In the area of metamaterial research, it is frequently assumed that effective medium parameters provide all the information necessary to determine the light-matter interaction. But even if the effective medium parameters can describe faithfully how the material can manipulate wave in the long wavelength limit, can the same set of parameters describe faithfully how the wave can manipulate the material? It is known that the total electromagnetic induced force acting on a metamaterial can be calculated using the Maxwell stress tensor if the light scattering property of the metamaterial can be described by standard effective medium parameters. However, we show that the optical stress inside a metamaterial systems cannot be determined by the information of effective permittivity and permeability and it can only be correctly calculated using the Helmholtz stress tensor which takes into account of electrostrictive and magnetostrictive effects. Using multiple scattering theory, we derived for the first time the analytical formulas for electrostrictive/magnetostrictive tensors for two dimensional all dielectric metamaterial systems, which are found to depend explicitly on the symmetry of the underlying lattice of the metamaterial as well as a local effective wave vector. These analytical results enable us to calculate light-induced body forces inside a composite system using the Helmholtz stress tensor within the effective medium formalism in the sense that the fields used in the stress tensor are those obtained by solving the macroscopic Maxwell equation with the microstructure of the metamaterial replaced by an effective medium. The optical stress induced by an external light source at the boundary of two metamaterials is also subtle. We investigate the optical stress induced at the interface formed by two kinds of metamaterials and it is found that the stress is strongly affected by the electrostriction and magnetostriction effects. We show that the symmetry of the underlying lattice can dramatically influence interface stress to the extent the optical stress in the interfacial area is in fact "indeterminate", in the sense for given values of effective permittivity and permeability, the interfacial stress is unknown unless we know the microscopic details or their manifestations through electrostrictive tensors. Related to this problem, we find that light in a negative-index material background in general does not pull an object immersed in it.

Metamaterials bring subwavelength resonating structures to overcome the limitations of conventional matter. The realization of active metadevices requires electrically reconfigurable components operating over a broad spectrum with a wide dynamic range. The existing capability of metamaterials, however, is not sufficient to realize this goal. Here, using large area graphene capacitors incorporated with metallic split ring resonators, we demonstrated electrically controlled metadevices on large-area flexible substrates. In this device architecture, metallic resonators are capacitively coupled to the graphene electrodes that introduce voltage-controlled dissipation. Electrostatic tuning of charge density on graphene in the order of 10¹⁴ cm^-2 enabled us to switch the resonance behavior of the split ring resonators by 50 dB with an operation voltage of 2V. Large modulation depth, simple device architecture, and mechanical flexibility are the key attributes of the graphene-enabled metadevices that could find a wide range of applications ranging from active signal processing to switchable cloaking.

As natural chiral materials demonstrate limited circularly dichroic contrasts, enhancement of these polarization dependent signals has long been the focus of chiral metamaterial research. By manipulating the geometric chirality of resonant plasmonic nanostructures, we are capable of enhancing light confinement to amplify chiral modified, nonlinear signals from quantum emitters. The metamaterial demonstrates a linear transmission contrast of 0.5 between left and right circular polarizations and a 20× contrast between second harmonic responses from the two incident polarizations. Nonlinear and linear response images probed with circularly polarized lights show strongly defined contrast. As a second set of experimentation, the chiral center of the metamaterial is opened, providing direct access to place emitters to occupy the most light-confining and chirally sensitive regions. The resulting two-photon emission profiles from circularly polarized excitation displays mirrored symmetry for the two hybrid enantiomer structures. The efficiency of the nonlinear signal directly correlates to the chiral resonance of the linear regime. The nonlinear emission signal is enhanced by 40× that of the emitters not embedded in the metamaterial and displays a 3× contrast for the opposite circular polarization. Such manipulations of nonlinear signals with metamaterials open pathways for diverse applications where chiral selective signals are monitored, processed, and analyzed.

Tunable optical metamaterials and metasurfaces are an emerging frontier, with promising applications including optical modulation, routing, and beam steering. Dynamic control in such meta-devices can be achieved by incorporating active media, e.g. liquid crystals or phase change materials, into the optical structures. Vanadium dioxide (VO₂), a prototypical phase change material, has a thermally induced insulator-metal transition that results in a considerable change in its optical properties. It has been shown that the complex refractive index of VO₂ changes drastically during phase transition.
Here we demonstrate tunable metasurfaces created using defect engineering via ion irradiation through lithographically defined masks, which locally modifies the phase transition properties on a subwavelength scale. Our metasurfaces consist of irradiated and unirradiated (intrinsic) VO₂ regions in a square checkerboard arrangement on a sapphire substrate. The phase transition properties of the metasurface are determined by the ion fluence and the duty cycle of the irradiated regions. Naively one might expect the reflectance of our metasurface to be the average of the reflectance values of the irradiated and intrinsic VO₂. Instead we observe an effective response of the metasurfaces, with an “effective phase transition temperature” between the phase transition temperature of the irradiated and intrinsic VO₂. This is due to the subwavelength nature of our patterned features. Hence we can treat the patterned VO₂ film as an effective medium which has a well-defined temperature- and wavelength-dependent complex refractive index. We apply effective medium theory to model the behavior of our metasurfaces, and observe good agreement with the experimental results. By combining defect engineering with electron-beam lithographic patterning techniques, we can design effective media with engineered anisotropy and gradient indices. Our approach will be broadly applicable to the development of tunable optical meta-devices.

We report on the realization of ultrathin free-standing all-dielectric metasurfaces, with sharply resonant optical properties in the near-infrared (telecoms) wavelength range, in which the optical forces generated among constituent elements are sufficient to induce reversible nanoscale structural deformation. With mechanical Eigenfrequencies in the hundreds of megahertz range, the optomechanical response of such structures provides for fast, strongly nonlinear tuning of optical properties at µW/µm² intensities.

We present a novel methodology to implement metasurfaces that realize the optical properties of chiral media which possess differential operation with respect to left- (LCP) and right-circular polarization (RCP). Chirality is very recurrent in biological media and organic compounds. These compounds have molecules which don&’t superimpose onto their mirror image lifting the degeneracy between LCP and RCP causing the chiroptical response. Thus, generating and sensing optical chirality is valuable to stereochemistry and molecular biology in addition to its electrodynamic applications. However, the chiroptical effect is generally weak in natural crystals and detectable only when strong phase differences between LCP and RCP accumulate over a long optical path. Strong chiroptical effects have been demonstrated in metamaterials using 3D nano-structures with broken mirror symmetry. Yet, they are complicated to implement because they require fabrication of multi-layers with angular rotations of nano-structures along successive layers. Here, we present a simplistic and efficient technique to produce chiroptical effects with gap-plasmonic metasurfaces. A planar array of metallic antennas is fabricated on top of a metallic film reflector and spaced by a dielectric layer. The metal\dielectric\metal sandwich excites slow gap-plasmonic waves that cause a significant phase and polarization change to the back-reflected beam. As a result of antennas&’ anisotropy, the metasurface responds differently to RCP and LCP. Through introducing a phase-shift between the reflected LCP and RCP components of light, the metasurface performs the chiral response of rotating polarization angle (PA) around propagation direction. We implement two metasurfaces that rotate PA of linearly polarized light by 45⁰and -45⁰, respectively. The structure doesn&’t require using complex chiral antennas. We use simple rectangular antennas, and the chiral response is obtained through superposition of reflected beam from a 16-antennas supercell by careful design of location and orientation of each antenna. We obtain a chiral effect that depends on the supercell structure rather than the properties of composite materials. Therefore, the operation is very tolerant against fabrication inaccuracies and\or temperature effects, and high quality results are experimentally obtained. Thus, gap-plasmonic metasurfaces can provide simple, compact and efficient technology for applications include bio-sensing, DNA structural analysis, crystallography, and secure quantum communications.

Symmetries play a fundamental role in physics and devices physics. In this talk, I will discuss the fundamental role of symmetries at the nanoscale resonant level in constructing nanophotonics optical devices. I will in particular discuss the possibility to construct new optical modes that do not decay despite residing the continuum of radiation modes. These modes, called bound states in the continuum, are very peculiar modes with topological properties that can enhance the functionality of nanophotonics optical devices. Their design, fabrication and characterization will be presented.

Metamaterials are artificial media that produce optical phenomena not present in naturally occurring materials. However, three-dimensional (3D) metamaterials suffer from extreme propagation losses, limiting their utility. Two-dimensional (2D) metasurfaces and, in particular, hyperbolic metasurfaces (HMSs) for propagating surface plasmon polaritons, have the potential to alleviate this problem. Because SPPs are guided at a metal-dielectric interface (rather than passing through metals), these HMSs have been predicted to have lower loss while still exhibiting phenomena observed in 3D metamaterials.
We report the first experimental realization of a hyperbolic metasurface [1] formed by lithography and etching nanostructures into sputter-deposited, single-crystalline silver films. The resulting devices display broadband negative refraction and diffraction-free propagation. Additionaly, we find that the HMS exhibits strong, dispersion-dependent spin-orbit coupling, enabling polarization- and wavelength-dependent routing of chiral SPPs. Because we begin with extremely low-loss silver films[1,2], the measured propagation distances of up to 30 mu;m in the HMS shows that the 2D nature of our devices coupled with the single-crystalline silver films offers a substantial, one to two orders of magnitude improvement over 3D metamaterials. These results provide tools for implementing high-performance plasmonic nanostructures with widespread conventional and quantum optics applications.
[1] Alexander High*, Robert C. Devlin*, Alan Dibos, Mark Polking, Dominik S. Wild, Janos Perczel, Nathalie P. de Leon, Mikhail D. Lukin and Hongkun Park. Visible-frequency hyperbolic metasurface. Nature522, 192-196 (2015). *equal contribution.
[2] S. Kolkowitz*, A. Safira*, A. A. High, R. C. Devlin, S. Choi, Q. P. Unterreithmeier, D. Patterson, A. S. Zibrov, V. E. Manucharyan, H. Park, M. D. Lukin. Probing Johnson noise and ballistic transport in normal metals with a single-spin qubit. Science347, 1129 (2015)

Optical metamaterials are being exploited to manipulate light on the nanoscale but integrating such metamaterial structures with a nanoscale source such as a quantum dot (QD) to gain integrated multiple functions such as enhancement of QD excitation and/ or emission rates and directing the emitted photons on to a waveguide in an on-chip optical system remains a challenge. Past efforts have been largely focused on individual functions such as: (i) enhancement of excitation and/ or emission rates via incorporation of the QD in a photonic cavity [1];(ii) guiding of the emitted photons via appropriate placement of the QD by an antenna[2] directed towards (iii) a lossless waveguide[3]. Here we propose and analyze a novel design that incorporates these multiple functions simultaneously at multiple wavelengths into an architecture comprising a QD in a coupled dielectric nanoparticle resonator based optical circuit.
We realize these multiple functions by tailoring the wavelengths of the magnetic and electric multipole modes in high index dielectric nanoparticles as a function of size, shape and material parameters to control their coupling to the electronic states in the QD at the designed excitation and emission wavelengths. The optical response of the system is analytically modeled using the multipole expansion method [4] based on classical electromagnetism. We present specific results for a system comprising TiO₂ nanoparticle optical resonators and a QD excited at 532nm and emitting at 980nm. The nanoparticle radius and separations are chosen such that the magnetic hexapole (TE_3,1) mode is in resonance at the excitation wavelength of 532nm to provide local electric field enhancement at the QD while simultaneously the magnetic dipole (TE_1,1) and electric dipole (TM_1,1) modes are in resonance at the QD emission at 980nm to provide guiding. In this case we find a 20-25 fold enhancement of electric field intensity at the QD at the excitation wavelength along with simultaneous Purcell enhancement, guiding and on-chip lossless propagation at the emission wavelength. The different functionalities in our design show sufficient robustness with respect to fabrication tolerances such as nanoparticle positions which, along with the inherent lossless nature of the dielectric material makes the design suitable for scaling. Our generic analysis of the multipole resonances and light-matter interactions makes this approach readily applicable to optimization of dielectric materials and architectures relevant to optical and other wavelength regimes.
References
[1] S. Buckley et.al. Rep. Prog. Phys. 75, 126503 (2012).
[2] A. E. Krasnok et.al. Optic Express 20, 20599 (2012).
[3] A. Yariv, et.al. Optic Letters 24,11 (1999).
[4] J. M. Gerardy et.al. Phys. Rev. B 25, 6 (1982).

New apertured and apertureless methods will be described with relationship to the excitation and detection of dark and bright surface plasmon polaritons. The methods are based on interrogating meta surfaces with multiple probes. As has been very elegantly demonstrated by Xifeng Ren et al, [Applied Physics Letters 98, 201113 (2011)] an apertured Near-field Scanning Optical Microscopy (NSOM) probe acts as a point source of surface plasmon polaritons with a deterministic position and minimum requirement for the light source. On the other hand Dobman et al, [Adv. Optical Mater. 2014, DOI: 10.1002/adom.201400237 ] have shown that a second type of bound surface plasmon polariton, that cannot be excited from the far-field, propagates well across a metasurface. Such a bound plasmon polariton can only be excited, if light is used, from a near-field probe which produces all k vectors. Furthermore, point excitation of plasmons without background light and with all k vectors and all energies also will be reported using tunneling probes incorporated into a multiprobe system with scattering or collection NSOM for mapping the transport of plasmon polariton propagation on a variety of meta surfaces. All of these probes are constructed to allow for contact of one probe with another which is readily accomplished.

Photonic gradient metasurfaces are ultrathin electromagnetic wave-molding metamaterials that provide a route for realizing flat optics. Recently, we reported on a novel class of metasurfaces - spinoptical metamaterials - which gives rise to a spin-controlled dispersion due to the optical Rashba effect. The optical spin as an additional degree of freedom offers controlled manipulation of spontaneous emission, absorption, scattering, and surface-wave excitation. Spin-symmetry breaking in nanoscale structures caused by spin-orbit interaction, leading to a new branch in optics - spinoptics is presented. The spin-based effects offer an unprecedented ability to control light and its polarization state in nanometer-scale optical devices, thereby facilitating a variety of applications related to nano-photonics. However, the up-to-date metasurface design, manifested by imprinting the required phase profile for a single, on-demand light manipulation functionality, is not compatible with the desired goal of multifunctional flat optics. Here, we report on a generic concept to control multifunctional optics by disordered (random) gradient metasurfaces with a custom-tailored geometric phase. This approach combines the peculiar ability of random patterns to support extraordinary information capacity, and the polarization helicity control in the geometric phase mechanism, simply implemented in a two-dimensional structured matter by imprinting optical antenna patterns. By manipulating the local orientations of the nanoantennas, we generate multiple wavefronts with different functionalities via mixed random antenna groups, where each group controls a different phase function. Disordered gradient metasurfaces broaden the applicability of flat optics as they offer all-optical manipulation by multitask wavefront shaping via a single ultrathin nanoscale photonic device.

On-chip metamaterials with a refractive index of zero shows extreme physical properties such as infinite phase velocity and wavelength. It also has several potential integrated-photonics-related applications including super-couplers, surface emitting lasers, and phase-mismatch-free nonlinear optics. Silicon-on-insulator (SOI) platform received much attention recently due to its compatibility with complementary metal-oxide-semiconductor (CMOS) technology, which makes the mass production of photonic devices reliable. Current implementations of on-chip SOI-based zero-index metamaterials involve either metallic structures or high aspect-ratio silicon pillars, which require many processing steps, and is complicated to integrate with other photonic devices on a standard SOI wafer. Additionally, metamaterials involving metallic structures cost highly.
Here, we design an on-chip zero-index metamaterial consisting of square array of air-holes in the 220-nm thick top-silicon layer of a standard SOI wafer. This structure can be fabricated through the single-step E-beam lithography and fully-etching, which reduces the processing steps and the fabrication difficulties significantly.
Simulated effective permittivity and permeability cross zero simultaneously and linearly at the design wavelength of 1550 nm, with a finite effective impedance. Computed band structure shows that this epsilon-and-mu-zero behavior corresponds to a Dirac-cone dispersion at the center of the Brillouin zone (Gamma point), indicating a relatively isotropic zero index. This Dirac-cone dispersion is formed by the degeneracy between a quadrupole mode and two degenerate dipole modes at the Gamma point. All these results indicate that the zero index of this metamaterial is low-loss, isotropic and with a good impedance matching to free space and standard optical waveguides.
To directly demonstrate the zero index of this metamaterial, we plan to measure the refraction of a prism made of this metamaterial. We also plan to retrieve the complex index and impedance of the metamaterial from the complex transmission and reflection coefficients measured using on-chip Mach-Zehnder-interferometer-based setups.
In conclusion, we demonstrate an on-chip CMOS-compatible all-dielectric metamaterial with a low-loss, isotropic, and impedance-matched zero index at 1.55 um.

When an array of small metallic particles is embedded into a dielectric matrix one should expect a pole in the polarizability of the medium at certain energy, when the negative real part of the dielectric function of the metal compensates the double value of the positive real part of the dielectric function of the surrounding dielectric material. This gives rise to so called Froehlich resonance in the optical properties of such metamaterial.
We investigated a resonance in optical absorption, which originates from localized plasmon excitations in a self-organized system of metal AsSb nanoparticles embedded in a semiconductor AlGaAs matrix.
The AsSb-GaAlAs metamaterial was produced by a low-temperature molecular-beam epitaxy on the (001) GaAs substrates followed by a high-temperature annealing. Our transmission electron microscopy study revealed a system of almost spherical AsSb inclusions in the crystalline AlGaAs matrix. The diameter of the inclusions was 4-7, 5-8 and 6-9 nm after annealing at 400, 500 and 600C, correspondingly. The filling factor was constantly 0.17%.
The Froehlich plasmon resonance in our metamaterial was revealed at 1.48 eV with a bandwidth of 0.18 eV. The absorption coefficient within the resonant band was as large as 9000 cm-1. No significant changes in the parameters of the resonance have been observed for different particle sizes, which is consistent with Mie scattering theory. In theoretical calculations we used well documented data for the dielectric properties of AlGaAs and Drude model for the electron system of the metal AsSb nanoinclusions. A reasonably good description was achieved with plasmon resonance energy of 7.38 eV and damping time of 3 fs in bulk AsSb, which were fitting parameters of the model.

