of the wavelength of light, but current experiments on nanostructured materials challenge this assumption. Theorists are still debating the correct model permittivities observed at the sub nanoscale. I shall discuss the theories, how they can be implemented to calculate optical properties, and how they also have consequences for the Van der Waals interactions and heat transfer between nanoparticles.

The versatile methodology of transformation optics (TO) allows us to control electromagnetic waves in almost arbitrary manner, leading to a number of exotic optical designs for cloaking, imaging, waveguiding, and light harvesting purposes. Due to the form invariance of Maxwell&’s equations after coordinate transformation, the corresponding material parameters can be calculated to achieve the desired light trajectories. However, as there are no specific criteria to choose transformation functions and control the range of parameters, in many cases the resulting materials are highly anisotropic and suffer singularities. Quasi-conformal mapping solves this problem to a large extent. However, the design process is no longer analytical but numerical, and the disregard of the minimized anisotropy sometimes may cause undesirable problems.
Here we introduce a new strategy to design isotropic TO devices using analogy method. As light trajectories and wave fronts construct a set of orthogonal grids in isotropic media, analogous problems satisfying Laplace equations are designed, where flow lines and equi-potential lines are perpendicular to each other. When we design TO devices for different functionalities, different analogous systems can be chosen. For instance, in cloaking, water flow around an isolated object can be used for reference on how light trajectories should act to avoid the cloaked region; and in bending, the equi-potential lines in a cylindrical capacitor tell us which route power flow should follow. Our method manifests an intriguing analogue between different systems across multiple disciplines, including optics, fluid dynamics and electrostatics, and will significantly facilitate the TO design from a new perspective.
Two novel devices are presented as proof-of-concept examples. The first one is an analytical carpet cloak. The analogy we use comes from an ideal flow passing over a cylinder. In the prototype, light propagates along the trajectories of flow lines, by which they smoothly avoid touching the cloaked object. To remove singularities in the refractive index profile, subsequent processes are performed based also on analogy considerations. Either changing the geometry or adding an additional flow will modify the profile of the flow pattern and therefore lead to a fully analytical design of carpet cloak. Although some approximation is made to truncate the boundary contour, the cloak is strictly isotropic, non-singular and conceals a large area comparing to its size. The second example is a directional lens antenna, whose emission mimics the electric field lines from a point charge in front of a flat conducting sheet. The conductor plays the role of the aperture of antenna, giving rise to an emission of high directivity. Both devices can be readily realized with the current techniques of metamaterials. Using the novel analogy method introduced here, we can design more complex devices with achievable material parameters.

Metamaterials composed of a double-periodic metal-dielectric multilayer stack can show a negative refractive index in the UV spectral range for TM polarized light, as we proposed theoretically [1]. Two light propagation regimes can be distinguished for this geometry: parallel and perpendicular to the layer stack. For light propagation parallel to the waveguides, light couples to an anti-symmetric, backward surface plasmon polariton waveguide mode. Negative index behavior, i.e. an anti-parallel phase and energy velocity, is maintained over a broad angular range around the planar direction.
For light propagation predominantly normal to the waveguides, the light experiences a periodically varying permittivity. Therefore, the eigenmodes in this situation are described by Floquet-Bloch waves. The fundamental harmonic of this Bloch wave is characterized by a negative index [1]. However, not all higher harmonics share this “left-handed” behavior. In order to determine the effective response of the metamaterial for light propagation in this direction, it is essential to quantify the relative contribution of each harmonic separately.
Using a plane wave expansion technique we calculate the Fourier coefficients of the different harmonics directly. From these Fourier coefficients, we find that not the fundamental, but the first-order harmonic, characterized by a positive index, is excited with the largest amplitude in the multilayer system. This makes the assignment of a unique refractive index to this multilayered system an ambiguous task. To study the relative coupling efficiencies of the harmonics we perform finite-difference time-domain simulations for light incident on a metamaterial prism geometry. The Bloch wave inside the metamaterial is refracted over multiple angular peaks, each corresponding to a separate harmonic. We find that rather than the fundamental harmonic, indeed the first-order harmonic dominates the transmitted wave.
Refraction measurements were performed on a metamaterial microprism. The prism was sculpted from a multilayer slab using 30 keV Ga focused ion beam milling at an oblique angle of incidence. The slab consisted out of 2.5 unit cells (Ag/TiO2/Ag/TiO2: 50/44/32/44 nm, optimized for lambda;0=410 nm), which were deposited using physical vapor deposition. The 6x6 mu;m prism had a sloping angle of 4°. The far-field refraction angles were measured in a Fourier microscope. The experimental refraction data corroborate that most of the transmitted light originates from the first-order positive index harmonic, rather than the fundamental negative index harmonic of the Bloch wave. Fundamental understanding of the contributions and control of higher-order harmonics as presented here is essential to engineer efficient “left-handed” negative index response for applications in flat lenses and other optical components.
[1] E. Verhagen, R. de Waele, L. Kuipers and A. Polman, Phys. Rev. Lett. 105, 223901 (2010)

Negative index metamaterials (NIMS) have been explored as specialized structures built over small areas (100&’s of µm^2) using techniques, such as focused ion beam lithography, that are useful primarily for research purposes. This talk reports some recent advances in materials and methods that offer high speed routes to similar types of systems, in ways that can scale to nearly arbitrarily large areas, in a manufacturing mode. We describe, in particular, 3D-NIMs formed by transfer printing that incorporate 11-layers and have sub-micron unit cell dimensions, over areas > 75 cm^2, corresponding to >10^5x10^5 unit cells, all with excellent uniformity and minimal defects. Detailed studies of the physical deposition processes used to form these stacks reveal important considerations in materials selection that allow extension to NIMS with operation in the visible regime.

Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. The recently introduced generalized laws of refraction and reflection have opened up a new exciting opportunity to develop ultrathin, lightweight flat optics for next-generation photonics applications. Unfortunately, experimental demonstrations of this principle, which have typically relied on cross-polarized scattering by a layer of resonators, achieved only very limited efficiency with most of the incident power remaining in the ordinarily refracted/reflected beams. In this work [1], we first demonstrated ultrathin, broadband, and highly efficient (reaching 88%) metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. Building on these results, we then created metamaterial structures exhibiting near-perfect anomalous refraction. These structures redirect up to 61% of the incident power into the anomalous beam and practically eliminate the ordinary component. This work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities with practically useful efficiencies for applications in the technologically difficult terahertz-frequency regime.
[1] Nathaniel K. Grady et al., “Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction,” Science 340, 1304 (2013).

Insulator-to-metal transition usually occurs in materials such as semiconductors and transition metal oxides. Metamaterials provide an opportunity to conveniently realize such a phenomenon. In this work, we demonstrate insulator-to-metal transition in hybrid active planar metamaterials, and further employ this concept to accomplish broadband terahertz modulation with high modulation depth and possibly high speed. The hybrid metamaterials consist of a square or rectangular metal grid integrated with semiconductors (e.g. silicon or gallium arsenide) at the gap. The grid exhibits a strong resonance, and below the resonance frequency it behaves as an insulator capable of high transmission. The grid transits to a metal wire grating when the integrated semiconducting islands become conducting, and thereby is opaque below its plasma frequency. This transition can be realized through photoexcitation, in which the required fluence is dramatically reduced as compared to a bare semiconducting film. More importantly, it can be also conveniently accomplished by carrier depletion/injection using a voltage bias. This demonstration overcomes the narrowband performance of prior metamaterial-based modulators and providing new opportunities for a host of applications.

The metamaterials field is at the forefront of the research environment due to its capability of finding new and exciting applications in various wavelength regimes. Here we present the possibility of using the metamaterials concept in order to devise, fabricate and characterize a high-transmittivity membrane that acts as a half wavelength plate for the terahertz (THz) regime.
The THz research is a new and emerging field. Its appeal is given by the possibility of measuring, using radiation with frequencies between 0.2 and 10THz, different characteristics of various chemicals. Such analysis is more and more integrated into everyday life, also because the THz radiation is non-ionizing and thus harmless to living organisms. They are currently used in full-body scans in airports, for example.
With all the advances in this field, one of the main bottlenecks is the lack of high-quality devices that can control the beam characteristics. Due to the generation method of the THz beams, they are linearly polarized. Allowing for a controlled rotation in the polarization angle without manipulating the emitter is not, to our knowledge, possible.
In this work we show a complete modeling-fabrication-measurement cycle for obtaining large dimensions, high transmittvity half wavelength plates for the THz regime. The polarization state of a beam passing through this device can be controlled such that a linearly polarized beam rotates its polarization plane with the desired value. This effect is due to inducing a π phase difference between a beam travelling along the device&’s fast axis with respect to one travelling along its slow axis. This way, at the exit from the device, the two beams will generate a beam polarized along a desired plane. The special case where the polarization plane flips with 90 degrees is the one demonstrated in this article. To be noted that the same device can be used to obtain a beam whose polarization plane can be freely chosen by only changing the angle between its axis and the incident beam polarization.
The device consists of five layers, out of which three are patterned metallic ones while the other two act as dielectric spacers. It behaves as expected, thus obtaining π phase shift in the desired frequency range and having the same transmission amplitude for the two polarizations. The difference in transmittivity between the theoretically predicted value of 95% and the experimentally achieved one of 60% is most probably due to higher than assumed losses in the dielectric spacing. Modeling of systems with higher losses indicating the correctness of our assumption as well as discussion about new strategies to be used in order to reduce these losses will be presented.

Optical antennas transform light from freely propagating waves into highly localized excitations that interact strongly with matter. While their radio frequency counterparts are straightforward to characterize, optical antennas are nanoscopic and high frequency, rendering amplitude and phase measurements challenging and leaving a wealth of information hidden. Here we present the capabilities of a novel compact spectral interferometric microscopy (SIM) technique, which exposes the amplitude and phase response of individual optical antennas across an octave of the visible to near-infrared spectrum. We use the technique to measure the transmission amplitude and phase of a gold ring-disk dimer exhibiting Fano interference. Utilizing the microscopy capability, SIM can identify those parts of the dimer that support a bright dipolar mode, even though the dimer is smaller than the wavelength. Meanwhile, the measured phase allows us to quantitatively estimate the proportions of scattering and absorption, which confirms the roles of the various dark and bright modes of the dimer&’s constituent nanoparticles. In particular, we are able to confirm that Fano interference only cancels a bright mode&’s scattering leaving the residual extinction due to absorption. SIM enables real time phase and amplitude studies of isolated quantum and classical antennas with potential applications across the physical and life sciences.

The study of wave phenomena in granular media is a rich and rapidly developing field of research. Close-packed, ordered arrays of elastic particles, often referred as granular crystals can interact via Hertzian contact and support a wide range of unconventional linear or nonlinear phenomena. Acoustic studies of granular media typically involve macroscopic particles whereas contact-based vibrations of microparticles remain largely unexplored. The adhesion which can be neglected on millimeter scale is significant on micron scales and therefore microparticles are expected to yield qualitatively different dynamics. We employ the convective assembly method to fabricate the centimeter-sized, two-dimensional granular crystal consisting of 1mu;m silica spheres adhered to the substrate. Furthermore, we fabricate the microsphere waveguide structure by template-assisted-self-assembly. Laser-induced transient grating technique is used to generate and detect surface acoustic waves in microsphere array samples. We demonstrate, both experimentally and by theoretical analysis, that the Rayleigh wave in the substrate interacts with the contact resonance of microspheres leading to hybridization and avoided crossing. Microsphere arrays can be modeled within the effective medium approach as locally resonant metamaterials for surface acoustic waves.

In this work, we investigate the dispersion of a sub-wavelength metal/dielectric multilayer based on the Bloch theory and the transfer matrix method. Such an artificially constructed sub-wavelength medium exhibits nonconventional hyperbolic dispersion, and is thus called hyperbolic metamaterials (HMMs). Next, we further manifest the evolution of the optical phase of the HMMs- in the low frequency region, the iso-frequency surface reveals a one-sheeted hyperboloid in the small momentum limit but gradually turns into a cylinder near the zone boundary; yet, as the frequency increasing, the iso-frequency surface expands and a closed ellipsoid phase emerges in the zero momentum regions, leading to a two-phase coexistence (ellipsoid phase and one-sheeted hyperboloid phase). As frequency further increases, on the one hand, the ellipsoid phase expands; on the other hand, the hyperboloid phase expands but then shrinks after and eventually merges with the ellipsoid phase and then splits into two lobes. In the end, we also calculate the photonic density of states (PDOS) of the HMMs to characterize their optical phases and phase transitions. The calculation results indicate that the evolution of the optical phase transition and the PDOS strongly depend on the filling fraction of the metal in the HMMs, paving a route towards manipulating the light-matter interaction.