Relative placement of nanoparticles allows for fine-tuning of plasmonic, electronic, and magnetic interactions at the nanoscale. DNA-programmable assembly is a powerful means for controlling nanoparticle placement in extended crystalline materials imparting independent control over lattice size, spacing, and composition. Investigations into light-programmable matter interactions of these nanoparticle assemblies demonstrate great promise for engineering three-dimensional metamaterials by utilizing the precise materials tunability of this technique. However, these nanoparticle superlattices previously existed either as mu;m-scale aggregates or epitaxially grown structures bound to a substrate.
The desire to integrate these nanoparticle assemblies into higher order constructs has led to the investigation of transferable nanoparticle superlattice materials. We describe a novel method for synthesizing freestanding and transferrable nanoparticle superlattice sheets made through DNA-programmable techniques to flat, curved, or dimpled substrates without sacrificing the control over lattice symmetry and parameter afforded by the DNA-programmable technique. Grazing incidence small angle X-ray scattering and optical spectroscopy studies performed on these novel materials suggest preservation of nanoparticle superlattice ordering through the sheet synthesis process as well as large area uniformity. Moreover, these remarkable silica-embedded structures can withstand harsh chemical, physical, and mechanical conditions—an important discovery that will likely increase their utility.
This technique will provide an avenue for researchers to consider using such nanoparticle superlattices for a wide variety of sensor, cloaking, and light manipulating studies and devices.

In 2006 we introduced the concept of epsilon-near-zero (ENZ) structure and its supercoupling effects [M. G. Silveirinha and N. Engheta, “Tunneling of Electromagnetic Energy through Sub-Wavelength Channels and Bends Using Epsilon-Near-Zero (ENZ) Materials”, Phys. Rev. Lett., 97, 157403 (2006). Since then numerous properties of ENZ materials have been investigated extensively, and also expanded to include other extreme scenarios. One of the interesting paradigms of such extreme metamaterials are the structures in which the relative permeability attains near-zero values, while the permittivity exhibits conventional positive values. Such engineered mu-near-zero (MNZ) materials offer an interesting platform for wave-matter interaction with exciting features. In the present work, we show, both theoretically and experimentally in the microwave domain, how the supercoupling phenomenon between highly mismatched waveguide sections is different from that of the ENZ scenario. One major difference is the required height of the MNZ transition channel, which needs to be disproportionaly wider than the height of the input and output waveguides, contrary to the ENZ case where the ENZ channel had to be much narrower. Importantly, in the MNZ regime the electromagnetic field in the channel is dominantly magnetic. This offers an interesting possibility for tailoring and enhancing the radiation by small magnetic emitters, which is usually weaker than that of electric dipole emitters for which the ENZ structure is suitable. We will present our theoretical results and experimental verification of the MNZ supercoupling effects using the microwave waveguides with the transition region constructed to exhibit MNZ response. There are other intringuing features specific to the MNZ structures, which will be discussed in the presentation.

Metamaterials have become one of the most exciting fields in optics due to their exotic optical properties and important applications that are not attainable from naturally occurring materials. In particular, broken symmetry in metamaterials can lead to extremely strong effects of anisotropy (broken rotational symmetry), chirality and bianisotropy (broken mirror symmetry), which go far beyond the properties of nature materials. In this talk, I will show that by combining chirality with strong anisotropy, new topological order emerges in metamaterials leading topologically protected photonic surface states that are immune from scattering by defects and sharp edges. In addition, I will talk about our recent works on optical metasurfaces, a new type of structured surfaces showing well controlled abrupt phase discontinuities for circularly polarized incident light arising from Berry phase. A number of novel device applications have been realized based on the Berry phase metasurfaces: a dual polarity metalens that can functions either as a convex or a concave lens, a helicity switchable unidirectional surface plasmon polariton coupler, and high definition, high efficiency holograms. Finally, I will show that extending the concept of Berry phase to harmonic generations can lead to continuous phase control over the local nonlinear polarizability, which goes beyond the conventional poling technique that only manipulate the sign of the nonlinear coefficient, i.e. binary phase profile.

Optical metamaterials provide novel ways of manipulating light, allowing effects such as negative refraction,¹ optical phase tuning,² off-specular reflection following generalized Snell-Descartes laws,² enhancement of the quantum yield of light emitters and tuning of their radiation spectrum and pattern,³ near-perfect optical absorption,⁴ topological optical darkness.⁵ These optical meta-effects open the way to the development of lightweight and ultracompact photonic solutions including: flat optical components, data encryption media, integrated high efficiency lighting and optically-monitored sensors.
Most of the existing optical metamaterials operate in the near infrared to radiofrequency range and are based on noble metal plasmonic structures. Making them suitable for the visible or ultraviolet ranges is a still challenging task, since it demands a nanoscale structuration at the limit of lithography techniques. In addition, tomorrow&’s photonic solutions appeal at the development of the so-called “reconfigurable metamaterials”⁶ presenting reversibly switchable optical meta-effects under external excitation. Achieving reconfigurability together with optical meta-effects in the visible and ultraviolet requires to identify and properly assemble nano-elements, beyond the rather inert noble metals and even beyond plasmonic elements.
In this presentation, we first identify the most promising non-conventional nano-elements for building reconfigurable metamaterials at visible and ultraviolet frequencies. We make a special emphasis on those whose dielectric function is sensitive to external parameters (light, magnetic field, heat, pressure, chemical agentshellip;). Among them, semi-metallic nano-elements (Bi, Ga, Sb) are especially relevant due to: i) their solid-liquid phase transition that can be activated by thermostatic, laser, Joule or plasmonic heating, ii) the strong dielectric contrast between the solid interband polaritonic phase and liquid plasmonic phase in the ultraviolet and visible. We focus the second part of the presentation on the demonstration of optical meta-effects and reconfigurability in metamaterials built from nano-Bi.
1 Naik, G. et al.; Demonstration of Al:ZnO as a plasmonic component for near-infrared metamaterials. PNAS 2011, 109, 8834
2 Yu, N. et al.; Light propagation with phase discontinuities: Generalized laws of reflection and refraction. Science 2011, 334, 333
3 Yang, T.B. et al.; Real-time tunable lasing from plasmonic nanocavity arrays, Nature Comm.2015, 6, 1-7
4 Hägglund, C. et al.; Self-Assembly Based Plasmonic Arrays Tuned by Atomic Layer Deposition for Extreme Visible Light Absorption, Nano Lett., 2013, 13, 3352
5 Kravets, V.G. et al.; Singular phase nano-optics in plasmonic metamaterials for label-free single-molecule detection. Nature Mater. 2013, 12, 304
6 Turpin, J.; Reconfigurable and tunable metamaterials: a review of the theory and applications, Int. Journ. Ant. Prop. 2014, 429837

Metamaterials, designed composite structures with unprecedented properties and applications, have emerged as a new frontier of science and technology. As the properties of metamaterials are primarily from their structures rather than their chemical constituents, controlling geometrical effects such as symmetry and spatial arrangements play fundamental roles in metamaterials research. Metamaterial with designed symmetry that can be realized by top-down fabrication methods such as lithography, but often result in planar and small-scale metamaterials. In contrast, self-assembly approaches, may offer advantages of large scalability and cost effectiveness. However, there are significant challenges in achieving controlled assembly symmetries of metamaterials in large scale due to constraints of thermodynamics and crystal lattice mismatching between designed composites. Here we show a scalable synthesis route that enables high level control of spatial assembly symmetries, creating large-scale metamaterials with desired properties. Combing optical physics and materials chemistry, we demonstrate scalable assembly of non-trivial broken symmetries in metamaterials by optical feedback strategy. Moreover, by rational engineering symmetry configurations and coupling of unit cells, three dimensional (3D) isotropic optical negative index metamaterials can be scalable fabricated that is of paramount importance for applications such as superlens and transformation optics. Our method not only offers a scalable and sustainable manufacturing technique for 3D metamaterials, but also provides a platform for studying symmetry-based physics and applications.

Planar metal/dielectric multilayer geometries provide a unique platform to engineer the dispersion of light in many different ways. We present a novel geometry to realize a flat lens composed of a single-periodic metal/dielectric multilayer stack. We use the hyperbolic dispersion of a multilayer composed of very thin layers as a starting point, and gradually increase the layer thicknesses. We find that fully angle-independent dispersion can be achieved (wavelength 354 nm) for a multilayer composed of 53 nm silver and 25 nm titanium dioxide films, characterized by an omnidirectional negative refractive index n=-1.
Confocal and near-field microscopy show that such a thin multilayer slab acts as a flat lens, with unique features such as a submicron thickness, the absence of an optical axis and a lens-image separation of only 350 nm. The experimental data are in good agreement with analytical Green&’s tensor calculations of the focus profile above the metamaterial lens. We use analytical field calculations to calculate the direction of the Poynting vector, taking into account losses and dispersion, and find that the unit-cell and time-averaged Poynting vector of all individual harmonics of the Bloch wave are oriented in the same direction. The image is formed by the coherent superposition of refracted light from multiple harmonics in the metamaterial.
We show that by replacing the Ag layer by a transparent conducting oxide film the lens can be made to operate anywhere in the UV, visible, and near-IR spectral range. Specifically, we show that using aluminium-doped ZnO a flat lens can be made operating in the infrared telecom range near 1.5 micron. The electrical tunability of the focusing characteristics will be presented as well. The new design presented here provides the simplest flat lens geometry proposed to date.
Inspired by the versatility and elegance of the multilayer design, we present a dielectric multilayer material that serves as anti-reflection coating for stratified media, by matching the modal field profiles across the interface, thus solving the impedance matching problem often found with layered metamaterials. We then show how a dielectric multilayer structure can act as an angular filter, which can be used to control the emission characteristics of a light-emitting diode. Finally, we show how a layered metamaterial combined with a special cavity design creates a wave-attracting cavity resonance at optical frequencies. We show that for circularly polarized light the plasmonic spin-Hall effect can be exploited to achieve a deterministic wave attractor for light emitters in the cavities&’ near field

Plasmonics, which offers unparalleled capability for light confinement and guiding at the sub-wavelength scale, is an emerging technology for realization of compact devices for a wide range of applications in biosensing, energy conversion, nanofabrication, nanoscale resolution imaging and enhancement of spontaneous emission. Advances in materials synthesis has paved way for the realization of alternative plasmonic materials. Among the alternative plasmonic materials, transition metal nitrides such as TiN and ZrN have attracted significant attention owing to their plasmonic resonance at visible and NIR wavelengths, and robust physical properties including high melting point, which make them promising for devices that operate under harsh conditions.¹ We expand on the photonic devices employing alternative plasmonic materials by exploring their applications for realization of novel plasmofluidic devices.
Plasmofluidics, which is the synergy between optofluidics and plasmonics enables new possibilities in micro and nanofluidics including particle manipulation, sorting, sensing and ultrasensitive spectroscopy on a lab-on-a-chip platform.² We report a plasmofluidic device for on-chip concentration and sensing of particles in a self-contained lab-on-a-chip platform using arrays of plasmonic TiN nanoantennas.
The device comprise plasmonic nanoantennas fabricated on a glass substrate (coated with a thin layer of electrically conducting TiN film). The plasmonic nanoantennas comprise of TiN nanodisks with diameter of 270 nm, lattice spacing of 370 nm and thickness of 30 nm. An ITO-coated glass substrate is placed over the substrate having the plasmonic nanoantennas, with a 90 µm thick microfluidic channel in between. Illumination of the plasmonic nanoantennas with a 1064 nm NIR laser source results in collective photo-induced heating of the nanodisks, which in turn induces local inhomogeneities in the fluid&’s electrical properties. When an AC electric field is applied, a large-scale vortex is induced. The vortex rapidly captures and transports the suspended particles with high throughput towards the surface of the TiN nanoantennas where they are captured by local electric field effects. Trapping of particles in this device configuration is shown to occur in seconds effectively beating the diffusion limit. Our TiN-based plasmofluidic device provide an all-in-one multipurpose integrated analytical platform for capture, transport, trapping, sorting and label-free sensing of analytes.
References:
1. Boltasseva, A. & Shalaev, V. M. Materials science. All that glitters need not be gold. Science347, 1308-10 (2015).
2. Ndukaife, J. C. et al. Photothermal heating enabled by plasmonic nanostructures for electrokinetic manipulation and sorting of particles. ACS Nano8, 9035-43 (2014).

Noble metals such as Au and Ag have been used traditionally as metallic components in plasmonic and metamaterial devices in the visible spectral range because of the relatively low optical losses. However, conventional metals with high carrier concentrations are not suitable for near infrared (IR) plasmonic applications due to their relatively large optical losses, which are detrimental to the performance of plasmonic devices. Thus, it is necessary to develop less metallic materials (i.e., lower carrier concentrations) as an alternative for traditional metals in the near IR. Doped metal oxides have been recognized as low loss metallic components for plasmonic and metamaterials applications in the near IR because they offer a tunable carrier density in the range up to 10²¹ cm^-3. The zero-cross-over permittivity values of these metal oxides can easily be tuned by adjusting carrier density through doping in the range from 1.3 µm to 3 µm with a relatively lower optical losses compared to what is achievable with conventional metals in the near IR. We have investigated various types of metal oxides, such as Al-doped ZnO, Ga-doped ZnO, Sn-doped In₂O₃, and VO₂ using pulsed laser deposition. We will present details on the synthesis and optimization of these metal oxides along with tunable plasmonic devices based on these materials.
This work was funded by the Office of Naval Research (ONR) through the Naval Research Laboratory Basic Research Program.

Metamaterials enable unprecedented flexibility in manipulating electromagnetic waves. The optical response of most metamaterials is static, modified only by adjusting the geometric parameters of the constituent building blocks. Many functionalities of metamaterials may be greatly enhanced by hybridizing these materials with functional matter, like phase-change materials, where the dielectric properties can be controlled in real-time by an external stimulus such as an applied electric field, light, mechanical stress or temperature.
One of the most widely studied phase change materials is vanadium dioxide (VO₂), which exhibits a reversible insulator to metal transition (IMT) as the temperature is increased above a critical temperature T_C ~ 68°C. The low-temperature insulating phase of VO₂ is considered to be a Mott-Peierls insulator wherein both electron-electron correlations and dimerization of vanadium ions contribute to the opening of an insulating gap. Thus, the IMT is very sensitive to the stability of the electron hybridization and therefore on electronic doping, structural defects and lattice strain, making ion beam irradiation a particularly effective technique to modify the IMT.
We demonstrate how ion beam irradiation can be used to modify and engineer the VO₂ phase transition via the intentional creation of defects and lattice damage (“defect engineering”). Rutherford backscattering spectrcoscopy and optical measurements show, that the presence of a small amount of structural defects caused by ion irradiation significantly decreases the transition temperature - even below room temperature - of the irradiated regions. Unlike existing means to modify the IMT via doping during growth, ion beam irradiation can be combined with lithographic patterning to create complex optical meta-devices with designer phase transitions. Using this approach we demonstrate ultra-thin (thickness ~ lambda;/100) temperature-tunable perfect absorbers and reconfigurable polarizers for the mid-infrared.

Metasurfaces, the 2D equivalent of metamaterials, offer new functionality for the manipulation and study of light and light-matter interaction. Incorporating and taking advantage of optical nonlinearities in metasurfaces further expands the available toolbox because it facilitates access to light at different frequencies. Until recently, studies of nonlinearities in metasurfaces utilized the intrinsic nonlinearity associated with the metal used to construct the individual units in the metasurface array. Here, we will discuss a new approach that combines a highly nonlinear material with the field enhancing and light manipulation properties of metamaterial nanocavities. This combination allows for a much higher effective nonlinearity, can be dynamically tuned and it can span most of the IR wavelength range. As an example we fabricate a phased array metasurface based on second harmonic generation from metallic nanoresonators strongly coupled to intersubband transitions in semiconductor quantum wells. We realize a mid-infrared source with an arbitrary beam shape and polarization, having a total thickness of less than one micron. Using III-Nitrides semiconductor heterostructures, we scaled these highly nonlinear metasurfaces to the near-infrared.

As the expansion of fundamental and application research in plasmonics, there is an increasing demand on better materials. In addition to the demand for lowering optical loss, another important and required property is the processability. Although gold and silver show excellent plasmonic responses, they cannot be nanostructured with selective dry etching techniques. This limitation makes the fabrication of nanostructures of gold and silver complex and tricky. Titanium nitride (TiN) has been proven to be a promising material having the compatibility with nanofabrication techniques [1]. TiN, which is composed of abundant elements of titanium and nitrogen, has gold-like optical properties together with a high thermal stability and a mechanical toughness. The thin-film fabrication techniques have been established for TiN because of its technological and industrial importance.
In this study, we have fabricated two-dimensional diffractive arrays of TiN nanoparticles by taking advantage of its compatibility with nanofabrication technique. High-quality, epitaxial thin films of TiN were prepared by using a pulsed laser deposition method. The thin films prepared were structured to the arrays of nanoparticles with the pitch of 400 nm by the combination of nanoimprint lithography and reactive ion etching. The choice of periodic array comes from its ability to support collective plasmonic mode; thanks to the periodicity on the order of lightwaves, the localized surface plasmon polaritons excited on each nanoparticles are coupled through diffraction [2]. The collective mode is associated with the intense field spatially extended in the plane of the array, so that it is advantageous for many optical applications including fluorescence enhancement, surface-enhanced Raman scattering, and solar cells. The results of optical transmission indicate that the arrays support the collective plasmonic modes. Numerical simulation visualizes the intense fields accumulated both in the nanoparticles and in between the particles, confirming that the collective mode originates from the simultaneous excitation of localized surface plasmon polaritons and diffraction.
This study experimentally verified that the processing of TiN thin films with the nanoimprint lithography and reactive ion etching is a powerful and versatile way of preparing plasmonic nanostructure.
References
[1] G. V. Naik, J. Kim, and A. Boltasseva, Opt. Mater. Express 1, 1090 (2011).
[2] G. Vecchi, V. Giannini, and J. G. Rivas, Physical Review B 80, 201401(R) (2009).