In recent years, there has been increased interest to the occurrences of conical points in the dispersion of classical waves at a non-zero frequency. Such “Dirac cones” resembling the dispersion of electronic excitations in graphene are typically found at the edges of the Brillouin zone of two-dimensional photonic and phononic crystals. More recently, the existence of Dirac cones at the center of the Brillouin zone has been demonstrated [1,2]; furthermore, it was shown that at frequencies near the conical point the material may be modeled as a zero index effective medium. In this report, we will discuss the occurrences of conical points at zero wavevector in the dispersion of Lamb waves in homogeneous elastic plates without periodic structures. Dirac cone dispersion results from accidental degeneracies between longitudinal and transverse thickness resonances [3]. The dispersion can be fine-tuned to yield a Dirac cone by coating a plate with a layer of different material and varying the thickness of the latter. Similarities and differences with respect to Dirac cone dispersion in phononic crystals and possible observable effects will be discussed.
[1] X. Huang, Y. Lai, Z. H. Hang, H. Zheng and C. T. Chan, Nature Materials 10, 582-586 (2011).
[2] F. Liu, X. Huang, and C. T. Chan, Appl. Phys. Lett. 100, 071911 (2012).
[3] R. D. Mindlin, An Introduction to the Mathematical Theory of Vibrations
of Elastic Plates (World Scientific, Singapore, 2006).

The location and identity of molecules on the surface of plasmonic nanostructures is critical for defining the interaction between the two materials, which is important for any application utilizing hybrid organic-plasmonic systems. Unfortunately, the size of both organic molecules and many plasmonic nanostructures of interest are well below the wavelength of light, which means that the coupled molecule-plasmon system will appear as a diffraction-limited spot when imaged by far-field optical microscopy. As a result, probing where molecules are bound to the nanostructure remains a distinct experimental challenge. This talk will describe how super-resolution imaging can resolve the positions of individual fluorescent ligands bound to the surface of plasmonic nanostructures, allowing both the shape of the nanostructure to be mapped out and the ligand locations to be determined. While this talk will focus primarily on biosensing applications, the approach can be broadly generalized to any fluorescently-labeled hybrid organic-plasmonic material.

It is well known that a practical resolution limit of any conventional optical systems originates from the fact that electromagnetic waves diffract upon propagation. As a result, the visualization of features smaller than the wavelength of the illuminating light requires the development of new imaging techniques. Such techniques are of paramount importance for, for example, sub-wavelength imaging of normally invisible subcellular structures in biological studies and for sub-wavelength photolithography.
Metamaterials-based optics was predicted to solve the problem of diffraction-limited resolution of conventional optical components. In particular, one of the first key applications of metamaterials was predicted to be a so-called hyperlens. Hyperlenses overcome the diffraction limit by transforming evanescent waves responsible for imaging sub-wavelength features of the object into propagating waves. Once converted, those formerly decaying (evanescent) components commonly lost in conventional optical imaging, now can be collected and transmitted using standard optical components. However, there are several challenges that need to be addressed to adapt this metamaterial-based technology for practical applications. For instance, to date optical metamaterials have primarily been demonstrated in the form of thin films with thicknesses on the order of, or less than, the optical wavelength on an appropriate substrate. In order to use them in applications such as remote sensing and imaging devices the light in- and out-coupling issue must be solved. To address this challenge, we are developing fiber-coupled metamaterials to realize fiber-coupled hyperlenses for single cell endoscopy and other forms of sub-wavelength bio-imaging and sensing.
Fiber optics is a mature technology that enables long-distance, low-loss light delivery. Moreover, fiber designs can be optimized for efficient excitation light delivery and nonlinear optical signals collection. In particular, powerful femtosecond pulses that are needed to generate nonlinear optical responses (e.g., second harmonic signal or two-photon absorption) in the sample experience nonlinear and dispersive propagation penalties that stretch the pulses in time and hence reduce their peak intensity and impair their ability to generate nonlinear signal. The primary processes responsible for this pulse distortion are group velocity dispersion and self-phase modulation. Thus, it should be possible to achieve subwavelength nonlinear imaging.
Therefore, merging this mature fiber optics technology with the emerging metamaterials technology would likely revolutionize applications such as medical endoscopy as well as many other imaging and nanofabrication techniques. In this talk, we will discuss our recent results on design, optimization, fabrication and characterization of these devices.

We have developed a plasmonic nanopore sensor, which combines a single solid-state nanopore with the hot spot of a plasmonic nanoantenna. The nanopore enable label-free electrical detection of single biomolecules that translocate through the pore. The plasmonic antenna effectively focuses light to a highly intense nanoscale hot spot right at the nanopore. Among many exciting possibilities added by the nanoantenna are light-controlled forces provided by the high optical field gradients in the hot spot. Our combined optical and molecular dynamics simulations suggest that these plasmonic forces can be used to trap single DNA molecules and to control their motion through the nanopore. We are currently performing experimental investigations of this concept of plasmon-controlled nanopore translocations, which, if successful, may help to enable single-molecule label-free DNA sequencing as well as open up for other entirely new nanopore-based applications.
Gold nanoantennas are also excellent in converting local light intensity to heat. Hence, the plasmonic nanopore enables light-triggered rapid control of the nanopore temperature, with anticipated use for investigations of protein denaturing and refolding at the single-molecule level. By monitoring changes in the ionic conductance of the nanopore we could quantify plasmon-induced temperature changes in the hot spot of a single optical nanoantenna.[1] Furthermore, spatially scanning the plasmonic nanopore enables subdiffraction-limited profiling of optical intensity landscapes, as demonstrated for a tightly focused low-intensity laser beam.[1]
[1] MP Jonsson and C Dekker. Nano Letters 2013, 13, 1029-1033

In this work, we demonstrate a plasmonic chip based technology for performing ultra-sensitive infrared (IR) absorption spectroscopy in aqueous solutions, capable of monitoring biomolecule interactions at the sub-monolayer level in real-time. In leveraging engineered plasmonic antennas for IR absorption enhancement, our results represent a dramatic advance over previous studies, being the first demonstration of their use for biologically significant measurements in solution. Notably, such kinetic measurements are fundamentally challenging in traditional IR spectroscopy due to (i) sensitivity limitations and (ii) the extremely strong absorption bands of liquid water, which routinely prohibit analysis in aqueous solutions. Our plasmonic system overcomes these challenges, enabling us to monitor in real-time protein and nano-particle binding events at high sensitivity. Compared to traditional label-free approaches our technology is based on molecular bond-specific IR absorption signatures, therefore it even enables observation of minute volumes of water displacement during molecular interactions. These measurements are made possible by the plasmonic enhancement of absorption bands by factors of 10^3 to 10^4 in conjunction with a non-classical form of internal reflection that together boost sensitivity while limiting interference from solution absorption. In comparison with the current state of the art in IR spectroscopy, which require sampling volumes several thousand times larger and cumbersome, bulky macroscopic optics, our results offer overall sensitivity improvements of over an order of magnitude in an ultra-compact chip based technology. These features are significant in a broad context for a number of reasons.

Top-down lithographic techniques currently dominate the field of nanostructure fabrication due to their high level of precision and versatility, however, these techniques become significantly more expensive and time-consuming as the scale of interest decreases below 50 nm. In an effort to construct plasmonically coupled meta-molecules on a sub-30nm size scale, a disk-shaped aggregate of the Tobacco Mosaic Virus coat protein (TMVcp) is utilized as a template for three-dimensional assembly of gold nanoparticle rings in aqueous solution. Two distinct methods are used to achieve this. The first exploits a combination of electrostatic and hydrogen-bonding interactions between gold particles passivated with bis-p-(sulfonatophenyl)phenylphosphine (BSPP) and arginine residues on the coat protein surface. This produces 23 nm rings on the disk surface. The second method utilizes simple bioconjugation techniques for the attachment of α-lipoic acid to the TMVcp N-terminus, providing a handle for gold nanoparticle assembly with unprecedented geometric control and specificity. The result is 25 nm rings that consist of 12-14 nanoparticles with a 2 nm interparticle spacing.
Theoretical models suggest that these rings may display metamaterial behaviour in the optical spectrum and preliminary ensemble spectroscopic measurements reveal intriguing optical properties. Optical effects can be tuned by the introduction of a nanoparticle in the center of the rings through a pH dependent electrostatic attraction between the negatively charged gold particles and the positively charged arginine groups at the center of the TMVcp disk aggregate. The presence of an inter-particle gap between each of the particles implies a large concentration of electromagnetic ‘hot-spots&’ which can be exploited for numerous applications since the BSPP ligand binds weakly and is, therefore, easily displaced by other molecules. These structures represent the first steps into solution-phase plasmonic materials and highlight the versatility of virus-templated self-assembly for the exploration of size scales that have only been studied in theory.

Metal-insulator-metal (MIM) waveguides are versatile building blocks for metamaterials as the propagation of light is intrinsically coupled to surface plasmon polaritons of which the dispersion can be strongly controlled. We present the bottom-up and top-down fabrication of three-dimensional architectures composed of MIM waveguides composed of Ag/Si3N4 and of Ag/Si. Both planar and coaxial geometries are presented. We demonstrate how these structures act as three-dimensional metamaterials operating in the ultra-violet spectral range. We show how a proper geometry leads to a refractive index that is negative in the UV, and demonstrate the special case of n=0, which is tuneable from the UV into the near-IR spectral range. The n=0 metamaterial shows very good impedance matching and enables novel micro/nano-optical components for e.g. transmission enhancement, wavefront shaping, controlled spontaneous emission and superradiance.

The fabrication of three-dimensional metamaterials with a negative index of refraction is one of the holy grails in metamaterial research. Recently, a design of a negative-index metamaterial in the visible/UV was proposed theoretically [1]. The negative index in this geometry results from the coupling of the plasmonic modes in double periodically stacked thin layers of alternating metal and insulator. Another design, based on the same physical concept, composed of an array of coupled plasmonic coaxial waveguides has also been reported for which simulations predict a polarization-independent negative index [2]. The (negative) refractive index of this metamaterial can be tuned by changing the geometry and has a relatively high figure of merit. So far, the fabrication of this new metamaterial has remained elusive due to the small dimensions of the three-dimensional architecture. Here, we show the first experimental realization of a three-dimensional coaxial metamaterial in the visible/UV spectral region.
Fabrication of the coaxial metamaterial is a multi-step process starting with patterning a 1 µm thick Si(100) membrane with 100 keV electron beam lithography, using the high-resolution resist HSQ. Rings are fabricated with a height up to 250 nm, outer diameter >80 nm, and wall thickness as small as 10 nm. Using these rings as etch mask, the coaxial structure is transferred into the Si membrane with SF6 and CHF3 reactive ion etching. Hollow pillars of Si having a >80 nm outer diameter with >20 nm thick walls are made in this way. The wall thickness can be reduced down to 3-5 nm by thermal oxidation of the Si, followed by oxide removal in HF. The Si pillars are then infilled with Ag using a newly developed thermal evaporation method in which the rotating sample is irradiated with 300 eV Ar ions during Ag evaporation. This new technique leads to fully conformal infiltration of Ag without nanovoids. Finally, the sample surface is polished using 30 keV Ga focused ion beam milling under glancing incidence. The resulting metamaterial sample, with dimensions of 20x20 µm, is composed of a periodic array of Si rings embedded in Ag.
Optical transmission measurements on the coaxial metamaterial show clear transmission in the UV/blue spectral range of 10-50%. The phase shift of light transmitted through the sample is measured using a home-built Mach-Zehnder interferometer operating at visible/UV frequencies. Measurements were performed on a 70 nm thick metamaterial having 135 nm outer and 86 nm inner diameter separated by a 200 nm pitch in a hexagonal array. Phase shifts of -52° and -80° are obtained at lambda; = 496.5 and 514.5 nm respectively. We convert the measured phase shifts to an effective refractive index by comparing the measured results with simulations and analytical calculations.
[1] E. Verhagen, R. de Waele, L. Kuipers and A. Polman, Phys. Rev. Lett. 105, 223901 (2010)
[2] S.P. Burgos, R. de Waele, A. Polman, and H.A. Atwater, Nature Mater. 9, 407 (2010)

Strongly confined surface modes on corrugated metallic surfaces, usually termed spoof plasmons, can help to overcome problems in THz research such as the lack of strong THz radiation sources. Prism coupling of THz radiation to spoof plasmons supported by a spoof plasmon surface (SPS) consisting of a linear array of subwavelength grooves (period = 60 mu;m) is demonstrated here and applied to THz refractive index sensing [1]. The grooves are filled with various fluids, namely nitrogen, gasoline, liquid paraffin, glycerin and water, and the optical response of the SPS is measured via an Otto prism configuration in a THz Time-Domain Spectrometer (THz-TDS). Sharp changes in reflectivity and phase are used as readout responses to measure any change in the effective refractive index of the SPS caused by the fluids. The fluids tested here have a wide range of refractive indices, allowing us to assess the efficacy of SPS sensing for a variety of materials. Figures-of-Merit as high as 49 and a sensitivity of 0.49 THz/RIU are achieved experimentally with excellent agreement with numerical calculations. The optical properties of the SPS can be easily tailored, making it suitable for a wide range of sensing applications.
[1] Ng, B., Wu, J., Hanham, S. M., Fernandez-Dominguez, A. I., Klein, N., Liew, Y. F., Breese, M. B. H., Hong, M. and Maier, S. A. (2013), Spoof Plasmon Surfaces: A Novel Platform for THz Sensing. Advanced Optical Materials. doi:10.1002/adom.201300146