Transition metal nitrides have recently garnered much interest as alternative materials for robust plasmonic device architecture including applications in solar absorbers, heat-assisted magnetic recording, photothermal cancer therapies, etc. Titanium nitride (TiN) is one such potential candidate with proof of concept demonstrations in several of these areas. One advantage of the transition metal nitrides is that their optical properties are tunable according to the deposition conditions. The controlled achievement of tunability, however, is also a challenge. Although the formation of TiN has been the subject of numerous previous studies, a thorough analysis of the deposition parameters necessary to form metallic TiN films optimized for plasmonic applications has not been demonstrated. Similarly, such TiN films have not been subjected to detailed optical measurements which could be used in FDTD device simulations to optimize plasmonic device designs.
To be able to design, simulate and build robust and optimal device structures, we have conducted a systematic and thorough examination of the effect of varied substrates, temperatures, and reactive gas compositions on sputtered TiN thin films with a specific focus on the resulting optical properties at visible to NIR frequencies. We measured the optical properties of each film via spectroscopic ellipsometry with more “metallic” films demonstrating a larger negative value of the real part of the permittivity. We have correlated these optical measurements with both the films&’ deposition conditions and microstructures; the different deposition conditions have resulted in TiN films with different optical responses. By sputtering under different conditions we are able to tune the value of the permittivity from small positive values, through small and moderate negative values, and finally all of the way to large negative values which are comparable to those measured in gold.
In summary, we have performed an initial optimization of sputtered TiN films with desirable plasmonic behavior on a variety of commonly used substrates and are now able to tune the films over a wide range of values of permittivity at longer visible into near-IR wavelengths. Initial plasmonic demonstrations have proven the films to be of high optical quality.

Exciting collective oscillations of conduction electrons in metal nanostructures by electromagnetic fields, known as localized surface plasmon resonance (LSPR), provides an important approach to manipulate light in sub-wavelength spaces. With the unique capability of LSPR to strongly interact with resonant photons and overcome diffraction limit, plasmonic metal nanostructures have been employed in many applications, including surface-enhanced spectroscopy, high-resolution microscopy, optoelectronics and photocatalysis. Au nanostructures have been extensively studied as plasmonic materials, which exhibit excellent plasmonic effects and stability in different environments. Their resonant frequencies, however, are limited in the visible and near-infrared (NIR) regions, due to the existence of interband transitions. As the application of plasmonic nanostructures extending to the ultraviolet (UV) region, a counterpart of Au that can be operate in UV is desired but not available until our recent experimental and theoretical “re-exploration” of Rh nanostructures. Rh, a noble metal with excellent stability, has been widely used as catalyst, but little attention has been paid to its plasmonic properties. Thus, a systematic investigation of structure-property relationship of Rh nanostructures and UV plasmonic properties is needed. This presentation reports our recent progress on Rh-based UV plasmonic materials. We developed modified polyol methods with both seedless and seed-mediated protocols to synthesize Rh nanostructures. By tuning the reaction conditions, Rh nanostructures with well-defined shapes and tunable sizes were successfully synthesized. Their plasmonic properties were studied by spectroscopic methods as well as theoretical simulations. UV-vis extinction spectroscopy revealed the existence of LSPR in the Rh nanostructures and showed the shift of resonant frequencies with different sizes, which is in good agreement with our collaborators&’ simulations. The plasmonic properties of these Rh nanostructures were also studied by surface-enhanced Raman spectroscopy. The UV plasmonic Rh nanostructures and the understanding of structure-property relationship could benefit both fundamental study and practical use of UV plasmonic materials.

All optical control of light holds great promises for optical switching, photonic circuits, telecommunication and molecular sensing. Ideally light modulation schemes would possess wide spectral range, large tunability, small optical losses, material stability, and ultrafast modulation speeds. However, combining all these merits simultaneously in practical operation is still very challenging. Here we report a lossless, super-broadband and enhanced optical modulation enabled by indium-tin-oxide nanorod arrays (ITO-NRAs). Large modulation of differential and absolute transmissions of ITO-NRA in the ultraviolet to visible (UV-vis) range was achieved by pumping at its metallic, near-infrared (NIR) localized surface plasmon resonance (LSPR) wavelength. The spectral feature is easily tunable by sample geometry design, and the ITO-NRA shows extremely high pump fluence tolerance due to its chemical stability and high melting point. The optical modulation exhibits both an ultrafast, sub-picosecond component and a slow, multi-nanosecond component. The spatially uniform ITO-NRA also exhibits highly coherent acoustic vibrations at gigahertz (GHz) frequency that further expands the modulation bandwidths. Our work brings new opportunities for active optical control via alternative plasmonic materials.

Metal and metal oxide electrodes play a significant role in many cutting edge applications including photonics, membranes, biological supports, sensing, electrochromics, and in various green technologies, such as, photocatalytics, Li-ion batteries and photovoltaics. There is strong interest in the ability to create one-dimensional nanoscale metal and metal oxide electrode structures that provide high surface area, tunability of the electrode - organic interfaces, and low tortuosity for improved electron / hole transport characteristics. Interest in patterning polymer based nanodevices and creating sub-100 nm metal and transparent conducting oxide (TCO) based nanostructured electrodes (NSEs) has led us to modify the traditional imprint lithography technique to enable synthesis of an array of sub-30 nm diameter polymer nanostructures. In this approach, a hard e-beam lithographed Si or SiC master is used to directly imprint a large area nanopattern onto polyacrylonitrile (PAN) film. The PAN film is then cured at ~ 200 °C to synthesize nanostructures. Large area nanostructured hybrid silver and indium tin oxide (ITO) arrays with feature sizes below 100 nm have been fabricated. The optical and electrical properties of these core shell electrodes including the surface plasmon frequency can be tuned by suitably changing the dielectrics and their dimensions. The surface plasmon wavelength of the nanopillar Ag changes from 650nm to 690nm depending on the dimensions of the pillars. Adding layers of ITO to the structure shifts the resonance wavelength toward the infrared region by an amount depending on the sequence and thickness of the layers in the structure. Quantum confinement of the free carriers in ITO is expected within the 30nm thick shell and contributes significantly in the overall optical and electrical properties. The NSITO is more transparent across the entire spectrum and shows lower specular reflection. The band edge of the NSITO is red shifted and shows a second smaller optical band gap ~3.25 eV in addition to bulk optical band gap at 3.55 eV. Recently we have shown that the optical band gap can be fine tuned by changing the shell thickness on the sidewall. NSITO also shows lower specular reflection and calculated carrier concentration is ~ 5 - 6 times larger than the flat ITO sample and a strong function of the shell thickness.We developed an array of electrodes with traditional SiO₂ / Ag multi-layer configuration within the nanorods in different orders. Expectedly the optical characteristics of these electrodes are strong functions of geometry of the layers and surrounding dielectrics and can be explained using traditional hybrid plasmonic model based on Drude model. Detailed FDTD modeling and electrical characterization indicate that by changing the layer dielectrics optical and electrical properties can be modified.

Invisibility cloaks have potential uses ranging from masking objects from light to shielding objects from shockwaves. To be cloaked, an incident wave is reassembled to continue onward as if there was no disruption by, or to, the object. Any wave type may be cloaked as long as materials with the appropriate characteristic wave interactions are made available. Invisibility cloaks are advancing, but are limited by their mathematical representations.
Currently, there are two principal, fundamentally different, approaches for the mathematical representation of a cloak. One is termed transform cloaking, and the other approach is scatter-cancellation cloaking. Each approach can only provide a partial approximation to the perfect cloak in real world applications. Transform cloaking bends a wave around an object. Scatter-cancellation cloaking uses the scattering off of many bodies to negate the incident wave perturbation. Scatter-cancellation cloaking operates more effectively at smaller scales, and is more effective over a wider wavelength range, but is shape-constrained. Transform cloaking can function for any size or shape object; however, while theoretically better for individual frequencies, it is problematic to approximate with real world materials. Challenges to the construction of transform cloaks arise from the continuous mapping function used to bend the wave by compressing the cloak/object space. For example, this compression would require materials with unrealistically high and rapidly varying indices of refraction. Here, a synergistic, combination of these approaches is proposed and demonstrated.
A novel hybrid cloak model is designed by optimizing the position and properties of intrusions embedded in a cloaking space composed of a matrix of shells surrounding the object. Variation of the intrusion geometry, density and spatial location is used to optimize the scattering contribution to the cloaking efficacy. Variation of the index of refraction within individual shells is used to optimize and control the transform contribution to the cloaking efficacy. The resultant hybrid cloak was analyzed for phase shifts; time delays for the exit wave to arrive beyond the cloaking region and to reach a steady state; and the extent of wave scattering. The combined cloak is compatible and transferable to real world material characteristics and outperformed each separate model alone.

The research field of strong exciton-plasmon coupling has drawn much interest over the last decade. The strong coupling leads to formation of split hybridized energy states, which are different from those of constituent components. Traditionally, strong coupling is studied by means of optical spectroscopy, which combines features of both (hybridized) HOMO and LUMO molecular states. In order to decouple contributions of HOMO and LUMO to the overall spectra, we combined optical spectroscopy with cyclic voltammetry -the electrochemistry technique that probes the oxidation and reduction potentials (e.g. transitions between the HOMO and the "vacuum" states).
Experimentally, we have studied interaction of highly concentrated dye molecules with thick silver films and films of Ag nanoislands featuring strong surface plasmon resonance. (i) In thick Ag films, broadening and splitting of the HOMO state, which was observed in the cyclic voltammetry experiments, was also seen in the optical spectroscopy spectra. (ii) In Ag nanoisland samples, strong exciton-plasmon coupling manifested by Fano resonances observed in the reflection spectra. Lowering of the HOMO energy state (in comparison to that in thick Ag films) was observed in the cyclic voltammetry experiments. More experimental results and analysis will be presented at the conference.
The authors acknowledge NSF PREM grant DMR 1205457, NSF IGERT grant DGE 0966188, ARO grant W911NF-14-1-0639, and AFOSR grant FA9550-14-1-0221.

We will present a simple and robust crystal growth technique, which yields large area single-crystal films of silver ideally suited for fabricating high-finesse plasmonic metamaterials. The stacked gold/silver films with the total thickness of 140 nm were epitaxially grown on LiF substrates around 500°C by using a sputtering method. We confirmed the high quality of the thin films by measuring the surface roughness using scanning probe microscope (SPM) and the optical constants using ellipsometry.
Metamaterials are a class of artificial materials designed to interact with light in ways no natural materials can.^1,2 Due to its resonant nature, the response of the metamaterials is very sensitive to the presence of dissipative losses in the metallic resonators. The losses are particularly strong in the plasmonic regime and are hampering the use of metamaterial devices for photonic applications. The several approaches to overcome the losses including the search for better plasmonic media among metallic alloys, semiconductors, and conductive oxides,^3-5 as well as direct compensation of losses by combining metamaterials with various optical gain media⁶ were reported. Those solutions, however, aim to minimize Joule losses, while in practice dissipation rates are often much higher than expected from the Ohm&’s law alone. The additional significant drawback comes from surface roughness and grain boundary scattering due to polycrystalline nature of metal films,⁷ and therefore employing single-crystal of noble metals can make a major contribution to the improvement of plasmonic losses.
We demonstrated that single-crystal gold thin films grown on LiF substrates had smooth surfaces with root mean square (RMS) roughness of 0.2 nm and nanostructured metamaterials fabricated with these films had strong resonant response in the near-IR spectral range.⁸ In this work, we have developed a single-crystal silver thin film growth technique in order to reduce the losses further and extend the response wavelength. The SPM measurements showed the gold/silver films had remarkably smooth surfaces with RMS roughness less than 0.1 nm. The ellipsometry measurements showed that strong interband transitions occurred at the wavelength of 300 nm. The estimated quality factor of localized surface plasmon resonances from the measured optical constants is 20, which is two times larger than the single-crystal gold films on LiF substrates. We believe that this technique could make it readily accessible to both plasmonics and metamaterials communities.
References
[1] V. G. Veselago et al, Nature Mat. 5, 759 (2006).
[2] N. I. Zheludev, Science 328, 582 (2010).
[3] D. A. Bobb et al, Appl. Phys. Lett. 95, 151102 (2009).
[4] M. G. Blaber et al, J. Phys: Cond. Mat. 21, 144211 (2009).
[5] A. Boltasseva et al, Science 331, 290 (2011).
[6] S. M. Xiao et al, Nature 466, 735 (2010).
[7] M. Kuttge et al, Appl. Phys. Lett. 93, 113110 (2008).
[8] V. A. Fedotov et al, Optics Express, 20, 9545 (2012).

Plasmonic interferometers, which typically consist of slit-groove and hole-groove structures on a metallic film, have found a wide variety of applications, such as enhancement of light transmission through small apertures and label-free biosensing. In such devices, the scattering of light by nanostructures on the metallic surface may be employed for two distinct purposes: either the efficient coupling of free space light into surface plasmon polariton (SPP) modes, or the reverse process, in which SPPs are scattered into free space light. More recently, plasmonic interferometers have been proposed to engineer the radiation pattern of emitters (e.g., quantum dots and fluorescent molecules). For example, both slit-groove and hole-groove structures were applied to beam fluorescent emission based on the interference of the scattered light by the groove structures. In addition to coupling light into and out of free space, the groove structures can also act as a lossy mirror which reflects SPPs. In this way, we can identify two major optical paths in the emitter and plasmonic interferometer combination system: emitted light that directly propagates through the nanoaperture, and light scattered by the aperture itself, which propagates as an SPP initially towards the groove but is then reflected back towards the nanoaperture. Due to the interference of these two optical paths, the transmitted fluorescence spectra can be modified.
In this work, using a thin layer of trivalent chromium doped magnesium oxide (Cr³⁺:MgO) emitters on top of a metallic film, we systematically studied the spectral modification caused by a variety of plasmonic interferometer configurations. Nanostructures were first milled onto a thin silver film using a focused ion beam. A thin Cr³⁺:MgO emitter layer was then deposited on top of the silver. Initially, the transmitted fluorescence spectra through the slit-groove structure were measured and normalized by the reference spectrum from a single slit. Due to the interference of the directly emitted light and the reflected SPPs by the groove structure, clear spectral modification was observed, which could be modified by tuning the slit-groove distance. However, such spectral modifications are not strong considering the high losses upon reflection of the SPP. To further enhance the spectral modification, bull&’s eye structures with one-, two- and three-grooves were also studied, resulting in much stronger interference effects. We also note that the spectral modification is sensitive to the refractive index near the surface of the emitter layer. Therefore, the proposed emitter-coupled plasmonic interferometers have potential applications for biosensing and optical modulators.

Since several years ago, light is considered as a new way for futuristic computation, overcoming current limitations of electronics [1]. Researchers are currently working in the development and implementation of photonic devices that behave as a counterpart of electronic devices, in the so-called field of Metatronics [2]. One of the main challenges of this field is the implementation of optical circuitry. In this sense, photonic equivalent resistors, capacitors and inductors are needed. Engheta and coworkers [3] showed that nanostructures with convenient effective optical properties present impedance properties similar to that of these lumped elements. However, the main handicap of these devices lies on their complex implementation as well as their passive properties.
Current research on semiconductor and semimetal nanostructures is showing new impressive properties, such as optical magnetism [4]. In this work, we focus our attention on the impedance properties of bismuth (Bi) nanospheres. By a simulation tool implementing the finite elements method (COMSOL copy;), we observe that these nanoparticles have a remarkable capacitive impedance in the near-infrared part of the spectrum. Changing the particle size, the value of the impedance can be tuned through a wide spectral range. The simplicity of the structure and its fabrication makes it a good candidate as integrated nanocapacitor in potential optical circuits. In addition, by heating the nanoparticle over its melting point (T=2710C), its impedance changes fastly from purely capacitive to a purely inductive.
[1] D.A.B. Miller. Are optical transistors the logical next step? Nature Photon. 4, 3-5 (2010).
[2] H. Caglayan, S.-H. Hong, B. Edwards, C. R. Kagan, and N. Engheta. Near-Infrared Metatronic Nanocircuits by Design. Phys. Rev. Lett. 111, 073904 (2013).
[3] Y. Sun, B. Edwards, A. Alugrave; and N. Engheta. Experimental realization of optical lumped nanocircuits at infrared wavelengths. Nature Mater.11, 208-212 (2012).
[4] J.M. Geffrin, B. García.Cámara, R. Goacute;mez- Medina, P. Albella, L.S. Froufe-Pérez, C. Eyraud, A. Litman, R. Vaillon, F. González, M. Nieto-Vesperinas, J.J. Sáenz, F. Moreno. Magnetic and electric coherence in forward-and back-scattered electromagnetic waves by a single dielectric subwavelength sphere. Nature Commum.3, 1171 (2012).

Very recently sizable body of research has focused on transverse spin of evanescent electromagnetic waves. In beams that can be described by paraxial limit the spin angular momentum of light is a measure of its helicity and is either parallel or anti-parallel to the direction of propagation. In the case of non-paraxial or evanescent waves, however, fields with spinning axis transverse to the direction of propagation can exist. Here we calculate the transverse spin associated with a locally excited chiral surface plasmon polariton and show that it can exert chirality-specific forces on chiral objects of opposite handedness. We then comment on features of such a chiral force including the relative magnitudes of in-plane and out-of-plane forces.

Formation of metal nanostructures has attracted attention due to unique optical properties based on surface plasmon resonance (SPR). Various functional optical devices consisting of metal nanostructures have been proposed, for example, waveguide, metamaterial and so on. The performances of the devices are dependent on the geometrical structures and the arrangements of the metal nanostructures. However, the efficient fabrcation process of the geometrically-controlled metal nanostructures has not been established.
To fabricate metal nanostructures, a template process using nanoporous materials is used to be used. Anodic porous alumina is well known as a typical nanoporous material. Anodic porous alumina is obtained by anodizing Al in an acidic electrolyte. Metal nanostructures are formed by electrodepositing metal into the nanoholes in the anodic porous alumina. One of the advantageous points of using the anodic porous alumina is controllability of geometrical structures. The diameter and arrangement of the nanoholes in the anodic porous alumina can be easily controlled by changing anodizing conditions. In addition, the nanoholes have high aspect ratio. Until now, we have reported the fabrication of metal nanostructures and its application to plasmonic devices [1,2]. In this presentation, the fabrication process of geometrically-controlled metal nanostructures (nanohole and coaxial nanocable) with high aspect ratio using the anodic porous alumina is described. By using anodization and electrodeposition process, the metal nanostructures with high aspect ratio could be obtained. In addition, the application of the structures to three-dimensional plasmonic waveguides is described. It was observed that the light was effectively propagated in the plasmonic waveguides. It is expected that this fabrication process based on the anodic porous alumina can be applied not only to fabricate the plasmonic waveguides but also to form metamaterials requiring geometrically-controlled metal nanostructures.
[1] T. Kondo, H. Masuda, K. Nishio, J. Phys. Chem. C, 117, 2531 (2013).
[2] T. Kondo, N. Kitagishi, T. Yanagishita, H. Masuda, Appl. Phys. Express, 8, 062002 (2015).

Hyperbolic metamaterials (HMM) are non-magnetic anisotropic nanostructures that can support highly confined wavevector modes in addition to surface plasmon modes within the structure due to hyperbolic dispersion. They feature hyperbolic (or indefinite) dispersion because one of their principal components has the opposite sign to the other two. Their properties include the strong enhancement of spontaneous emission, diverging density of states, negative refraction and enhanced superlensing effects. Such metamaterials represent the ultra-anisotropic limit of traditional uniaxial crystals, having dielectric properties in one direction (ε > 0) but metallic properties in the other (ε < 0) and supporting high-wavevector propagating waves (bulk plasmon modes) due to hyperbolic dispersion. Here, we report the excitation and collection of those modes at optical frequencies using grating coupling principle. The design, fabrication and characterization of grating-coupled HMMs (GCHMMs) in a wide wavelength range, from visible to near infrared are presented. HMMs have many promising applications due to the existence of high-k modes, a concept that has been theoretically and numerically investigated by many research groups. However, the experimental verification of their existence in a multilayer setup is challenging because the high modal indices and deep subwavelength confinement.
We will discuss current and potential applications of GCHMMs in nanophotonics and bio-medical research. To demonstrate the suitability of our structure for sensing applications, experiments have been performed using a flow cell-based 2D GCHMM. A maximum sensitivity of 30,000 nm per RIU is obtained at near IR wavelength range, which is highest sensitivity reported in conventional plasmonic sensors.
REFERENCES
[1] Sreekanth, K. V., De Luca, A. and Strangi, G., “Negative refraction in graphene-based hyperbolic metamaterials,” Appl. Phys. Lett., 103, 023107, 2013.
[2] K. V. Sreekanth, A. De Luca, and G. Strangi, “Experimental Demonstration of Surface and Bulk Plasmon Polaritons in Hypergratings” Scientific Reports (Nature Publishing) 3, 3291(2013)
[3] K. V. Sreekanth, K. Hari Krishna, A. De Luca and Giuseppe Strangi “Large Spontaneous Emission Rate Enhancement in Grating Coupled Hyperbolic Metamaterials” Scientific Reports (Nature Publishing) 4, 6340 (2014)

Plasmonic metamaterials are ideal platforms for enhanced Raman spectroscopy techniques that can be used to detect and identify molecules at extremely low concentrations. Engineered structures that localize optical fields enable chemical analysis with high sensitivity and at low detection levels, and can be designed to do so over large detection areas. Here, we present two metamaterial-like architectures designed for surface enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) sensing. These architectures consist of colloidal Ag nanoparticles assembled on either a flat metal surface or an AFM tip. In both cases, a high quality plasmonic cavity is generated between the nanoparticle and the underlying metal film, where we locate our analyte. We demonstrate large-area SERS maps of patterned molecular monolayers with ~1 mu;m resolution and signal variance of < 10%. We also demonstrate sub-diffraction resolution TERS maps of monolayer-patterned surfaces with pattern features of ~0.6 mu;m.

Phase change materials add functionality and increase the range of applications for metamaterial and plasmonic structures by allowing dynamic control of the optical properties. Vanadium dioxide (VO₂) exhibits a structural phase transition at 67°C with a large change in optical and electrical properties. VO₂ has been studied extensively as a model correlated electron system and has also been used for active plasmonic and metamaterial devices. Tuning the phase transition temperature of VO₂ is desirable for many applications; by adding tungsten the phase transition temperature can be reduced by approximately 20°C per atomic percent. We characterize the optical properties, electronic structure and phonon behavior of pure and tungsten doped VO₂ thin films on silicon, above and below the transition temperature. The optical constants are measured by spectroscopic ellipsometry in the wavelength range from 0.37-37 µm. We observe softening of the Raman and infrared active phonon modes and a non-linear trend in optical properties with increased tungsten concentration. A non-linear shift in the electronic transition energies between the vanadium and oxygen energy levels is also observed. We will present the experimental results and discuss them in the context of other work and use simulations to explain the behavior.

Molding the wavefront of light is a basic principle of any optical design. In conventional optical components such as lenses and waveplates, the wavefront is controlled via propagation phases in a medium much thicker than the wavelength. Metasurfaces instead typically produce the required phase changes using sub-wavelength-sized resonators as phase shift elements patterned across a surface. This “flat optics” approach promises miniaturization and improved performance. Here we introduce metasurfaces which use dielectric ridge waveguides (DRWs) as phase shift elements in which the required phase accumulation is achieved via propagation over a sub-wavelength distance. By engineering the dispersive response of DRWs, we experimentally realize high resolving power meta-gratings with broadband (l = 1.1-1.7 microns) and efficient routing (splitting and bending) into a single diffraction order, thus overcoming the limits of blazed gratings. In addition, we demonstrate polarization beam splitting capabilities with large suppression ratios.

We demonstrate a new technique to make a metal film patterned with a dense array of sub-10-nm annular gaps. This metasurface is created by high-throughput atomic layer lithography, which utilizes the angstrom-scale thickness resolution of atomic layer deposition to pattern ultra-thin nanogaps in metal films. The measured optical transmission spectra through the coaxial waveguide array show intense resonances at infrared frequencies. By shrinking the gap size from 10 to 1 nm, the resonance wavelength can be pushed toward longer wavelengths in the mid-infrared (MIR) regime. For the smallest gap size of 1 nm, the field intensity in the gap can increase by over 1000 times. Such extreme confinement and enhancement of MIR radiation is a unique feature of this resonant nanogap structure. Our experimental results can be explained by theoretical calculations and agree well with full three-dimensional computational modeling. Because the resonances of these ultra-thin coaxial waveguides can be tuned over broad frequencies by changing the geometry - gap size, ring diameter, film thickness - of the structure, the intense fields of this highly confined mode will benefit many applications in nonlinear optics, surface-enhanced spectroscopies, optical trapping, and metamaterials.

Chiral metamaterials are artificial materials constructed from chiral elements. Their structural chirality leads to chiroptical effects, such as optical rotation and circular dichroism. The unique optical property of chiral metamaterial enables applications such as negative refraction, manipulation of light polarization, super-chiral field based sensing, etc. The ideal chiral metamaterial or meta-device for practical applications should be prepared in large scale on substrates with high and tunable chirality. Here, we report a simple and scalable method to fabricate chiral metamaterial, which combines dynamic shadowing growth and self-assembled colloidal monolayers. Fan-shaped chiral nanostructures are created by Ag vapor depositions on monolayers of polystyrene nanospheres in three different azimuthal directions. The shape and chiroptical response of nanostructures strongly depend on the monolayer orientation with respect the incident Ag vapor. A systematic study reveals that fan-shaped nanostructures created at specific monolayer orientation can exhibit extremely large chiroptical response. Such fan-shaped nanostructures can be realized in large scale (~ 1 cm²) by Ag depositions on monocrystalline monolayers whose orientation is carefully aligned. Moreover, the chiroptical response of the Ag fan-shaped nanostructures can be further improved by annealing in the vacuum. After the annealing process, the nanostructure changes its shape slightly due to the atomic diffusion of Ag and coalescence process, and the polystyrene nanospheres are transformed into a flat layer, which both contribute to the improvement of chiroptical response. Furthermore, the arrays of Ag fan-shaped nanostructures can be transferred into polydimethylsiloxane, which acts as a flexible and transparent substrate. The chiroptical response of the obtained flexible chiral metamaterial can be easily tuned by mechanical deformation. Such large-area chiral metamaterials with high and tunable chiroptical response can be used as excellent meta-devices.

Metallic nanostructures can manipulate light at the nanoscale through the excitation of surface plasmon polariton(SPP), which is the collective oscillation of conduction electrons coupled to lightwaves. The science on SPPs, i.e., plasmonics, has been extensively explored to open a broad range of applications. Recent progress in top-down nanolithography now allows us to fabricate plasmonic metallic structures on the scale comparable to the lightwaves. Various metallic nanostructures, such as bow-ties, Yagi-Udas, and periodic particle arrays to name a few, have been produced to achieve the designed plasmonic responses. However, it is still a challenge to fabricate structures one-order smaller than the lightwaves in a large scale. In parallel with the progress in top-down techniques, bottom-up strategies, such as self-assemblies and electrochemical etching, have been studied to fabricate smaller structures as complementary techniques.
In the present study, we have fabricated metallic meso-structures by using mesoporous silica as a template. Mesoporous silica is a group of porous materials usually prepared from a combination of surfactant and silicon alkoxide; a self-assembled structure of surfactant is utilized as a template for the mesoporous structure of silica gel derived from the alkoxide. Highly-oriented mesoporous silica thin films with hexagonally-packed meso-cylinders aligned parallel to the surface were prepared on rubbing treated silica substrates following ref. [1]. The periodic structures on the surface of mesoporous silica were exposed by wet etching process [2]. Oblique angle deposition of gold (Au) on the mesoporous silica films resulted in the anisotropic and discrete Au structures of rod-like islands aligning along the meso-cylinders. The typical pitch of the grating structure examined by Fourier transform of the scanning electron microscope (SEM) images is around 10 nm. Optical transmission of each sample was measured for p/s polarized light by using halogen white lamp. The transmission is highly polarization-sensitive, reflecting its anisotropic morphology. The results qualitatively agree with the results of simulations by COMSOL multiphysics. The electric field distribution in simulation indicates that a strong electric field is accumulated in the gaps between the Au rods. Thanks to the meso-grating structure, the sample has a large number of nanogaps.
In summary, we have offered a new fabrication strategy for plasmonic structure one-order smaller than the lightwaves. The meso-grating structure with a typical pitch of 10 nm has a high density of nanogaps and polarization and wavelength selective optical response, and it can be applied to sensing and other optical devices.

Dirac cone, which has been widely studied in optical metamaterials and photonic crystals, reveals many interesting physics and leads to some novel applications in manipulating electromagnetic wave propagation. Dirac cone was firstly found at the zone boundary due to structural degeneracy. Such Dirac cone can relate to non-zero Berry phase and nontrivial photonic topological insulator (PTI) is achieved by opening the Dirac cone. Thus robust waveguide which is backscattering to disorders or defects can be obtained. Another kind of Dirac cone is recently found at the zone center, which is induced by triply accidental degeneracy in high-symmetric photonic crystals. The photonic crystal behaves as a zero-refractive-index metamaterials at the Dirac frequency. Directional emission and channel cloaking were achieved.
Here, we show our experimental realization of photonic topological insulator and zero-refractive-index metamaterials. Firstly, we proposed a PEC waveguide of uniaxial metacrystal to construct a prototype of PTI. The natural coupling between waveguides modes leads to strong effective bianisotropy. Non-resonant meta-atoms were designed to meet the ε/mu;-matching criteria and the fabricated microwave PTI was fulfilled in a broad frequency range. Gapless spin-filtered edge states were demonstrated experimentally by measuring Ez and Hz fields, as well as their phase differences. The phase difference were stable and independent of the source position. Robust transport of the edge states was also observed when an obstacle was introduced near the edge. Furthermore, we also proposed the staggered photonic metacrystals to cope with the intrinsic materials dispersion problem. The topological behaviors can be maintained even for dispersive metamaterials. Nontrivial PTI with large gap spin Chern number was found due to band inversion at high symmetric reciprocal k-points. Two proposals of robust spin-filtered power splitter and slow-light waveguide were discussed.
Secondly, we showed that zero-refractive-index mematerials can also be achieved in nonperiodic photonic quasicrystals. The states in the conical dispersions are extended and have a nearly constant phase. Theoretical simulations and experimental results showed evidences that the photonic quasicrystals do behave as a near zero-refractive-index metamaterial. In addition, as to quantitatively demonstrate the zero-refractive-index behaviors at optical wavelength, we fabricated a concave lens comprising silicon nanowires that was arranged in square lattice. The device possessed Dirac cone at telecommunication wavelength and light focusing effect was observed. The focal spot, which was close to the center of curvature of the lens, showed an intuitional evidence for little phase change of zero-refractive-index metamaterials. The effective refractive index was retrieved quantitatively by using lens&’ maker formula.

The metasurface - an ultra-thin layer of elements which have unique property of locally tailoring the electromagnetic field at the nanoscale- is an ideal platform to explore novel concepts and applications. Here we experimentally demonstrate a three-dimensional ultra-thin metasurface invisibility cloak that covers on an arbitrarily-shaped object and makes it undetectable in the visible light. Our metasurface cloak consists of subwavelength-scale nanoantennas which provide a distinct phase shift to the reflected electromagnetic waves. Based on this phase control capability, the phase of the light scattered at each local point on the metasurface is the same as that reflected from a flat mirror. The cloak is completely invisible under intensity or phase measurements. We also demonstrate a metasurface photon spin detector based on strong photonic spin-orbit interaction. It is induced by strong light bending effect provided by metasurfaces. We show that with this strong interaction the photon spin angular momentum can be further transferred to the collective motion of electrons in a conductive metasurface, which leads to a direct detection of intrinsic photon spin states.

We will discuss our recent progress on metasurfaces, with particular emphasis on the introduction of large nonlinearities in multi-quantum well layers, nonreciprocal wave interaction induced by time-modulation and/or nonlinearities and nonlinear absorption, and unusual scattering features introduced by balanced loss and gain features. Our results show that these novel concepts may provide new directions for metasurfaces to control to a new degree the incoming light. Giant nonlinear response on a surface, local, subwavelength control of the nonlinear emission properties of the surface, largely nonreciprocal transmission and isolation, loss-free planar focusing, and cloaking of large obejcts, will be among the features that we will discuss, enabled by the proposed concepts.

Metafilms are 2-dimenstional flat optical elements that consist of a dense array of optical antennas with subwavelength period that are capable of arbitrarily controlling of various optical properties such as reflection/transmission/absorption amplitude, phase, polarization conversion, and diffraction. Due to their versatility, there has been considerable research on fundamental properties and applications of metafilms, including perfect absorbers, beaming and focusing, and polarization-dependent surface plasmon launching, to name a few.
As a next logical step, design and implementation of tunable metafilms have attracted significant interest. Tunable metafilms feature dynamic manipulation of absorption characteristics, active beam steering, and on-demand polarization control. Various active materials such as transparent conducting oxides (TCOs), transition metal dichalcogenides, phase changing materials, III-V semiconductors have been examined as candidate materials.
In this presentation, electrically tunable metafilm absorbers in the mid-infrared regime based on TCO are presented. A thin film of TCO is sandwiched between the top metallic gratings and a bottom planar metal substrate with a thin dielectric spacer. Upon application of an electrical bias, the TCO can for either a depletion layer or an accumulation layer, in which the optical property is significantly altered. Around the plasma frequency of the TCO where the electric permittivity approaches zero, the electric field amplitude in the TCO is substantially increased, giving rise to effective modulation. As such, it is of critical importance to align the material resonance of TCO (the plasma frequency) and the structural resonance of metafilm. The experimental results show that the absorption of the metafilm absorber can be modulated around 18% with electrical bias of 5 V across 20 nm-thick-gate oxide.
In conclusion, we demonstrate dynamic tuning of metafilm absorber by using the electro-optical effect in the TCO sandwiched by two metal structures. We believe the proposed device can pave a way to further research on various optical applications such as modulators, IR shutters, and thermal emission control.

Random lasers rely on coherent multiple scattering of light in a random medium. Traditional random lasers are composed of dielectric materials with wavelength-size resonant features. Recently, random lasers that incorporate metallic nanoparticles have been demonstrated, leading to enhanced scattering and resonant field localization effects that significantly improve lasing performances.[1-3] In particular, due to the excitation of surface plasmon resonances (SPRs) with high field localization at metal-dielectric interfaces, the gain of SPR-driven random lasers can be boosted and the lasing threshold strongly reduced. In this paper, we demonstrate plasmon-enhanced random lasing in a novel metal-dielectric nanocomposite structure consisting of an interconnected random network of dielectric nanofibers doped with metal nanoparticles in a gain medium. In our work we report on the fabrication and characterization of a new plasmonic material for random lasing consisting of nanofiber random networks from an HPC (Hydroxypropyl cellulose) water-based solution containing Au nanoparticles of prescribed dimension and morphology as well as a laser dyes Rhodamin 6G (RHG) molecules. Randomly interconnected networks of active plasmonic nano-fibers with an average diameter of ~300 nm and variable thicknesses were fabricated by electrospinning from the HPC active solution. Optical characterization was performed by measuring the coherent backscattering cone that resulted in a transport mean free path of light in RHG-doped samples of about 6.8 microns. Additionally, transmission electron microscopy and dark-field scattering spectroscopy were utilized to investigate the Au nanoparticles&’ morphology and distribution as well as their plasmonic resonances across the visible spectral range. Random lasing was demonstrated by pumping the samples with a ps laser (532 nm,30ps). Above a pump-threshold fluence of 25mJ/mm² sharp spikes appeared in the emission spectrum, caused by random but deterministic resonances, which were coherently amplified within the dielectric fiber network. Our results demonstrate that samples doped with Au nanoparticles (60 nm) in the HPC fibers feature a significant reduction in the random lasing threshold by ~30% compared to undoped structures. Strategies to further improve lasing performances in plasmonic-driven random fiber networks will be presented and discussed. The cost-effective and facile synthesis of plasmon-driven random lasers demonstrated in this work has a large potential for the development of novel types (i.e., fiber-based) of random cavities and plasmonic lasers for optical biosensing and detection.
1. G. D. Dice and A. Y. Elezzabi, Journal of Optics a-Pure and Applied Optics, vol. 9, pp. 186-193, Feb 2007.
2. G. D. Dice, S. Mujumdar, and A. Y. Elezzabi, Applied Physics Letters, vol. 86, Mar 28 2005.
3. O. Popov, A. Zilbershtein, and D. Davidov, Applied Physics Letters, vol. 89, Nov 6 2006.

We outline the recent progress in developing new plasmonic materials that will form the basis for future low-loss, CMOS-compatible devices that could enable full-scale development of the metamaterial and nanophotonic technologies.