Gold nanoshells with different surface topography were synthesized in high yield by a templated surfactant-assisted seed growth method. When hexadecyltrimethylammonium chloride (CTAC) was used as surfactant in the growth solution, isotropic nanoparticles grew on polymer templates and fused together into relatively smooth nanoshells. On the other hand, a growth solution containing hexadecyltrimethylammonium bromide (CTAB) generated spiky nanoshells composed of sharp and dense spikes. The sharpness and density of spikes on the shell surface can be tuned by varying the amount of bromide ions in the growth solution. These highly structured gold nanoshells interact strongly with light in the near-IR region and proved to be effective surface enhanced Raman scattering substrate. Furthermore, the spiky gold nanoshells exhibited strong quadrupole plasmon resonance in the visible region. The excitation at the quadrupole plasmon resonance wavelength resulted in surprisingly high Raman enhancement. Finite-difference time-domain (FDTD) modeling was used to predict and understand the optical properties of the nanoshells and the extraordinary quadrupole-induced Raman enhancement.

Thermal radiation is light that is emitted from an object at a temperature above absolute zero. The spectrum and intensity of thermal radiation are a function of temperature and the emissivity of the object, which is frequency-dependent, as expressed in Planck&’s law. By integrating Planck&’s law over all frequencies, the Stefan-Boltzmann law of thermal radiation is obtained, which states that the total energy density emitted is proportional to the fourth power of temperature. This is congruent with every-day experiences that hotter objects emit more light, corresponding to a strictly positive differential thermal emittance.
We show that the differential thermal emittance can become negative in a geometry which exhibits an absorption resonance that can appear and disappear as a function of temperature. Our geometry involves a thin (~150 nm) film of vanadium oxide (VO2), a phase change material, on a sapphire substrate. Recently, we showed that a film of VO2 deposited on sapphire can operate as a temperature-tunable absorber. As absorptivity and thermal emissivity are thermodynamically related, such a structure is expected to have a temperature-dependent emissivity.
We measured the emissivity of our VO2/sapphire structure, and observed that for a particular set of temperatures and wavelengths, and emissivity of ~1 is obtained, corresponding to the previously-reported “perfect absorber” condition. We integrated the thermal emission spectrum over the 8-14 µm atmospheric transparency window, and found that in the vicinity of the VO2 phase transition the sample exhibits a large negative thermal emittance. Imaging with a thermal camera shows that the sample appears cooler even as it is heating up over the 73 - 85 °C range. This phenomenon can find uses in applications such as infrared camouflage and thermal tagging/identification.

Optomechanical structures are of great interest due to their ability to effectively transduce mechanical motion to optical fields, enabling ultra-high sensitivity sensors, actuators, oscillators, and much more. Typical optomechanical resonators are composed of Fabry-Perot cavities made using dielectric mirrors, whispering gallery micro-cavities, or photonic crystal defect cavities, structures that are typically larger than the wavelength of light.
Here, we present a novel opto-mechanical architecture with ultrahigh sensitivity that is composed of a plasmonic nanocavity coupled to a nanomechanical oscillator [1]. The structure consists of a linear plasmonic cavity supporting plasmonic Fabry-Perot resonances due to metal-insulator-metal surface plasmon polaritons that are confined in the 22-nm-wide gap between two suspended Si3N4 nanobeams (l=20 micron, w=800 nm, t=50 nm), coated with 120 nm of gold. The lowest-order orthogonal in-plane and out-of-plane resonant mechanical modes are observed at 2.0 MHz (out-of-plane) and 4.44 MHz (in-plane); high-order overtones are also observed. Mechanical quality factors are in the range Q=600-650. The measured resonances are in good agreement with numerical simulations.
Due to the strong plasmonic confinement in the narrow coupling gap, the plasmon-nanomechanical coupling strength is >2 THz/nm, an order of magnitude higher than that of non-plasmonic optomechanical oscillators. From the mechanical resonance spectra a displacement sensitivity as small as delta=10 pm, with a displacement sensitivity of 3.58e-14 m/sqrt(Hz) is measured, demonstrating the extreme sensitivity of the plasmonic mechanical oscillator design. Nonlinear effects are observed at high laser power and pave the way to optical control over nanomechanical motion.
The free-space coupling used in this new cavity design allows far-field readout of parallel arrays of coupled nanomechanical oscillators created by fabricating several nanobeams next to each other, as we will show. Finally, to extend the plasmonic transducer frequencies into the GHz regime, we study plasmo-mechanical coupling using localized plasmonic resonances in nanoscale coaxial Au/vacuum/Au cavities.
[1] R. Thijssen, E. Verhagen, T.J. Kippenberg and A. Polman, Nano Lett (2013), DOI: 10.1021/nl4015028

We present a study on the interaction of surface acoustic waves (SAWs) with the contact resonance of a two-dimensional array of 1 mu;m silica microspheres adhered to a substrate. A laser-based transient grating technique is used to generate SAWs and measure their dispersion. Briefly, the technique involves crossing two short optical pulses on the sample which interfere and create a sinusoidal interference pattern. Absorption of the laser light by the thin metal layer creates a thermal expansion, which launches counter propagating acoustic waves with wavelength equal to the period of the interference pattern. Diffraction of an incident probe beam monitors the frequency and decay of the acoustic wave. By changing the crossing angle we vary the acoustic wavelength from ~7 - 50 mu;m. The measured dispersion curves show “avoided crossing” behavior as a result of the hybridization of the contact resonance of the microspheres with the SAW in the substrate at a frequency of ~200 MHz. The experimental dispersion is matched to an analytical model with the contact stiffness as the only fitting parameter. The measured contact resonance frequency is compared with estimates based on the Derjaguin-Muller-Toporov contact model. The study opens the door for a new class of locally resonant metamaterials based on the contact resonance of microparticles. Potential applications for linear and nonlinear SAW devices will be discussed.