We demonstrate a novel and simple geometrical approach to cloak a scatterer sitting on a ground plane from an incoming plane wave (carpet cloaking) [1]. By using an extremely thin dielectric metasurface, distorted wavefronts are reshaped to mimic the reflection pattern of the flat ground plane. In order to achieve cloaking, the reflection angle has to be equal to the incident angle everywhere on the scatterer. To achieve this, we use the generalized Snell&’s law to calculate the required phase gradient [2-4]. The final design works by providing wavefronts with a local additional phase to compensate for the phase difference induced by the geometrical distortion. We design our metasurface in the microwave range using loss-low, sub-wavelength dielectric resonators. We verify the design by full-wave time-domain simulations and show that results match the theory particularly well. This approach is general and can be applied to hide any object on a ground plane using a metasurface of class C₁ (first derivative continuous). Besides, it is valid not only at microwave frequencies but also at higher frequencies all the way up to the visible.
References:
[1] L. Y. Hsu, T. Lepetit, and B. Kante, “Extremely Thin Dielectric Metasurface for Carpet Cloaking,” PIER 2015 (Accepted)
[2] N. Yu, P. Genevet, M. A. Kats, F. Aieta, J.-P. Tetienne, F. Capasso, and Z. Gaburro, “Light propagation with phase discontinuities: Generalized laws of reflection and refraction,” Science 334, 333-337 (2011)
[3] J. Li, and J. B. Pendry, “Hiding under the carpet: A new strategy for cloaking,” Phys. Rev. Lett. 101, 203901 (2008).
[4] J. Valentine, J. Li, T. Zentgraf, G. Bartal, and X. Zhang, “An optical cloak made of dielectrics,” Nat. Mat. 8, 568-571 (2009).

Enhancing nonlinear light-matter interaction has been highly sought-after for a variety of applications including lasing, efficient single photon sources, and all-optical light modulation. Recently, resonant plasmonic structures have been considered promising candidates for enhancing nonlinear optical processes due to their ability to greatly enhance the optical near-field; however, such enhancement is typically limited to a confined volume near the metal surface. This prevents the inherently large nonlinear susceptbility of metal from being efficiently exploited. Here, we realize strong near-field enhancements within silicon resonators by stringently designing a Fano-resonant metasurface based on volumetric Mie resonances. Due to the excellent field overlap and high damage threshold, we measured a third harmonic generation (THG) enhancement factor of 1.5×10⁵ with respect to an unpatterned silicon film and an absolute conversion efficiency of 1.18×10^-6 with an infrared pump intensity of 3.2 GW/cm². Combining the large nonlinearity with a sharp linear transmittance spectrum, we observe transmission modulation through the metasurface with a modulation depth of 36%.

The terahertz spectral range of the electromagnetic spectrum—from about 100 GHz to 15 THz—has long been a challenging region in between the successful realms of electronics and photonics, because of the lack of efficient and compact sources and detectors for terahertz radiation. We experimentally demonstrate efficient broadband THz generation, ranging from 0.1 - 4 THz, from a thin layer of SRRs with few tens of nanometers thickness by pumping at tele-communications wavelength of 1.5 microns (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. By comparison with theory, we have shown a giant sheet nonlinear susceptibility that far exceeds known thin films and bulk inorganic materials such as ZnTe. Our approach provides a potential solution to bridge the “THz technology gap” by solving the four key challenges in the THz emitter technology: efficiency; broadband spectrum; compact size and tunability.
In summary, we have shown that a single nanometer-scale layer of SRRs merges nonlinear metamaterials and THz science/technology, representing a new platform for exploring artificial magnetism induced nonlinear THz generation. This leads to broadband THz emission from deep-subwavelength meta-atoms.

Metamaterials are artificially engineered materials offering the unique opportunity to tailor their optical properties and functionalities. We propose the use of metamaterials as antireflection coatings for silicon-based solar cells. Effective medium approximation is often used to describe the optical permittivity of metamaterials. In this talk, we demonstrate that a quarter wavelength-thick silicon nitride layer can be replaced by a linear array of silicon nanowires smaller than 40 nm wide and having an appropriate filling fraction determined by the effective medium approach. However, this model is valid only for such deep subwavelength-sized structures. In fact, as soon as the size of the nanowires is large enough to support structural Mie resonances, other effects must be taken into account. We investigate the transition from the non-resonant to the resonant regime (while avoiding 1^st order diffracted beams in the medium above the array). We observe the deviation from the lowest-order effective medium approximation and we provide design guidelines to achieve the same optical properties and spectral features for different sizes of the wires. The resonant regime is of technological interest because larger nanowires are easier to fabricate on a large scale. We also show that one can take advantage of resonant effects to design arrays of nanowires having different sizes outperforming traditional antireflection layers. In addition, we use the physical understanding of the optical properties of resonant metamaterials to design effective layers made of 3D nanostructures whose thickness is still around the quarter wavelength value. We simulate the optical response of such metamaterials using Transfer Matrix Method calculations for non-resonant structures and full-field Finite-Difference Time-Domain calculations performed with commercial software package (Lumerical) for resonant structures. We fabricate linear arrays and 3D nanostructures using Focused Ion Beam on silicon wafer. Reflectivity has been measured using an optical microscope. This work provides an easy design guideline to fabricate metamaterials able to suppress reflectivity from silicon in a broad visible wavelength range, thus enhancing silicon-based solar cell efficiency. We estimate an increase of 23% in the efficiency by integrating our structures on a standard solar cell device structure.

Circularly polarized light (CPL) is utilized in various optical techniques and devices. However, using conventional optical systems to generate, analyze and detect CPL involves bulk optical elements, making it challenging to realize miniature and integrated devices. Recently, ultracompact optical elements and devices including CPL sources, quarter waveplates, polarizers, and beam splitters have been successfully demonstrated. Despite significant advances, the ability to detect and discriminate CPL without additional optical elements such as waveplates and polarizers remains challenging. Here, we report an ultracompact CPL detector that combines large engineered chirality, realized using chiral plasmonic metamaterials, with hot electron injection. We demonstrate the CPL detector&’s ability to distinguish between left and right hand CPL without the use of additional optical elements. Implementation of this CPL photodetector could lead to enhanced security in fiber and free-space communication, as well as emission, imaging, and sensing applications for CPL using a highly integrated photonic platform.

Nonlinear optics play an important role in many applications in photonics and quantum optics, such as in frequency conversion, sensing, and entangled-photon generation. The strong field confinement obtained by the transition to an integrated platform has led to unprecedented nonlinear figures of merit and the miniaturization of nonlinear devices. However, phase-matching remains an essential component to nonlinear processes and represents a significant obstacle, with many different free-space and on-chip techniques being developed to circumvent its constraints.
Recently, a 1-dimensional metamaterial with a refractive index of zero has been applied to nonlinear propagation in a zero-index medium. This metamaterial exhibits surprising phase-matching behaviour, in particular, the simultaneous generation of forward- and backward- propagating light.
We have designed a pair of 2D integrated metamaterials with an effective refractive index of zero: a square array of silicon rods on a silica substrate, and the inverse structure of a square array of air-holes in a silicon slab. These isotropic structures both exhibit a refractive index of zero for all in-plane propagation directions. The proposed devices are CMOS-compatible and can be fabricated using standard lithographic processes on an SOI wafer. Using full-wave simulations, we study the propagation and generation of nonlinear signals within these metamaterials and explore their unique phase-matching behaviour in multiple simultaneous directions. We leverage the 2-dimensional nature of these metamaterials to explore the dependence of the nonlinear signal on the size as well as shape of the nonlinear material. The presented results have important implications for future phase-matching schemes and integrated nonlinear applications.

Independent control of the photonic and plasmonic modes in visible wavelength in metasurface structures represents a fundamental goal in the field of nanophotonics, due to the potential implications in a wide range of fields, including sensing, quantum plasmonics, and tunable absorption. While bottom-up assembly has been commonly used to position colloidal nanoparticles close to each other in order to enhance near-field plasmonic coupling, and top-down lithographic techniques have been extensively applied in the construction of periodic nanostructures with sharp photonic resonance features, there are few reports that combine these two approaches to achieve control over both photonic and plasmonic coupling in the same structure due to the distinct length scales that must be manipulated.
In this work, we show that optical metasurfaces can be constructed using an approach that combines top-down and bottom-up processes, wherein gold nanocubes are assembled into ordered arrays via DNA hybridization events onto a gold film decorated with DNA-binding regions defined using electron beam lithography. This approach enables one to systematically tune three critical architectural parameters of the metasurfaces: (1) anisotropic metal nanoparticle shape and size, (2) the distance between nanoparticles and a metal surface, and (3) the symmetry and spacing of particles. Importantly, these parameters allow for the independent control of two distinct optical modes - a plasmonic gap mode between the particle and the surface and a photonic lattice mode that originates from cooperative scattering of many particles in an array. Through reflectivity spectroscopy and finite-difference time-domain simulations, we find that these modes can be brought into resonance and coupled strongly. The high degree of synthetic control in this design enabled the elucidation of design parameters that influence the degree of coupling, which can be utilized toward the construction of more sophisticated metasurfaces incorporating multiple types of plasmonic building blocks.

The engineering of artificial electromagnetic media composed of identical and resonant building blocks of subwavelength size has provided unprecedented functionalities of miniaturized optical devices for cloaking, molecular sensing, super-resolution imaging, nonlinear generation, light emission and perfect absorption. In this talk, we will present our results on the design, fabrication, optical characterization and electromagnetic calculations of Si-compatible hyperbolic metamaterials (HMMs) based on alternative plasmonic materials- titanium nitride (TiN). In particular, using magnetron sputtering and spectroscopic ellipsometry, we will discuss the fabrication and optical dispersion properties of Si-compatible HMMs composed of alternating TiN/SiO₂ sub-wavelength layers, and show the ability to tailor negative dielectric permittivity across the visible spectral range. Based on these structures, 20 nm-thick silicon quantum dots (Si-QDs) have been uniformly spin-coated atop the TiN/SiO₂ HMM and the broadband kinetics of the Si-QDs emission has been systematically investigated across the hyperbolic regime. Experimental results demonstrate up to 1.6-times enhanced decay rate of QDs emission when coupled to the high-k modes of HMMs compared with identical QDs on silica substrates. Finally, rigorous electromagnetic modeling has been performed to validate the experimental emission rates of embedded dipoles. Our findings provide a promising first step towards the engineer of novel Si-compatible broadband sources for applications to on-chip optical communication, processing and sensing.

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-structuring the constituent layers at length scales below the wavelength of light. The resulting metafilms and metasurfaces offer opportunities to dramatically modify the optical transmission, absorption reflection, and refraction properties of device layers. This is accomplished by encoding the optical response of nanoscale resonant building blocks into the effective properties of the films and surfaces. To illustrate these points, I will show how nanopatterned metal and semiconductor layers may be used to enhance the performance of solar cells, photodetectors, and enable new imaging technologies. I will also demonstrate how the use of active building blocks can facilitate the creation of active metafilm devices.

Metamaterials with a refractive index of zero have emerged as a new tool for phase control in nanophotonics. Waves propagate within such metamaterials with infinite phase velocity, resulting in uniform phase throughout. Recently two-dimensional zero-index metamaterials have been integrated with on-chip silicon photonics, allowing phase-free propagation over large areas. However, zero-index modes are inherently lossy: since the momentum of the wave is zero, it lies above the light line, and therefore couples to waves in free space. In particular, momentum conservation implies that the light is scattered vertically, perpendicular to the surface. We can leverage this scattering to couple guided waves into free space while controlling the phase, treating the zero-index material as a metasurface.
We use full-wave simulations to study the out-of-plane radiation from zero index metamaterials in the near-infrared. The metamaterial consists of a silicon photonic crystal on a silica substrate, where the geometry of the structure is tuned to achieve simultaneously zero electric and magnetic response. Controlling the nanostructure allows for fine control of the effective mode index over a continuous range of positive and negative values, corresponding to positive and negative refracted angles. Since the periodicity of the crystal is smaller than the free space wavelength, the beam scatters without diffraction.
This platform provides additional flexibility beyond beam steering. By introducing spatial variation in the effective index, we can control the phase of scattered light to achieve sophisticated wavefront engineering. We explore two examples: a graded index photonic crystal for focusing, and a spiral waveguide to generate orbital angular momentum.

Plasmonic metasurfaces, which are arrays of metallic nanostructures in which the macroscopic electromagnetic properties arise from the collective interaction of the individual nanostructures, have the potential to lead to improvements in the efficiency of optoelectronic devices when employed as electrodes. Interactions between absorbers and metallic nanostructures can give rise to intriguing optical phenomena disparate from either of the isolated materials, such as absorption-induced transparency, strong coupling between excitons and surface plasmons, plasmonic splitting, and enhanced absorption. Because absorbers can dramatically change the optical properties of metallic nanostructures, and vice versa, understanding their interaction is necessary in order to design plasmonic metasurfaces that are of benefit to optoelectronic device performance. Here, we report a previously unidentified optical mode called, “absorption-induced scattering” that occurs ubiquitously for absorbers in the presence of scatterers. Microscope-coupled dark-field spectroscopy in conjunction with extensive electromagnetic simulations for absorber-coated plasmonic metasurfaces are employed to elucidate the origin of absorption-induced scattering. We show that the origin of this phenomenon is electromagnetic coupling between the optical transitions of absorber materials and scattering modes, regardless of whether the scattering modes are localized, collective, or plasmonic in origin. The general requirements for absorption-induced scattering to occur are: 1) the presence of a scattering object and an absorber material; 2) spectral overlap between the optical transitions of the absorber and the scattering modes; and 3) close proximity between the scattering object and absorber. Scattering arising from plasmonic modes is shown to increase the intensity of absorption-induced scattering, in particular when the plasmonic mode spectrally overlaps the absorption band edge of the absorber. Additionally, absorption enhancement in the absorber coatings and metasurfaces is characterized through a combination of integrating sphere reflectance measurements and electromagnetic simulations and it is shown that absorption-induced scattering can give rise to greatly enhanced absorption (~1.5× and ~7× enhancement within the absorber coating and metasurfaces, respectively).

The achievement of a compact and ultrathin perfect absorber, which absorbs all of the normally-incident radiation at a design wavelength, is desired for applications including optical sensing, photodetection, thermal emission control, thermophotovoltaics, and photovoltaics.
In order to achieve perfect absorption, a structure needs to exhibit non-zero electric and magnetic polarizabilities, which enables the forming of an effective Huygens sheet and the input impedance matching to free-space. The designs proposed until now are mainly focused on artificial high impedance surfaces on perfect electric conductor planes and lossy dielectrics on artificial perfect magnetic conductor planes. In addition, designs based on artificial three-dimensional chiral inclusions, complex bianisotropic particles exhibiting omega coupling, dielectrics coated on plasmon supporting ground planes, and substrate induced bianisotropy for plasmonic nanospheres have been studied.
Here, we propose a resonant ultrathin metamaterial optical perfect absorber that is based on periodically arranged rectangular C₄ symmetric blocks of polysilicon on a quartz substrate. This simple and metal-free design allows for easy fabrication of the proposed device, which is achieved by a low pressure chemical vapor deposition of polysilicon on quartz followed by standard electron beam lithography and dry etching processes.
In numerical simulations, we decrease the depth of gratings to form C₄ symmetric three-dimensional periodic structures and show that the TM₁₁ mode can be made degenerate with the TM₁₂ and TM₃₁ modes. This triple mode degeneracy allows us to achieve polarization independent perfect absorption between the wavelengths of 600 nm and 700 nm for polysilicon on quartz. The mismatch between the radiation and absorption quality factors, which is a result of the inherent material properties of polysilicon, limits the maximum achievable absorption with C₄ symmetric structures to 80%, 90%, 97%, and 70% around 490, 520, 600, and 800 nm, respectively. Our experimental results agree well with the numerical ones and show an absorptivity of approximately 75% around 490 nm, 80% from 520 nm to 700 nm, and 50% around 800 nm.
We expect that this design principle can be used for a variety of high-index materials to realize perfect absorbers across a wide range of wavelengths. The simple and metal-free design can enable low-cost manufacturing and easy integration.

The unconventional electromagnetic properties make possible by metamaterials stimulate us to reconsider and reevaluate the established rules of optics in both the linear and nonlinear regimes. One of the most intriguing phenomena in nonlinear metamaterials is “backward phase matching,” also known as the “nonlinear mirror,” which describes counter-propagating fundamental and harmonic waves in a negative-index medium. Predicted nearly a decade ago, this process is still awaiting a definitive experimental confirmation at optical frequencies. Here we report optical measurements showing backward phase matching by exploiting two distinct modes in a nonlinear plasmonic waveguide, where the real parts of the mode refractive indices are 3.4 and -3.4 for the fundamental and the harmonic waves respectively. Two distinct dielectric layers with similar linear, but different nonlinear behaviors and electrical conductivities are exploited to prevent the vanishing effect, which arises from the opposite mode symmetry along the transverse direction between the two modes involved. We electrically induced a change in the nonlinear susceptibility to further enable and confine the constructive coupling between the fundamental and harmonic modes inside the plasmonic waveguide. The observed peak conversion efficiency at the excitation wavelength of ~780 nm indicates the fulfillment of the phase matching condition of k_2w = 2k_w and n_2w = minus;n_w, where the coherent harmonic wave emerges along a direction opposite to that of the incoming fundamental light. Our work will open doors for an entirely new regime of nonlinear optical processes in artificially structured media, where many established rules have to be fundamentally modified to accommodate the unconventional material parameters not seen before.

Bulk metamaterials suffer from challenges including complicated three-dimensional micro- and nano-fabrication, as well as severe ohmic losses and strong dispersion associated with the resonant response. Single-layer planar metasurfaces ease the fabrication but their interaction with light is very limited, resulting in low efficiencies, for instance, in polarization conversion and anomalous reflection/refraction for flat optics. Few-layer metasurfaces provide a promising solution to these challenges. They consist of two or three layers of planar metasurfaces separated by thin low-loss dielectric spacers, enabling destructive/constructive interference due to the multireflection among layers [1], while near-field interactions remain negligible [2]. Here we present few-layer metasurfaces serve as ultrathin optical antireflection coatings without the requirement of index matching of materials being used, by taking advantage of the tailored reflection/transmission and their dispersion within the individual metasurfaces [3-5]. We further show that tri-layer metasurface structures are capable of ultra-broadband conversion of linear polarizations through the anisotropic response of individual layers [6], which may enable a class of practical flat optics.
H.-T. Chen, “Interference theory of metamaterial perfect absorbers,” Opt. Express20, 7165-7172 (2012).
L. Huang, D. Roy Chowdhury, S. Ramani, M. T. Reiten, S.-N. Luo, A. K. Azad, A. J. Taylor, and H.-T. Chen, “Impact of resonator geometry and its coupling with ground plane on ultrathin metamaterial perfect absorbers,” Appl. Phys. Lett.101, 101102 (2012).
H.-T. Chen, J. F. Zhou, J. F. O'Hara, F. Chen, A. K. Azad, and A. J. Taylor, “Antireflection coating using metamaterials and identification of its mechanism,” Phys. Rev. Lett.105, 073901 (2010).
H.-T. Chen, J. F. Zhou, J. F. O'Hara, and A. J. Taylor, “A numerical investigation of metamaterial antireflection coatings,” THz Sci. and Technol.3, 66-73 (2010).
B. Y. Zhang, J. Hendrickson, N. Nader, H.-T. Chen, and J. P. Guo, “Metasurface optical antireflection coating,” Appl. Phys. Lett.105, 241113 (2014).
N. K. Grady, J. E. Heyes, D. Roy Chowdhury, Y. Zeng, M. T. Reiten, A. K. Azad, A. J. Taylor, D. A. R. Dalvit, and H.-T. Chen, “Terahertz metamaterials for linear polarization conversion and anomalous refraction,” Science340, 1304-1307 (2013).

The efficient analysis and engineering of the polarization state is imperative in diverse disciplines, including physics, materials science, biology and quantum optics. For instance, scientists apply circularly polarized light to manipulate the spin state of electron for quantum information processing. Chrysina gloriosa (jeweled beetles) under left-handed circularly polarized light illumination appear more brilliant than those under right-handed circularly polarized light illumination, and circular dichroism spectroscopy is of critical importance to identify the structure of chiral molecules. Metallic mirrors are basic elements and widely used in optical setup to control the path of light. However, the state of circular polarization is reversed, or even degrades to elliptical polarization when it is reflected off a surface. Therefore, the original handedness of the optical signals is lost after multiple reflections in a complex optical system. Here, we propose and demonstrate a new concept of circular dichroism metamirrors, which enables selective, near-perfect reflection of designated circularly polarized light without reversing its handedness, yet complete absorption of the other polarization state. Such a metamirror can be considered as the optical analogy of Chrysina gloriosa in nature, while exhibits nearly maximal efficiency. A general method to design the circular dichroism metasmirror is presented under the framework of Jones calculus. It is analytically shown that the building block of such a metamirror needs to simultaneously break the n-fold rotational (n >2) symmetry and mirror symmetry. By combining two layers of anisotropic metamaterial structures, we design a circular dichroism metamirror in the mid-infrared region, which shows perfect reflectance (94.7%) for left-handed circularly polarized light without reversing its handedness, while almost completely absorbs (99.3%) right-handed circularly polarized light. These findings offer new methodology to implement novel photonic devices for a variety of applications, including polarimetric imaging, molecular spectroscopy and quantum information processing.

Naturally occurring materials are often not suitable for photonics applications because they either have weak absorption or produce strong reflections, and have poorly defined spectral features beyond the visible spectrum. Absorbing materials based on metasurfaces with metallic elements have been developed to address these limitations in spectral ranges from the near infrared to microwave. However, to obtain an effective medium response, each metasurface element must be deeply sub-wavelength in size. Making such sub-wavelength elements requires the use of top-down nanolithography techniques, which prevents the scalability of absorbing metasurfaces to large areas and for visible spectrum response. In this work we show large-area metasurfaces with nearly perfect absorption (99.7%) over truly macroscopic areas (up to 5 cm) using a simple, scalable and conformal solution-based assembly technique. The metasurface elements are composed of colloidally-synthesized silver nanocubes electromagnetically coupled to a metal film. The resonance wavelength is easily tunable from the visible to the near infrared spectrum, and the absorbers show good performance at oblique angles. The solution-based deposition technique enables the control of absorption and emissivity over surfaces of arbitrary geometry and size. In addition, enabled by the broad spectral tunability and spectral selectivity, these metasurface absorbers can be integrated with existing technologies for enhanced photodetection and imaging.

Metallic mirrors maximize the electric field intensity at about quarter wavelength distance from the surface of the metal. Patterning the metal with sub-wavelength grooves can result in a metamaterial mirror that maximizes the electric field on the metal surface. In this work, we combine metamaterial mirrors with graphene, wherein graphene, due to its two dimensional nature, acts as an ideal surface electric field probe. By monitoring the Raman signal from graphene-coated metamaterial mirrors, the underlying groove dimensions were optimized to maximize the electric fields at the surface of the mirror. The measured Raman enhancement trends for various metamaterial designs agree well with theoretical predictions based on Finite Difference Time Domain (FDTD) simulations. Spectroscopic reflectance and photoluminescence (PL) measurements have been used to study the correlation between the fields inside the metamaterial and the absorption within graphene. Corresponding FDTD models were developed to predict both reflectance and PL for our structures and to correlate these predictions to the observed Raman enhancements.

Plasmon drag effect presents interest for plasmon-based electronics as it provides opportunities to monitor or control plasmonic effect by electric means. In order to better understand the origin of optically induced electric currents in plasmonic surfaces and explore their properties, we study the effect in sine-wave grating metal surfaces with subwavelength periods. Theoretical simulations are based on the coupled-wave analysis. Predictions for plasmon-induced electric voltage based on hydrodynamic loss approach well fit experimental observations in the conditions of propagating plasmon polaritons. Possibility to use the effect for sensor applications is discussed as well.

It is well known that the spontaneous decay rate of a quantum emitter depends strongly on its environment. For example, a molecule placed inside an optical resonator, where the density of photonic states is greatly increased, can experience an enhancement of its spontaneous emission rate by many orders of magnitude. This is known as the Purcell effect, and has enabled many important applications in the field of quantum photonics.
This concept can be extended to chiral molecules, that is molecules that cannot be brought into congruence with their mirror images by any combination of rotations in 3D space, sugar being the most famous example. Such molecules exhibit circular dichroism, meaning that they interact differently with left- and right-handed circularly polarized light. By analogy to the Purcell effect, a "chiral Purcell effect" takes place when a left- or right-handed molecule is placed in an optical resonator of the same handedness.
Here we explore double-fishnet metamaterial resonators that can be used as nanoscale cuvettes supporting the chiral Purcell effect. These consist of metal-dielectric-metal multilayer stacks perforated by periodic arrays of square holes. We fabricate such structures using electron-beam lithography and characterize them optically using a high-brightness Fourier-transform spectrometer. Experimental results are compared to finite-difference time-domain simulations in order to understand the optical modes supported by these structures. We find that enhanced chiral fields are generated inside each cavity when such structures are illuminated with circularly polarized light.
These enhanced chiral fields allow us to explore the rich physics of light-matter interaction in a metamaterial cavity, and pave the way for the development of efficient circularly-polarized single-photon sources, which would be invaluable for quantum communication and quantum information processing. Additionally, these resonators have applications in chiroptical spectroscopy; by selectively increasing the emission rates of molecules of a particular handedness, they can be used as sensors to accurately and reliably detect small quantities of chiral biomolecules.

Graphene plasmons have recently attracted large attention because of their excellent electrical tunability, which enable applications such as fast light modulation, improved biosensing, and quantum optics in robust solid-state environement. In this presentation, we review recent theoretical and experimental advances in these directions and explore further possibilities, such as sensing at the single-molecule level, electrical detection of single plasmons, and ultrafast transient plasmonic phenomena.

A promising solution for active control of light on the nanometer scale are plasmons in graphene, which offer ultra-short wavelengths, long lifetimes, strong field confinement, and tuning possibilities by electrical gating [1,2]. The huge momentum mismatch between graphene plasmons and photons, however, presents a major technological challenge. Here, we present and discuss the coupling of incoming light into propagating graphene plasmons based on resonant optical antennas, constituting an essential step for the development of graphene plasmonic circuits [3]. By interferometric near-field microscopy we mapped the propagating plasmons launched by the antennas. Visualizing the plasmon wavefronts, the near-field images show how graphene plasmons can be focused by tailoring the antenna geometry, and how plasmon refraction can be achieved by spatially modulating the graphene conductivity. Future applications will benefit from the unprecedented low damping of the graphene plasmons we found in high-quality graphene boron nitride heterostructures [4].
[1] J. Chen, et al., “Optical nano-imaging of gate-tuneable graphene plasmons”, Nature 487, 77 (2012).
[2] Z. Fei, et al., "Gate-tuning of graphene plasmons revealed by infrared nano-imaging", Nature 487, 82 (2012).
[3] P. Alonso-González, et al., “Controlling graphene plasmons with resonant metal antennas and spatial conductivity patterns”, Science 344, 1369 (2014).
[4] A. Woessner, et al., “Highly confined low-loss plasmons in graphene-boron nitride heterostructures”, Nat. Mater. 14, 421 (2015)

Metasurfaces utilizing engineered metallic nanostructures have recently emerged as an important means to manipulate the propagation of light waves in a prescribed manner. However, conventional metallic metasurfaces mainly efficiently work in the visible and near-infrared regime, and lack sufficient tunability. In this work, combining the pronounced plasmonic resonance of patterned graphene structures with a subwavelength-thick optical cavity, we propose and demonstrate novel graphene metasurfaces that manifest the potential to dynamically control the phase and amplitude of infrared light with very high efficiency. It is shown that the phase of the infrared light reflected from a simple graphene ribbon metasurface can span over almost the entire 2p range by changing the width of the graphene ribbons, while the amplitude of the reflection can be maintained at high values without significant variations. We successfully realize anomalous reflection, reflective focusing lenses, and non-diffracting Airy beams based on graphene metasurfaces. Our results open up a new paradigm of highly integrated photonic platform for dynamic beam shaping and adaptive optics in the crucial infrared wavelength range.

The transport of excitons (bound electron-hole pairs) is a fundamental process that plays a crucial rule both in natural phenomena such as photosynthesis and in artificial devices such as organic solar cells. Similarly, excitons can play an important role in heat transport, and understanding and manipulating their role has possible applications ranging from thermoelectric effects to heat-voltage converters, nanoscale refrigerators, and even thermal logic gates. However, most systems composed of organic molecules are disordered and possess relatively large dissipation and dephasing rates, such that exciton transport typically is inefficient.
Strong coupling of excitons to a cavity mode is achieved when the energy exchange rate between exciton and electromagnetic field modes becomes faster than the decay and decoherence rates of either constituent. We will show how exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective strongly coupled mode (a polariton) allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other [1].
We furthermore show that by designing the electric field profile of the electromagnetic mode that provides the strong coupling, the transport properties can be tuned to achieve exciton harvesting and funneling, i.e., to guide excitons from a collection area to a specific location. We demonstrate this effect using the localized plasmon resonances of a single metallic nanosphere and a three-sphere structure. The latter provides pronounced hot spots where the electric field is strongly concentrated. We show that excitons are efficiently transported between these hot spots, bypassing the rest of the system [2].
[1] J. Feist and F. J. Garcia-Vidal, “Extraordinary exciton conductance induced by strong coupling”, Phys. Rev. Lett. 114, 196402 (2015).
[2] C. Gonzalez-Ballestero, J. Feist, Esteban Moreno, and F. J. Garcia-Vidal, “Harvesting excitons through strong coupling”, arXiv:1502.04905 (2015).

We show that highly concentrated dye molecules constituting gain media (which are used to compensate loss or provide for stimulated emission) exhibit strong coupling with surface plasmons and micro-cavities. The phenomenon magnifiests itselff in splitting of the dispersion curve and the avoided crossing. This effect has to be taken into account at accurate description of loss, gain and stimulated emission in plasmonic systems and miniature lasers.

We present a new theory of plasmon-enhanced Raman scattering by mapping the problem onto the canonical model of cavity optomechanics, in which abi-directional interaction takes place between molecular vibration and plasmon. The optomechanical coupling rate, from which we derive the Raman cross-section, is computed from the Raman activity of the molecules and the plasmonic field distribution. When the excitation is blue-detuned from the plasmon onto the anti-Stokes vibrational sideband, the electromagnetic force can lead to parametric amplification of molecular vibrations, revealing an enhancement mechanism never contemplated before. The optomechanical theory (i) provides a quantitative framework for the calculation of enhanced cross-sections; (ii) recovers known results; and (iii) enables the design of novel systems that leverage dynamical backaction to achieve additional, mode-selective enhancement. Finally, it yields a new understanding of plasmon-enhanced Raman scattering and opens a route to molecular quantum optomechanics.
Our model is based on the insight that the change in polarizability of a molecule under deformation (Raman tensor) leads to an optomechanical coupling to the plasmonic cavity, which is typically a localized surface plasmon (sometime called a “hot spot”). Since the plasmon decay time can have a value comparable to the vibrational period (both tens of femtoseconds) this model predicts that the localized plasmons in surface- and tip- enhanced Raman scattering are not only responsible for a huge electromagnetic field enhancement but should also exert a delayed “backaction” force on the molecular vibration. When the incoming excitation laser is blue detuned from the plasmonic resonance, this force leads to amplification of the molecular vibration, while red detuned excitation leads to its damping (or “cooling”). Quantifying the optomechanical coupling rate by density-functional theory (DFT) and finite-element modeling (FEM) numerical simulations, we find that efficient dynamical backaction can occur in realistic systems, leading to parametric amplification of the molecular vibration.
This new insight is of major relevance for the design of novel nanostructures pushing the limits in sensitivity and resolution of nanoscale Raman spectroscopy and imaging. In particular it suggest counter-intuitive guidelines such as the search for narrow (i.e. high-Q) plasmonic resonances not overlapping with the excitation wavelength. More radically, the theory lays the foundations of molecular cavity optomechanics and opens unforeseen research directions. The rich physics of cavity optomechanics is now accessible in systems of nanometric dimensions featuring coupling rates several orders of magnitude higher than state-of-the-art microfabricated devices.

Weyl points are degenerate energy states resulting from band crossing of linear dispersions in three dimensional momentum space. Unlike Dirac points in the two dimensional systems, Weyl points have been shown to be stable and the associated surface states are predicted to be topological surface states with non-trivial Chern number [1]. These topologically protected surface states may potentially lead to various interesting phenomena such as backscattering immune transport.
We fabricated and characterized gyroid photonic crystals in the Mid-IR regime with Weyl points present in their band structures. Full wave simulations of photonic bandstructure revealed that single gyroid has a complete band gap. By introducing the inversion of a gyroid, double gyroids brought line nodes (a line of degenerate states) into the bandgap. Breaking parity of the gyroids by introducing an air sphere, the line nodes lifts its degeneracy and form a pair of degenerate points instead, which are the Weyl points. Based on simulations, high index material is necessary for gyroids to have such property and a-Si was chosen as a suitable material. Two-photon lithography was utilized to fabricate polymer gyroids at a unit cell size of 5 µm and 10x10x10 unit cells in total, on KBr and intrinsic silicon substrates. We ALD&’ed the polymer gyroids with 40 nm layer of alumina, FIB&’ed the sides to expose the polymer and removed the polymer with O₂ plasma. We CVD coated and in-filled alumina gyroids with 100nm of a-Si conformally. Our initial FTIR characterization with incidence angle from 56^o to 74^o of a-Si single gyroid observed an increase in reflection by 10% from 7.5 µm to 9 µm, as well as an absorption dip of 10%, which agrees with the predicted band gap wavelength by simulation. FTIR characterization of double gyroids indicated increase in absorption at wavelength corresponding to line nodes. Observation of Weyl points from parity breaking double gyroids will be discussed, which are done by angle resolved spectroscopy with quantum cascade laser as the source.
1. L. Lu, L. Fu, J.D. Joannopoulos, M. Solja#269;icacute;, “Weyl points and line nodes in gyroid photonic crystals”, Nature Photonics 7, 294-299 (2013)

In the last fifteen years, tunable photonic crystals have represented an important class of intelligent materials whose optical properties can be modulated by external stimuli, such as chemical, thermal, electrical, magnetic, mechanical stimuli or light.^1-3 In this context, the use of hydrogels^4-7 allows an easy fabrication of these smart structures, able to respond to humidity, chemical concentration, pH and temperature. Here, we report a new easy-to-fabricate photonic crystal (PC) able to respond to different stimuli, including electric field. The PC is obtained by partial sulfonation with concentrated sulfuric acid of polystyrene nanoparticles (PS-NPs) crystals prepared by evaporative self assembly.^8,9 The reaction with sulfuric acid involves the external shell of each PS-NP and the shell thickness increases with the sulfonation degree. Cristal architecture, constituted from closely packed core-shell sulfonated PS-NPs, shows a hydrogel-like behavior. As a consequence, depending on the sulfonation degree, this highly hydrophilic porous structure can absorb a large quantity of polar solvents producing a swelling of the opal assemblies and an increase of its softness. The reflected wavelength range can be tuned from near infrared to blue color, by changing the ionic strength of the solution in which the opal is immersed. A similar change in color is observed when a mechanical pressure is applied on the hydrated opal, thanks to its soft nature. Additionally, these sulfonated opals show a promising response to electrical field, allowing a rapid color tuning from near infrared to blue with low voltage. This responsive structure could be used as sensor for chemicals or mechanical pressure, as well as for PCs based displays.
References
(1) Takeoka, Y. J. Mater. Chem. C2013, 1, 6059.
(2) Ge, J.; Yin, Y. Angew. Chemie Int. Ed.2011, 50, 1492-1522.
(3) Aguirre, C. I.; Reguera, E.; Stein, A. Adv. Funct. Mater.2010, 20, 2565-2578.
(4) Lee, K.; Asher, S. J. Am. Chem. Soc.2000, 15260, 9534-9537.
(5) Xu, X.; Goponenko, A. V.; Asher, S. a. J. Am. Chem. Soc.2008, 130, 3113-3119.
(6) Lee, Y.-J.; Braun, P. V. Adv. Mater.2003, 15, 563-566.
(7) Kang, Y.; Walish, J. J.; Gorishnyy, T.; Thomas, E. L. Nat. Mater.2007, 6, 957-960.
(8) Yang, Z.; Li, D.; Rong, J.; Yan, W.; Niu, Z. Macromol. Mater. Eng.2002, 287, 627-633.
(9) Niu, Z. W.; Li, D.; Yang, Z. Z.; Hu, Z. B.; Lu, Y. F.; Han, C. C. ChemPhysChem2003, 4, 865-868.