One fundamental goal of metamaterials is to envisage and then fabricate complex shapes which, by virtue of their structure, impart properties not previously accessible. The challenge for optical metamaterials in the visible region is that they generally require structures with feature sizes less than the wavelength of light, and hence three dimensional structural-control at the nanoscale. However, at these small sizes, it is particularly challenging to fabricate complex structures because surface-energy minimization generally leads to compact high-symmetry shapes. The existing techniques for metamaterial synthesis for visible wavelengths (e-beam lithography, DNA origami) produce high fidelity structures with potentially arbitrary shapes, but at low yields and/or inflexible designs.
In this work we present a fabrication method for nanostructures with programmed shape and material composition with wafer-scale yields [1]. The range of potential shapes will be discussed and for instance includes sub-100 nm multiple turn plasmonic helices. The growth method is based upon a physical vapor deposition process that combines independent control over the nanoseed pattern, substrate manipulation, and temperature. With it we grow nanoparticles with complex 3D morphologies, with smaller than 20nm feature size, and that contain several functional materials. In one wafer-run we are able to obtain high yields of colloidal metamaterials and nanowires that contain magnetic, semiconducting, metallic and insulating materials within the same structure.
The technique's versatility has allowed us to create films and metafluids based on chiral plasmonic metal nanoparticles. We demonstrate the effect of nanowire composition on the chiroptical response of such metamaterials, systematically explore the effect of particle shape, and thereby tune the chiral response through the visible regime. When optimized, suspensions of plasmonic nanohelices can produce a metafluid exhibiting molar circular dichroism an order of magnitude greater than any previously reported. We also demonstrate a unqiue plasmonic-insulator-plasmonic material, which cannot be fabricated by any other means. The optical rotation of this hybrid structure is large enough to suggest its use as a component in a metamaterial to achieve negative index of refraction at visible wavelengths.
[1] A.G. Mark, J. Gibbs,T.-C. Lee, P. Fischer. Nature Materials (2013). doi:10.1038/nmat3685

Metamaterial technology offers unique opportunities for “engineering” previously inaccessible values of refractive indices and achieving unprecedented control over light propagation. Negative or near-zero index of refraction, subwavelength imaging, and cloaking, are just few peculiar phenomena enabled by metamaterials. Beyond these linear optical properties, Metamaterials are predicted to enable fundamentally new manifestations of many nonlinear optical phenomena, including the backward phase-matching condition in negative index materials, dynamic control of their dielectric permittivity and magnetic permeability using nonlinearity, and dramatic enhancements of nonlinear effects due to resonant phenomena in negative and near-zero index materials. Most applications, especially nonlinear optical components, require large-scale, tunable, three-dimensional structures, much larger than the wavelength of light. However, existing fabrication methods such as focused ion beam or conventional electron beam lithography based on physical patterning are inherently limited to small-area two-dimensional samples.
We have combined a holographic photo-patterning approach that provides a simple and low-cost way to fabricate large areas of ordered structures with multi-pinhole illumination to produce complex periodic patterns. A typical holographic photopatterning combines the techniques of holography and laser induced polymerization in which the pre-polymer syrup is exposed to the spatial interference pattern introduced by single or multiple coherent laser beams. In our method, we prepare a photosensitive pre-polymer syrup - a mixture of monomer, photoinitiator, co-initiator and solvents - which is then sandwiched between two glass slides. A post-exposure UV curing procedure fully develops the structure and enhances a phase separation between the polymer and the solvent. Upon opening the sandwiched sample, the solvent evaporates and a periodic refractive index modulation is created in the recording media. Photo-polymerization will lead to higher polymerization in the high intensity regions of the interference pattern. While in a more traditional approach, spatially periodic patterns are achieved using two or three beam interference, our approach uses a mask with multiple pinholes that can be designed to produce a prescribed pattern in a repeatable and robust way. We demonstrate two- and three-dimensional structures fabricated using this technique. Finally, the polymeric material with the recorded spatial pattern can be transferred onto large area flexible structures.

We have developed a genetic algorithm method for designing the diffractive optics used in phase mask lithography and performed exposures through this phase mask to realize various complex 3D structures, include optical metamaterials comprised of both dielectric and metallic constituents. As a complement to this method, we are also exploring layer-by-layer assembly strategies whereby the 3D photonic structure is created by printing thin 2D sheets containing the desired optical functionalities. The design of these structures is also being guided by genetic algorithm strategies, which enable and element of manufacturability to be introduced into the design process.

Metamaterials are artificial materials designed to achieve properties which may not be available in the naturally existing materials such as negative refractive index. This has become a very exciting topic for the research and various investigations are being carried out in this area. Some of the focuses are on the fabrication of metamaterials on new substrates, over a large area format and with lower process cost. In this work electromagnetic metamaterials at microwave frequencies are implemented using the paint process on the fabric substrates. Metamaterial patterns are transferred on the fabrics by applying conductive paint through stencils containing metamaterial patterns. Stencils are made using a laser cutter machine which is able to cut patterns over a large area. Metamaterial structures contain electrical split ring resonators with metal line widths between 0.5 to 1.0 mm. Polarization sensitive and insensitive metamaterials are simulated, fabricated and measured at the frequencies between 2.1 GHz and 16 GHz. Both brush and spray based painting methods are used to apply conducting paints on the different fabrics which are commonly used in wearing clothes. Polarization insensitive metamaterial designed at 2.1 GHz shows a measured transmission depth of -10 dB at the resonant frequency. Metamaterials designed at 16 GHz is polarization sensitive and shows the transmission depth of -9 dB. Transmission depth and frequency can be tailored by the design of metamaterial structures. The paint process allows to fabricate metamaterial over a large area at a low cost.