Recently, exciting new physics of plasmonics has inspired a series of key explorations to manipulate, store and control the flow of information and energy at unprecedented dimensions. In this talk I will report our recent efforts on controlling light absorption and emission process through quantum effects in sub-20nm scale coatings. For example, we experimentally demonstrated strong absorption of 20nm thin oxides in the visible spectrum assisted by silver films. We found such a broadband light absorption below the bandgap of the oxide is a manifestation of quantum electron tunneling that penetrate into the thin oxide layer, and it is controlled by the static dielectric constant of the oxide instead of dopant. We also found quantum emitters on a graphene-hBN heterostructure can be switched on and off at mid infrared, by transferring energy into surface phonon polaritons, and this effect can be electrically tuned by biasing the graphene layer. I will also discuss application of these nanostructure for efficient light harvesting and controllable emission, with potential impact in high resolution mid-IR spectroscopy and imaging.

High refractive index resonant semiconductor nanoantennas have recently become the center of attention as a means of light manipulation at the nanoscale. They offer strong and tunable electric and magnetic Mie resonances that afford effective light confinement while maintaining lower dissipative losses compared to their metallic counterparts. Semiconductor nanowires can also benefit from mature fabrication technologies and the potential of convenient integration with on-chip optoelectronic systems.
By engineering the electric and magnetic resonances in these dielectric resonators, it is possible to control the directionality of the scattered light from the nanoantenna so that the backward or forward radiation from dipole emitters is strongly suppressed. In this study, we investigate the directionality control of the dipole emission with Si nanowires in the visible to near-IR region of the spectrum. To understand the interplay between the emission of the dipole itself and the emissions of the electric and magnetic dipole resonances excited in the nanoantenna, we conducted analytical and numerical analyses and resolved the power coupled to and re-radiated by the antenna throughout the spectrum. We observed that although the backscattered power from the antenna is suppressed at the Kerker wavelength (where the electric and magnetic dipole emissions destructively interfere in the backwards direction), the highest forward to backward emission ratio is obtained when the antenna shows a certain backscattering to cancel out the intrinsic direct backward emission of the dipole emitter. We also show that the distance between the emitter and the nanowire is critical for the directionality of the radiation from the system as it determines the percentage of the transferred power from the dipole to the nanowire, which is important for the spectral position of the optimum destructive interference in the backwards direction.
We believe that the results of this study will help us understand and further engineer the radiation properties of single optical emitters coupled to dielectric antennas and will be critical for applications requiring control over the absorption, reflection, emission, and directional manipulation of light at the nanoscale.

A new class of two-dimensional metamaterials (metasurfaces), called planar optical elements (POEs), can be classified as having a microscopic thickness (#8818; lambda;), macroscopic transverse dimensions (#8819; 100lambda;), and being composed of an array of nanostructured light scatterers. The array is engineered to produce a desired collective response in the phase, polarization, and amplitude of the scattered optical wave. POEs can be found in broad range of micro- and nano-photonic technologies [1, 2]. For example, POEs have been built for use as waveplates in vortex beam generation [3], and for negative reflection and refraction [3,4]. For imaging purposes, the lens-type POE is referred to as a “flat lens”, and an achromatic flat lens has recently been constructed [5]. In this paper, we pay attention to the general design criteria one should consider when constructing a flat lens.
Recent advances in fluorescence and near-field microscopies have enabled optical imaging with spatial resolution beyond the diffraction limit, and POEs may play an important role in the future of super resolution optical microscopy. We use numerical methods to directly integrate the scalar Kirchhoff diffraction integrals, without employing any analytic approximations, and explore what general characteristics of a flat lens produce the “tightest” possible point spread function.
For an aplanatic lens, which, given an axial plane wave as an input, will produce a spherical output wave, the resolution criterion is sometimes taken from Rayleigh, 0.61lambda;/NA, and sometimes from Abbe, lambda;/2NA, where NA is the numerical aperture. In this paper, we show that there is a connection between these two resolution criteria. The former criterion is due to the Fraunhofer approximation, while the latter is due to heuristic arguments made early in the development of optical microscopy. In practice, all high- flat lenses are inherently nonparaxial (large diffraction angles), so the Rayleigh limit does not apply. By analyzing the results of our parametric study, we can make some general remarks about the resolution limit of an aplanatic, high- flat lens. Furthermore, we consider the effect of changing the response of the flat lens, from one which produces spherical waves (aplanatic) to one which produces the so-called “perfect” wave [6].
[1] A. V. Kildishev, A. Boltasseva, & V. M. Shalaev, Science339, 1289 (2013)
[2] N. Yu & F. Capasso, Nat. Mat. 13, 139 (2014)
[3] N. Yu et al., Science334, 333 (2011)
[4] X. Ni et al., Science335, 427 (2012)
[5] F. Aieta, M. A. Kats, P. Genevet, & F. Capasso, Science347, 1342 (2015)
[6] J. J. Stamnes, Waves in Focal Regions: Propagation, Diffraction, and Focusing of Light, Sound, and Water Waves (Adam Hilger, 1986)

Design of perfect absorbers and emitters has been a primary focus of the metamaterials community owing to their ubiquitous applications in energy harvesting, efficient photodetection, and thermal emission control to name a few. A broadband and angle-insensitive response represents one particular challenge for metamaterial perfect absorbers and emitters. Here we report on sparse (10% fill fraction), flexible arrays of InSb nanocones as broadband, angle-insensitive, perfect absorbers in the infrared (1.5-5.5 mu;m).
The perfect absorption of the InSb nanocone arrays is due to strong free space coupling into resonant leaky waveguide modes, most notably the resonant TM₁₁ mode. The large absorption coefficient, arising from interband transitions, and high refractive index of InSb throughout the SWIR-MWIR makes it the ideal material for an IR-active waveguide with high loss and high confinement, respectively. Due to the spectral dependence of the resonant mode on the waveguide radius, the taper of the InSb nanocones leads to a broadband resonant response; the taper design coupled with the large resonant absorption cross section enables a broadband perfect absorber with a tailorable absorption band determined by the upper and lower nanocone radius. Additionally, the minimal dispersion of the TM₁₁ leaky waveguide mode leads to an angle-insensitive response. Electromagnetic simulations reveal that a 10% fill fraction, PDMS-embedded array of 20 mu;m tall InSb nanocones with an upper radius of 100 nm and a lower radius of 550 nm can achieve an average absorption fraction of 99% from 1.5 to 5.5 mu;m. Good agreement between experiment and simulation of broadband absorption in sparse arrays of InP nanocones has been demonstrated in the visible.
Active tunability of the InSb nanocone array absorption spectrum is achieved via the inclusion of a thin, planar VO₂ layer on the upper interface of the InSb nanocone array (same dimensions as above), which can be thermally modulated between its insulating and metallic phases. At low temperatures (insulating VO₂), electromagnetic simulations predict an average absorption fraction of 87% from 1.5 to 5.5 mu;m; at high temperatures (metallic VO₂), the InSb nanocone arrays have a predicted average absorption fraction of 43%. These findings indicate that the absorption spectrum of this design can be thermally modulated by 50%.
These theoretical results along with experimental fabrication and optical characterization of III-V nanocone arrays and other broadband dielectric perfect absorber designs will be discussed.

The design and fabrication of integrated circuits (ICs) has become increasingly complex, fragmented and globalized [1,2]. This has opened the door for unprecedented access to the chip supply chain leading to the current major security threat of tampering with the IC physical layout [1,2]. Adding or modifying as few as 10 logic gates can alter a circuit to compromise safeguards, information security or reliability [2]. Detecting such tampering is critical to ensuring the security of the commercial IC supply. Trusted Foundry Programs, which seek to control the fabrication phase of ICs have been implemented for U.S. defense chips, [1] but their sustainability is questionable given the rapidly increasing costs of leading edge foundries and they are not suitable for broad commercial applications. There is therefore a clear and pressing need for tools that enable rapid and low cost detection of IC tampering.
Imaging techniques can directly map the physical structures that define the logical functionality of an IC. The nanoscale dimensions of modern IC components combined with their massive number (> 1B transistors) present severe obstacles to applying conventional optical methods. The diffraction limit prevents their resolution in conventional imaging and exhaustively sampling every structure would require on the order of days to measure a single chip.
In this work we combine integrated nanoscale antennas with multi-spectral imaging to overcome these fundamental limitations. We leverage the fact that the physical layout of a digital IC is comprised of a series of standard cells each of which maps to a logical gate (e.g. AND, OR etc.) and integrate nanoscale antenna labels into the layout of the IC gates so that they can be readily differentiated by a few spectral measurements.
Our approach contrasts with typical nanoparticle labeling techniques where the response of the nanoantenna dominates over a weakly scattering background. Our labels instead couple to the functional metal wires in the near-field and their scattered signal interferes with comparable background in the far-field. We develop a computational approach to address the resultant complex design problem and enables the selection of near-optimal antenna labels in an automated fashion. We demonstrate the method using numerical simulations of a modern 45 nm IC fabrication process. Our approach enables mapping of the type and location of every logical gate in an IC in a few minutes by reducing the required spatial resolution and sampling rates by several orders of magnitude. Considering current ITRS trends our method can readily scale to technology nodes beyond “10 nm”.
References
1. Office of the Uner Secretary of Defense for Acquisition, Technology, Logistics, “Defense Science Board (DSB) study on high performance microchip supply,” 2005.
2. S. Bhunia, M. S. Hsiao, M. Bagna and S. Narasimhan, “Hardware Trojan attacks: Threat analysis and countermeasures,” Proc. of the IEEE 102, 1229-1247 (2014).

We discuss our recent efforts in designing nanophotonic structures for thermal radiation control, focusing in particular, on radiative cooling technology. We show that the use of frequency-selective emitter leads to superior performance. Thus, meta-material concepts can play a very significant role in radiative cooling. We also present some of the recent experimental demonstration of radiative cooling for a variety of applications.

The scaling of electronic and optoelectronic devices to smaller scales has been a central driver of science and technologshy;shy;y for the last several decades. Among the smallest and most densely integrated electronic devices are crosspoint junctions between two nanoscale metal electrodes separated by an insulating layer. This architecture relies on the electric-field induced transport of ionic species to reversibly grow and dissolve a nanoscale conductive filament capable of not only electronic but also optical functionality. We show that the presence of a filament in the crossbar junction can lead to an 80 nm tuning of the plasmonic resonance of a cross point device due to a modulation optical properties of the junction. This device represents a novel concept for optoelectronic devices: using the nanoscale motion of atoms to realize large changes in the optical constants in a nanoscale region of material. This demonstration paves the way for optoelectronic modulators with a footprint below 0.01 mu;m².

Metallic nanostructures with helical shapes feature a broad spectrum of interesting radiation and polarization characteristics that can largely be controlled by varying their geometrical parameters. In the field of plasmonics, there have been many fascinating studies that focused on the dichroic properties of metal nano-helices excited by light with circular polarization. In our work, using the rigorous Surface Integral Equation (SIE) method, we study light scattering and emission enhancement by Au nano-helices with geometrical dimensions comparable to the wavelength of visible light and we demonstrate that they behave as highly directional nano-antennas with largely controllable radiation and polarization characteristics in the optical regime. In particular, we systematically investigate the radiation properties of helical nano-antennas with realistic Au dispersion parameters in the visible spectral range, and we establish simple design rules that enable the engineering of directional scattering with elliptical and circular polarization. Apart from highly directional “quasi-axial” modes, we have found that Au nano-helices with geometrical pitch P = lambda;, where lambda; is the wavelength of incoming radiation, can give rise to backward circularly polarized scattering. We will discuss this interesting phenomenon and its applications to the engineering of light emission enhancement embedded dipolar sources. We will show that Au nano-helices excited by external plane waves can achieve emission enhancement values around 10 times in the visible and near infrared regime with elliptical polarization states at multiple emission angles. Given the realistic material and geometric parameters used in this work, our findings provide novel opportunities for the engineering of chiral sensors, filters, polarized lasers, and components for nano-scale active antennas with unprecedented beam forming and polarization capabilities.

Demonstration of the control of light by the introduction of abrupt phase discontinuities across a subwavelength scale has opened the doors to a new level of wavefront control (1,2). All-dielectric Huygen&’s metasurfaces hold significant promise for use as highly efficient optical metasurfaces. Homogeneous antenna arrays have been previously demonstrated, capable of imparting a uniform phase shift on a transmitted wavefront at near unity efficiency (3). However, strong inter-antenna coupling effects increase the complexity of the system, making the design of Huygen&’s metasurfaces capable of applications such as beam shaping and deflection difficult.
System complexity due to inter-antenna coupling necessitates the use of finite element method computation. We have designed a set of homogenous dielectric antenna arrays that take advantage of strong, overlapping electric and magnetic resonances, demonstrating a 2π phase shift range with very high transmission. Combining those individually modeled antennas into a heterogeneous array with a non-zero spatial phase gradient, however, proves to be difficult. Inter-antenna coupling results in the phase shift and transmission amplitude for antenna elements in the heterogeneous array to differ from their performance in homogenous arrays.
We have developed several techniques to mimic the heterogeneous array environment in the modeling of homogenous arrays, thereby minimizing the variation in performance of antenna elements. We demonstrate the effectiveness of these techniques for the specific application of beam deflection. We show that edge-to-edge spacing of antenna elements is a critical parameter for controlling inter-antenna coupling, rather than center-to-center spacing. Ellipsoidal cylinders are employed as antenna elements, allowing an additional degree of freedom relative to circular elements as well as several other critical benefits.
We present here the successful design, and computational modeling, of all-dielectric transmissive Huygen&’s metasurfaces, comprised of cylindrical, dielectric resonant nanoantennas, capable of very high beam deflection efficiency. One such array, for example, consists of 9 elements per period, with modeled reflectances ranging from 0.09% to 6.8%, with an average of only 2.8%. Absorption in the antennas is near zero. Full-field simulation shows the incident wavefront to be deflected nearly completely at the intended 7.5-degree angle. We have developed similar metasurfaces for a variety of operational wavelengths across the visible and infrared, as well as for a variety of deflection angles. Two practically realizable material systems are demonstrated, as well as spectral tuning via antenna height. Fabrication of these arrays is currently underway, and experimental confirmation of results is expected in the near future.
[1] Nanfang Yu, et al., Science, 334, 333 (2011)
[2] Longfang Zou, et al., Optics Express, 21, 1344 (2013)
[3] Isabelle Staude, et al., ACS Nano, 7, 2824 (2013)

In recent years a functionality of plasmonics was extended to data storage applications where plasmonic antennas are able to shrink the optical focused spot far below the diffraction limit down to a size of tens of nanometers. Those allows to develop a new technique called the heat assisted magnetic recording (HAMR) to overcome an existing limit of conventional magnetic recording with the areal density approaching 1 Tb/in². Heat assisted magnetic recording (HAMR) exploit a near-field transducer (NFT) to locally heat the recording medium to lower its coercivity by using very high anisotropy materials to maintain data thermal stability and the ability to record information. The NFT design is based on excitation of surface plasmons on a metal structure, which re-radiate with a sub-diffraction limited light spot and confined in the near field. In practice, commonly used the NFT materials are highly absorbing at optical frequencies effecting the self-heating of the NFT and limiting the lifetime of HAMR head.
One of the main obstacles in using the gold for HAMR application result mainly from its relatively low bulk melting temperature, thermal stability, high surface energy and lack of compatibility with standard CMOS processing techniques. As it has been previously shown, the mechanical properties of gold degrade at temperature around 100°C that is much below the operational temperature for HAMR head estimated at around 400°C.
Compared to gold, the transparent metal nitrides are excellent candidates to replace the noble metals in data storage applications. They are a thermally stable, mechanically hard and durable, can operate at high temperatures and CMOS-compatible. And most importantly, their plasmonic properties can be tuned to position their LSPR in the desired wavelength range to fit the wavelength range of commonly used laser diodes.
Here, the novel TiN-based NFT and light delivery system will be presented. Novel droplet transducer was implemented that ensures better impedance match with a recording media and, consequently, better coupling of power to a recording media even operating at the same excited resonance mode of antenna.
To maximize further the coupling efficiency to a recording media, the transducer was integrated with planar waveguide configuration where power from laser to the waveguide can be coupled through the end-fire arrangement. As any antenna has a particular radiation pattern, the proposed design offers great flexibility in the coupling of light to the NFT in order to match the radiation pattern of the antenna and thus to optimize the energy transfer. Both those can be realized by integrating a MZI planar waveguide with a laser to deliver a highly confined electromagnetic radiation to the novel TiN-based NFT. The proposed design allows in easy way, by adjusting the coupling angle from both MZI arms to a transducer, to match the radiation pattern of any antenna, and especially this one with the transition metal nitrides.

Plasmonics has moved from simple noble metal surfaces and nanoantennas into the functional realm. More complex plasmonic structures allow for tailoring resonances and functionality. Higher quality gold and silver with atomically flat surfaces and single crystalline atomic arrangements represent ultimate material quality. Hybrid materials enable chiral as well as nonreciprocal responses. Active switching is enabled by phase change materials as well as metal-to-insulator transitions. New materials for plasmonics include Yttrium and its hydrides, refractory materials such as TiN, as well as the highly reactive magnesium, whose particle plasmons can be switched on and off by hydrogen and oxygen. Novel fabrication methods such as two-photon femtosecond direct laser writing, colloidal etching lithography and interference lithography allow for low-cost and large-area fabrication of these functional materials.
We acknowledge the work and assistance of Nikolai Strohfeldt, Florian Sterl, Shahin Bagheri, Dominik Floess, Ramon Walter, Thomas Weiss, Xinghui Yin, M. Schäferling, Andreas Tittl, R. Griessen, T. Weiss, F. Neubrech, C.M. Zgrabik, E.L. Hu, T. Stauden, U. Kreibig, M. Wuttig, T. Taubner, A.K. Michel, A. Berrier, G. Richter, and Bettina Frank.