Split-ring resonators (SRRs) were designed to act as LC resonators and interact with a magnetic field of an incident light, resulting in the modulation of their relative permeability. Double-gap SRRs of Au or Ag with sub-100-nm wide lines were theoretically expected to exhibit resonance derived from the interaction with the magnetic field at visible frequencies [1]. Nevertheless, the optical properties of the SRRs at visible frequencies remain unclear because of the difficulty in fabricating the metal nanostructures. Here we fabricated uniform arrays of about 360 million double-gap Au SRRs with line widths of about 50 nm over a 5-mm square by reactive-monolayer-assisted thermal nanoimprint lithography. The SRR arrays were experimentally demonstrated to induce the resonance owing to the oscillation of free electrons excited by a magnetic field centered at 690 nm in the visible frequency region. An approximately 0.03-mu;m-thick resist layer of polystyrene (PS) was coated on a Au-plated substrate (Au 10 nm/ Cr 5 nm/ silica) modified with a benzophenone-containing photoreactive monolayer (PrM) and exposed to UV light to form a PS graft layer. The PS layer was transformed into the SRR structures by thermal nanoimprinting at 150 °C with a concave double-gap SRR array in a 5-mm square area. The PS graft layer suppressed thermally induced dewetting of a very thin 0.03-mu;m film and enabled the formation of SRR patterns uniformly. The obtained Au-plated substrate covered with a pattern PS resist layer was subjected to Ar ion milling to etch the Au layer selectively, exposure to vacuum ultraviolet light to remove the resist layer and PrM, and wet etching to remove the Cr adhesion layer. The uniform double-gap Au SRRs in a 5-mm square area were confirmed by scanning electron microscopy (SEM). The optical properties were studied by polarized transmission spectroscopy. Particularly, the transmission spectra for oblique incidence were measured to investigate the interaction with a magnetic field. An absorption band appeared at 690 nm as increasing the incident angle only when the incident light was s-polarized and had a component of a magnetic field normal to the SRRs. Comparing the transmission spectra of Au SRRs with those of Au nanorods having similar linewidths and lengths in detail, we proved that the absorption band centered at 690 nm was derived from the resonance owing to the free electrons caused by the interaction with the magnetic field of the incident light. These results contribute important experimental knowledge to research on visible-frequency metamaterials and to the design of optically functional metamaterials.
[1] Ishikawa, A.; Tanaka, T.; Kawata, S., Phys. Rev. Lett. 2005, 95, 237401.

Plasmonic gratings and lenses are routinely used to convert free-space beams into propagating surface waves and vice versa. So far, this approach has been limited to simple light beams, such as plane waves or Gaussian beams. We will present a powerful generalization of plasmonic structures to couple more complex wavefronts. This approach is based on the principle of holography: the coupler is designed as the interference pattern of the incident vortex beam and focused surface plasmon polaritons (SPPs). We have integrated these holographic plasmonic interfaces into commercial silicon photodiodes, and demonstrated that such devices can selectively detect the orbital angular momentum of light.1 We have also demonstrated new plasmonic couplers that generate upon illumination, new diffractionless SSPs, which unlike surface plasmon Airy beams propagate in a straight line: the cosine-Gauss beam (CGB)2 and the plasmonic bottle beam (PBB)3. The CGB is generated by a launcher consisting of intersecting metallic gratings that produce different degrees of confinement; diffractionless propagation distances approaching 100 microns have been demonstrated by NSOM imaging. By analogy to the three dimensional optical bottle beams, the PBB features a lattice of plasmonic bottles, i.e. closed regions of dark focii alternated with high intensity regions. Such bottle beam is created using three gratings by the interference of a two-dimensional nondiffracting cosine-Gauss beam with a quasi-plane wave. By controlling the propagation constant of the cosine-Gauss beam, we can change the numbers and the size of the plasmonic bottles. Such beams could become important for particle trapping. Finally we report a new type of holographic interface, which after being designed for a given wavelength, is able to manipulate the three fundamental properties of light (phase, amplitude and polarization) over a broad wavelength range. The resulting optical element is the superposition of two independent structures with very different length scales, i.e. a hologram with each of its apertures filled with nanoscale openings to transmit only a desired state of polarization. As an implementation we have fabricated a nano-structured holographic plate that can generate radially polarized optical beams from circularly polarized incident light and we have demonstrated that it can operate over a broad range of wavelengths. The author gratefully acknowledges collaborations with: P. Genevet, J. Lin, M. A. Kats, R. Blanchard, A. She, M. Petit, B. Cluzel and F. de Fornel.
1. P. Genevet, J. Lin, M. A. Kats and F. Capasso, Nature Communications 3, 1278 (2012)
2. J. Lin, J. Dellinger, P. Genevet, B. Cluzel, F. de Fornel, and F. Capasso, Physical Review Letters 109, 93904 (2012)
3. P. Genevet, J. Dellinger, R. Blanchard, A. She, M. Petit, B. Cluzel, M. A. Kats, F. de Fornel, and F. Capasso, Optics Express 21, 10295 (2013)

Controlling phase pickup of light with optical resonators with deep subwavelength dimensions could play a key role in shrinking the dimensions of optical elements. Various metasurfaces based on plasmonic structures have been demonstrated, while the possibility of employing dielectric nanostructures to control the phase front of light has remained relatively unexplored. Here we demonstrated the first Si-based metasurface structure operating in the visible spectral range capable of manipulating the phase front of light within an ultra-thin layer. Dielectric optical antennas based on silicon (Si) nanostructures can support leaky mode resonances, which can confine light within subwavelength, high-refractive-index nanostructures. The phase of light scattering from Si nanostructures varies rapidly across their optical resonances. By fabricating metasurfaces with building block of Si nanostructures, we have experimentally demonstrated a phased array for light steering and a high numerical aperture dielectric planar lens. The dielectric metasurface can avoid the intrinsic losses in metals and provide high diffraction efficiency. The ability to engineer the phase front of light at the subwavelength scale through a thin planar Si structure provides novel approach for nanoscale optical switches and polarization dependent optical elements.

Very recently, intensive attention has been attracted on metasurfaces, a class of planar metamaterials with exceptional abilities to mold light flow. The central idea of metasurfaces is to introduce the desired in-plane phase profile by patterning subwavelength resonant structures at the interface between two natural materials. The rationally designed phase offers an additional, yet a very important degree of freedom to fully control the wave propagation. For instance, anomalous reflection and refraction have been demonstrated in the infrared and optical region. Metasurface-based optical devices, such as vortex plates, wave plates and ultra-thin focusing lenses, have also been realized for different types of incident light, i.e., linearly polarized light, circularly polarized light, or vertex beams. So far, most studies on metasurfaces have been focusing on manipulating propagating waves. In contrast, only a few of them have been devoted to employing metasurfaces to control near-field waves, such as surface plasmons. Surface plasmons are collective electron density oscillations that are coupled to external light. Due to the strong confinement and large field enhancement, surface plasmons promise a variety of applications in super-resolution imaging and lithography, data storage, ultra-compact optical devices, as well as biomedical sensing.
In this work, we demonstrate that one-dimensional metallic gratings, a simple metasurface with practically feasible geometries, are capable of tailoring the dispersion, and thus the propagation characteristics of surface plasmons in an unprecedented manner. It is found that the constant frequency contour of surface plasmons can be tuned from being convex to flat, and to hyperbolic. As a result, normal, non-divergent and anomalous diffraction are achieved as surface plasmons propagate along the metasurface. In particular, arising from the hyperbolic constant frequency contour, surface plasmons undergo negative refraction as they propagate from a flat metal surface to a metasurface. The negative refraction works for broad incident angle and wavelength range. We have also designed metalenses, and explored the interaction between single nano emitters and the metasurface. These findings open an innovative avenue to designing on-chip plasmonic devices for imaging, photon harvesting and optical communication.