Previous experiments, including work in our group, have shown that surface plasmon polaritons (SPPs) preserve entangled state and do not cause measurable decoherence. However, all experiments were done when SPPs dispersion was in linear “photon-like” regime. We demonstrate in this experiment how transition to “true-plasmon” non-linear dispersion regime, which occurs near SPP resonance frequency, will affect quantum coherent properties of light.
To generate polarization-entangled state we utilize type-I parametric down-conversion, occurring in a pair of non-linear crystals (BiBO), glued together and rotated by 90 degrees with respect to each other. For projection measurement we use a pair of polarizers and single-photon avalanche diode coincidence count detectors. We put a plasmonic hole array in of the paths of down-converted light before polarizer.
For initial experiment we fabricate 1mm by 1mm plasmonic hole array (200nm of gold on optically polished glass) with a pitch of 700nm and elliptical holes (axes are 190nm and 240nm) using focused ion beam. Such geometry provides plasmon-enhanced transmission at 810nm (at normal incidence) - wavelength of down-converted light. Without hole array we measure visibility V=99% and Bell&’s number S=2.749±0.027. When we put this plasmonic hole array in our system and rotate it in such a way, that ellipse axes are at 45 degrees with respect to both polarizations, we get V=98% and S=2.743±0.067. When ellipse axes are parallel to both polarizations, we measure lower V=86% and lower S=2.695±0.053 as expected due to different transmission amplitudes of different polarizations.
As a sample with non-linear dispersion we fabricate three-layer IMI structure on optically-polished glass (amorphous Si - Au - amorphous Si). The results of measurements with this sample will be discussed.

Plasmonics and its applications have garnered ample attention over the years. These applications range from chemical and biological sensors to enhanced photovoltaics [1-2]. At the heart of these plasmonic devices are resonances that establish the devices&’ unique function. The ability to control these resonances is essential in advancing various applications of plasmonics. In order to control resonances, we must be able to quantitatively observe them. Typically, the approach to analyzing plasmonic resonances is to locate the peaks and troughs of transmission and reflection spectra. This qualitative approach is insufficient for non-symmetric or Fano resonances and worse for cases with multiple overlapping resonances. In order to ameliorate the present situation, we describe an effective Hamiltonian formalism [3] to study and tailor resonances of plasmonic systems at optical frequencies near-IR (1550 nm). By employing this method, we compute the complex poles of the scattering matrix to investigate resonance dynamics of coupled plasmonic bars. The complex poles provide a quantitative estimate of both resonance frequencies and linewidths. As such, we are able to track the evolution of these resonances and study the coupling behavior between multiple plasmonic bars. Strong coupling between these bars can be understood as plasmon hybridization [4]. By using the effective Hamiltonian model, we identify a negative coupling regime for a particular hybridization. We also demonstrate that symmetry breaking within our specific system allows for a very large degree of tunability [5].
References:
[1] H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nature Mater. 9, 205 (2010).
[2] N. Liu, M. Hentschel, T. Weiss, A. P. Alivisatos, and Harald Giessen, “Three-Dimensional Plasmon Rulers,” Science 332, 1407 (2011).
[3] N. Moiseyev, Non-Hermitian Quantum Mechanics (Cambridge University Press, United Kingdom 2011).
[4] E. Prodan, C. Radloff, N. J. Halas, and P. Nordlander, “A Hybridization Model for the Plasmon Response of Complex Nanostructures,” Science 302, 419 (2003).
[5] A. Kodigala, T. Lepetit, and B. Kante, “Engineering resonance dynamics of plasmon hybridized systems,” J. Appl. Phys. 117, 023110 (2015).

Alternative plasmonic materials have attracted considerable attention due to their advantages compared to conventional noble metals, including compatibility with Si processing, wide tunability of optical properties, and reduced losses. In recent years, they have emerged as a novel promising platform for linear and nonlinear nano-photonics applications. In this work, we demonstrate that post-deposition annealing of materials fabricated by magnetron sputtering allows large tuning of the structural and the optical dispersion properties of Indium Tin Oxide (ITO), Al-doped ZnO (AZO) and Titanium Nitride (TiN) nano-layers. By measuring their optical bandgaps, we show that thermal annealing treatments can largely modulate the carrier concentration in these materials, thus providing a path to reduce optical losses and allowing the engineering of the Epsilon-Near-Zero (ENZ) regime on Si. Besides, we perform TEM and XRD measurements to correlate the optical and the structural properties of these emerging materials. Finally, we investigate the effect of different annealing gases on the free carrier concentration and we show that oxygen vacancies are responsible for the free-carrier modulation in both ITO and AZO thin films. Our findings demonstrate the critical importance of annealing treatments to provide tunability of both optical and structural properties of ITO, AZO and TiN as well as loss reduction. The development of alternative plasmonic materials with free electron plasma response tunable from the visible to the mid-IR regimes will expand the reach of Si-compatible plasmonics, metamaterials and transformation-optics to high-density device integration on Si.

In Extraordinary Optical Transmission (EOT), a metallic film perforated with an array of [periodic] apertures exhibits transmission over 100% normalized to the total aperture area, at selected frequencies. EOT devices have potential applications as optical filters and as couplers in hybrid electro-optic contacts and devices. Traditional passive extraordinary optical transmission structures, typically demonstrate un-normalized transmission well below 50%, and are typically outperformed by simpler thin-film techniques. To overcome these limitations, we demonstrate a new breed of extraordinary optical transmission devices, by “burying” an extraordinary optical transmission grating into a dielectric matrix via a metal-assisted-chemical etching (MacEtch) process. The resulting structure is an extraordinary optical transmission grating on top of a dielectric substrate with dielectric nano-pillars extruded through the grating apertures, the EOT grating is in effect buried into the dielectric substrate creating a “buried - extraordinary transmission” (B-EOT) grating.
Here we demonstrate the phenomenon of colossal optical transmission, through B-EOT gratings where the transmission of light through the structure exceeds the transmission through a bare air-dielectric interface. These B-EOT structures not only show significantly enhanced peak transmission when normalized to the open area of the metal film, but more importantly, the peak transmission greater than that observed from the bare semiconductor surface, a phenomenon not seen in traditional EOT grating structures. The phenomenon results from the interplay of light interaction with structured dielectric and plasmonic modes supported by the metallic film. The structures were modeled using three-dimensional rigorous coupled wave analysis and characterized experimentally by Fourier transform infrared reflection and transmission spectroscopy, and good agreement between the two has been demonstrated. The drastic enhancement of light transmission in our structures originates from structuring of high-index dielectric substrate, with the dielectric pillars effectively guiding light through metal apertures.

Chemical and bio detection has attracted a great interest in the last few years due to growing demands from defense, energy harvesting, health care and environmental monitoring. Surface-enhanced Raman scattering (SERS) is one of the most effective and non-destructive spectroscopic tools that are available for both molecular and biochemical detection with fingerprinting specificity and high sensitivity. In recent years, advances in the field of plasmonics and metamaterials have boosted SERS spectroscopy using engineered SERS substrates of Ag and Au nanoparticles of various morphologies arranged in periodic and aperiodic geometries. However, state-of-the-art plasmon SERS substrates are often fabricated using expensive nanoscale lithography that limits technological scalability and drives higher costs. On the other hand, polymer nanostructures can be prepared by the electrospinning technique that is cost-effective, flexible and bio-compatible and relies on solution-based processing in the absence of harsh chemicals or precisely controlled environmental conditions (pressure, temperature, etc.).
In this work, we leverage HPC (Hydroxypropyl cellulose) polymeric nanostructures that are soluble both in water and organic solvents to demonstrate novel plasmonic composites based on Al, Ag and Au core-shell nanowires, dubbed plasmonic “nano-forests”. Specifically, using electrospinning we fabricate networks of cellulose nano-fibers with an average diameter of ~300 nm from a biocompatible solution and then we conformally coat the nano-fibers with Au or Ag nano-layers (40 nm) deposited by e-beam and thermal evaporation. After dissolving the HPC core of the nano-fibers in water, we demonstrate core-shell (air-filled) metal nanowires (with a shell thickness in the 10nm range) interconnected in a composite medium of controllable thickness. The fabricated plasmonic nano-forests were studied as novel and cost-effective plasmonic substrates for molecular and bacterial SERS detection. Dark-field scattering spectroscopy was used to investigate their plasmonic resonances across the visible spectral range and biochemical Raman scattering was measured in a confocal setup. Our work demonstrates strong and reproducible SERS signal from monolayers of p-mercaptoaniline (PMA) adsorbed on the nano-forests surfaces and estimates SERS enhancement values up to 10⁶for a single core-shell nano-fiber. In addition, we show that plasmonic nano-forests exhibit strong and highly reproducible SERS signal from bacterial cells (E.coli) with good selectivity between different strains (E.coli Strain K-12 and strain C). Our findings establish a very promising and novel SERS platform for cost-effective and biochemical detection and sensing.

Despite more than a decade of intensive scientific exploration, new plasmonic phenomena continue to be discovered, including quantum interference of plasmons, observation of quantum coupling of plasmons to single particle excitations, and quantum confinement of plasmons in single-nm scale plasmonic particles. Simultaneously, plasmonic structures find widening applications in integrated nanophotonics, biosensing, photovoltaic devices, single photon transistors and single molecule spectroscopy. Decay of surface plasmons to hot carriers is a new direction that has attracted considerable fundamental and application interest, yet a theoretical understanding of ultrafast plasmon decay processes and the underlying microscopic mechanisms remain incomplete.
Recently we analyzed the quantum decay of surface plasmon polaritons and found that the prompt distribution of generated carriers is extremely sensitive to the energy band structure of the plasmonic material. A theoretical understanding of plasmon-driven hot carrier generation and relaxation dynamics from femtosecond to picosecond timescales is presented here. Employing a Feynman diagram approach has been critical to determine the relevant processes. We report the first ab initio calculations of phonon-assisted optical excitations in metals, which are critical to bridging the frequency range between resistive losses at low frequencies and direct interband transitions at high frequencies. Previously a challenge in such calculations of metals has been to treat the intermediate virtual state as well as the energy conserving `on-shell' intermediate states that correspond to sequential processes. Here we compare the plasmon linewidth and decay rates estimated directly from the experimentally-measured complex dielectric functions with theoretical predictions for cumulative contributions from direct, surface-assisted, phonon-assisted transitions and resistive losses.
We also present calculations of energy-dependent lifetimes and mean free paths of hot carriers, accounting for electron-electron and electron-phonon scattering, lending insight towards transport of plasmonically-generated carriers at the nanoscale. We find that the noble metals have similar maximum carrier lifetimes ~30 fs and mean free paths (~50 nm) in the order Ag> Cu > Au, while aluminum has a smaller maximum lifetime ~10 fs and mean free path of 20 nm. Finally we will discuss calculations for multiplasmon and nonlinear processes in the ultrafast regime from the mid-IR (0.5eV) to visible (1.5eV) and in different geometries. These are compared with experimentally measured chi;³ processes in z-scan measurements and we find excellent agreement between theoretical predictions and measured values.

Because of their ability to strongly localize and enhance electromagnetic field, plasmonic structures have the potential for many nonlinear processes. However, for hybrid plasmonic structures with metallic and dielectrics components all possessing nonlinearities, the origin of their nonlinear responses remains ambiguous. In our recent study, film-coupled nanostripes, where gold stripes were spaced a few nanometers from a gold film by aluminum oxide (Al₂O₃) grown using atomic layer deposition, were proven to exhibit large third-harmonic generation (THG) enhancement [1]. It was found that the THG from the Al₂O₃ spacer was comparable to that from the gold, yet it was not possible to independently distinguish the two contributions. Motivated by this work, we propose a method that identifies the origin of the THG from the film-coupled nanostripes [2]. By considering the THG from each nonlinear components separately, we show that the THG from various constituents of the film-coupled nanostripes are distinguishable due to the fundamental difference in their radiation properties. The THG from the metallic component is suppressed by the structure itself, while that from the dielectric spacer is enhanced by the cavity modes supported by the plasmonic structure. The total THG is a superposition of all the nonlinear sources, whose radiation pattern is determined by the ratio between the third-order susceptibilities of the dielectric and the metal. Our method can be applied to analyze the origin of nonlinear signal of other hybrid plasmonic structures and can trigger experiments in the future.
References
[1] J. B. Lassiter, X. Chen, X. Liu, et al, “Third-Harmonic Generation Enhancement by Film-Coupled Plasmonic Stripe Resonantor” ACS Photon., 1(11), 1212 (2014)
[2] X. Liu, S. Larouche, P. Bowen, et al, “Clarifying the Origin of Third-harmonic Generation from Film-coupled Nanostripes” Opt. Exp.(acceptted)

Metal-containing nanoparticles are of interest for a broad variety of applications, ranging from photovoltaics and solid state lighting to sensing, spectroscopy and catalysis. The key property for most of these applications is the localized plasmon resonance. Each of these applications requires a specific resonance frequency, ranging from the ultraviolet to the mid-infrared, combined with either a high level of absorption or scattering. Therefore, it is of key importance to synthesize particles with a well-defined resonance wavelength and a tailored ratio of absorption vs. scattering. Here, we present the design, synthesis, compositional, structural and optical characterisation of polystyrene-Au and polystyrene-Ag core-shell particles with programmable optical properties. In this study, we performed optical simulations based on finite element method (CST^®) for the design of core-shell particles with a polystyrene core and silver or gold shell. We prepared the particles with desired core size and shell thickness using polystyrene particles tailored for both the deposition of Ag and Au through reduction of silver diammine and tetrachloroauric acid, respectively, in a two-step procedure. In the first step, small silver or gold seeds were selectively deposited on the polystyrene particle surface, covering all particles with homogeneously distributed metal seeds [1]. In the second step, the particles decorated with metal seeds were subjected to metal plating, resulting in well-defined polystyrene-metal core-shell particles. The core size could be tuned in the range between 200 nm and 500 nm, the shell thickness between 10 nm and 50 nm. Structure and composition of the resulting particles were extensively characterized. In addition, we determined the optical properties of the particle dispersions using extinction spectrophotometry. The resulting optical properties were compared with the simulations used for particle design. Discrepancies, e.g. caused by roughness of the metal shell or the formation of silver oxide, were studied and fed back into the simulations used for design of such particles. In addition, we performed a direct comparative analysis between dielectric-Ag and dielectric-Au core-shell particles, which has not yet been reported since previous studies use different template particles for Ag and Au deposition.
[1] D. Mann, S. Chattopadhyay, S. Pargen, M. Verheijen, H. Keul, P. Buskens, M. Möller, Glucose-functionalized polystyrene particles designed for selective deposition of silver on the surface, RSC Advances 2014, 4, 62878-62881.

The localized surface plasmon resonance of sub-wavelength metallic particles depends upon the particle composition, geometry, ambient dielectric environment, and electromagnetic field coupling to neighboring particles and surfaces. Due to the localized electric field enhancements around plasmonic particles, engineered arrays have been used to enhance infrared absorption signals by factors of 10⁴-10⁵ [1]. Surface enhanced sensing substrates typically have fixed geometry and static spectral responses. Recently, tunable plasmonic structures have been fabricated using microelectromechanical systems (MEMS) [2] or stretchable substrates [3, 4] that can tune the interparticle separation and electromagnetic field coupling.
We present the design, fabrication, characterization, and performance of a microelectromechanical systems tunable plasmonic spectrometer. The plasmonic element of this device is an array of holes in a suspended gold film, with a localized surface plasmon resonance in the mid-infrared. The hole array is suspended by MEMS bimorphs above a gold reflector, forming a tunable length Fabry Pérot interferometer. We report on attempts to use this device to measure the spectral response of molecules with vibrational modes in the “fingerprint region” of the mid-infrared. By changing the interferometer cavity length and scanning a reflectance minimum or maximum of the system across a spectral range containing vibrational bands of interest, we demonstrate the capability of selective enhancement or suppression of reflectance from adjacent vibrational spectral bands. This device shows promise as a scalable and versatile device for dynamic surface-enhanced sensing measurements.
We thank DARPA for their support of this work under contract number FA8650-15-C-7545.
[1] R. Adato, et al., PNAS. 2009, 106, (46), 19227-19232.
[2] C. Watts, et al. Adv. Mat.2012, 24 (23), OP98-OP120.
[3] I. Pryce, et al., Nano Lett.2010, (10), 4222-4227.
[4] S. Aksu, et al. Adv. Mat.2011, (23), 4422-4430.

We study the exciton emission from an ensemble of bare and gold/aluminum quinoline (Alq₃) coated GaAs-AlGaAs-GaAs core-shell nanowires (NW) using temperature- and intensity-dependent time-integrated (TI) as well as time resolved (TR) photoluminescence (PL).
The vertically aligned 150 nm diameter zincblende NWs were grown on GaAs substrate using the Au catalyzed vapor-liquid-solid method. Organic-plasmonic nanowire heterostructures (Au on Alq₃ coated NW) were fabricated by organic molecular beam deposition. PL measurements at low temperatures show a strong emission peak at 1.515 eV, a weaker peak at 1.495 eV and a weak emission at ~1.469 eV. The emissions are attributed to the free exciton transition, the carbon free to bound transition and tentatively to an emission from deeply trapped electron or hole states, respectively. Plasmonic GaAs-AlGaAs-GaAs NWs with a ~10 nm thick Au coating but without Alq₃ spacer layer reveal a significant reduction of the PL intensity for all emission bands compared with the uncoated NW sample. Organic-plasmonic core-multishell NWs with the same nominal Au coverage and an additional Alq₃ interlayer of 5 or 10 nm thickness show a noticeably stronger PL intensity which increases with rising Alq₃ spacer thickness. The observed behaviour suggests that Förster energy-transfer from free GaAs excitons to plasmon oscillations in the deposited gold film is mainly responsible for the PL quenching. TR PL measurements support our interpretation by showing an increase in the exciton decay times as we increase the spacer thickness. Au coated NWs also reveal a strong polarization dependent absorption which is mainly due to the significant dielectric mismatch between the nanowires and the adjacent vacuum environment.

Semiconductor nanowires are attractive building blocks for nanoscale electronics and optoelectronic devices. Catalytic, bottom-up nanowire growth is particularly versatile and allows high structural complexity and diversity of materials. Here, we show the ability to selectively produce single germanium nanowires at specific locations in a well-defined controlled process exploiting plasmon-assisted laser-induced Au-catalysed chemical vapour deposition. Using an optical dark field scattering technique during the growth process enables us to monitor in real time the evolution of the optical signature of the nanostructure under investigation. This allows us to observe, and therefore control, the early stages of germanium nucleation and the subsequent nanowire growth. We discuss detailed feedback mechanisms for enhancing structural control as well as the generality of the method for in-situ material characterisation and device integration.