Conventional imaging systems that use refraction optics are successful in enabling compact cameras at visible frequencies. However, at longer wavelengths (midIR- to radio frequency), imaging devices that rely on the refraction are either bulky or completely ground based due to the fundamental restriction of minimum pixel size by the free-space wavelength. Here we propose a new diffraction based concept of imaging that relies on meta-surfaces and computational post-processing. In combination with engineered meta-surfaces, our approach takes advantage of naturally available high refractive index materials to shrink the pixel dimensions and thus reduce the size of sensors. As an added advantage, the use of planar geometries in the place of curved lenses makes imaging devices focus free and provides complete volumetric distribution of the optical fields of the object space. The proposed design also allows using software (computational post-processing) in the place of hardware to mitigate the potential aberrations, further reducing the number of optical elements. In this presentation we propose the design to make compact sensors, demonstrate its capability of recovering volumetric field distribution using low dimensional projections, and discuss the robustness of the recovery procedure using rigorous coupled wave analysis (RCWA) and non-linear fitting techniques.

One of the great benefits of metamaterials arises from the flexibility in engineering their optical responses to achieve control over the propagation of light to an unprecedented level. Plasmonic metasurfaces, a sub class of metamaterials that consist only of a monolayer of planar metallic structures, have shown great promise for leveraging full control of light and low fabrication cost as they do not require complicated three dimensional nano-fabrication techniques. One of the interesting features of metasurfaces is the capability of generating abrupt interfacial phase changes across the interface, and therefore provide a unique way of controlling the wave front locally at the subwavelength scale. Such a shaping of a wave by a modification of the spatial phase and intensity distribution is very important for all applications that require a propagation of light like focusing, beam shaping, and reconstruction of 3D images, a technique which has been known as holography for long time.
Recently metamaterials were used to demonstrate wave plates for generating vortex beams, ultrathin metalenses and 2D holography and projection. However, none of these techniques has achieved a more complex functionality like reconfigurable focusing or 3D image reconstruction in the visible range. In particular 3D image reconstruction would be an important step since the essence of holography lies in its capability to display 3D holographic images.
Here, we will experimentally demonstrate two new applications namely a switchable dual-polarity lens and 3D holography by using plasmonic metasurfaces consisting of simple subwavelength metallic nanorods with spatially varying orientations. As the spatial phase can be continuously controlled locally at each subwavelength unit cell by the rod orientation, metasurfaces represent a new route towards the flexible modification of spatial phase and intensity profiles that can result in high-resolution on-axis 3D holograms with wide field of view. In addition, the undesired effects of twin images and multiple diffraction orders usually accompanying holography are eliminated.
For our demonstration we utilize the abrupt phase change that occurs for a circularly polarized light converted to its opposite helicity. The phase shift at the interface, ranging from 0 to 2π, is realized by a metasurface consisting of an array of plasmonic gold dipole antennas with subwavelength sizes and separations. The local phase of light transmitting through the metasurface is geometrical and solely controlled by the orientation angle of the individual dipole antennas which provides an easy way to obtain nearly arbitrary phase profiles. With this technique we demonstrate an ultrathin lens which can be altered from focusing to defocusing simply by changing the polarization state of the light. Furthermore, we will demonstrate a 3D computer generated holography image reconstruction by the same technique that will work over a large wavelength range.

Surface plasmon polaritons (SPPs), the excitation of collective motion of conduction band electrons on metal-dielectric interfaces, have been shown to exhibit intriguing properties of strong enhancement of local field and large in-plane momentum. It is important that the free space photon can be coupled to SPPs in a controllable manner. Unidirectional SPPs have been achieved by asymmetric design of the grating coupler, highly compact plasmonic antennas and metasurfaces with locally controlled phase profile. However, the direction of SPP excitation in these couplers is predefined and cannot be reconfigured. Here, we apply the concept of interfacial phase discontinuity for circularly polarizations on a metasurface to the design of a novel type of polarization dependent SPP unidirectional excitation. Selective unidirectional excitation of SPPs along opposite directions is experimentally demonstrated at optical frequencies by simply switching the helicity of the incident light.
The unidirectional SPP coupler consists of an array of elongated apertures with a constant gradient of orientation angle along a certain direction. Each aperture can be considered as a combination of electric and magnetic dipoles. Each dipole, under the illumination of circularly polarized light at normal incidence, can be decomposed into two circularly oscillating components, with one component having the same helicity as the incident light and a phase that does not depend on the orientation of the aperture, and the other with opposite helicity to the incident light and a phase that is twice the orientation angle of the aperture. For an array of apertures with a constant gradient in the orientation angle, the refraction and diffraction of light by the array show ordinary and anomalous refraction and diffraction orders. For a circularly polarized incident beam at normal incidence, the two ordinary first-order diffracted beams are symmetric about the central ordinary zero order beam along the surface normal. In contrast, the anomalous refracted beam and two first order anomalous diffracted beams are shifted towards the same direction relative to their ordinary counterparts. As a result, the two anomalous first-order diffracted beams are not symmetric about the surface normal, which forms the basis of unidirectional SPP excitation at certain optical frequencies.
We have experimentally demonstrated a polarization dependent unidirectional SPP excitation due to phase discontinuities introduced by an array of plasmonic apertures with spatially varying orientations. The directional SPP excitation shows very high extinction for circularly polarized incident light. The ratio for the excitation of SPPs propagating to opposite directions can be simply adjusted by the ellipticity of the input light. The physics underlying our work is a Berry geometrical phase that depends solely on the orientation of each nano-aperture, but not the structure and shape of each individual element.

Metamaterials is a transformative research to engineer science and achieve innovative materials and system platforms. With recent progress in fabrication to achieve complex configurations one would need an advanced computational paradigm to characterize complex materials. We introduce a powerful mathematical scheme based on H-matrix to engineer materials building blocks cell by cell. Namely we manage to solve complex metamaterials fast and efficient, orders of magnitude improvement in speed and memory in compared to conventional methods. Novel physics and applications in plasmonic, energy, and bio systems will be highlighted.

We report a new concept of engaging the coherent interaction of optical waves on metamaterial nanostructures for the efficient control with light of a plethora of optical phenomena from absorption to refraction, from optical activity to anisotropy, and from nonlinear response to luminescence. We provide a number of demonstrations of coherent control phenomena alongside a discussion of their spectroscopic and optical data processing applications.