Session F6: Focus Session: Nanostructures and Metamaterials II

Achromatic metasurface optical components by dispersive phase compensation

The replacement of bulk refractive elements with flat ones enables the miniaturization of optical components required for integrated optical systems. This process comes with the limitation that planar optics suffers from large chromatic aberrations due to the dispersion of the phase accumulated by light during propagation. We show that this limitation can be overcome by compensating the dispersion of the propagation phase with the wavelength-dependent phase shift imparted by a metasurface. We demonstrate dispersion-free multi-wavelength dielectric metasurface deflectors in the near-infrared and design an achromatic flat lens in the same spectral region. Our design is based on low-loss coupled dielectric resonators, which introduce a dense spectrum of modes to enable dispersive phase compensation. Achromatic metasurfaces will find applications as multi-band-pass filters, lightweight collimators, and chromatically-corrected imaging lenses.   

Scattering properties of semiconductor-based metamaterials with subwavelength cavities in the infrared region

All-semiconductor (III-V) metamaterials for the IR have recently been proposed for applications in superlensing and sensing [1]. These 1-D structures have been shown to have negative effective refractive index at wavelengths around 8 $\mu $m. Some other metamaterial structures for the visible range [2] employed SiC spherical inclusions in a plasmonic (MgB$_{2})$ host medium. In this paper we develop and model IR metamaterials that utilize both low-loss highly doped GaAs/InAs semiconductor thin films and 3-D structures, such as subwavelength resonant cavities. We predict the light scattering by these structures using experimentally measured dispersion relations for the doped semiconductor films. The frequency dispersions of permittivity are well fitted by the Drude model. Our future work will include the fabrication of the metamaterial structures and their optical characterization. \\[4pt] [1] S. Law, C. Roberts, T. Kilpatrick, L. Yu, T. Ribaudo, E. A. Shaner, V. Podolskiy, and D. Wasserman, \textit{Phys Rev. Letters}, \textbf{112}, 017401, 2014.\\[0pt] [2] A-G. Kussow, A. Akyurtlu, A. Semichaevsky, and N. Angkawisittpan, \textit{Physical Review B, }\textbf{\textit{76}}, 195123, 2007.   

Emission Testing Results of Thermally Stable, Metamaterial, Selective-Emitters for Thermophotovoltaics

In thermophotovoltaics, the energy from a heated emitter is converted to electricity by a photovoltaic diode. A selective emitter can be used to emit a narrow band of wavelengths tailored to the bandgap of the photovoltaic diode. This spectral shaping improves the conversion efficiency of the diode and reduces undesirable diode heating. In our research, we study selective emitters based on metamaterials composed of repeating nanoscale structures. The emission characteristics of these materials vary based on the compositional structure, allowing the emitted spectrum to be tunable. Simulations were performed with CST Microwave Studio to design emitters with peak wavelengths ranging from 1-10 microns. The structures were then fabricated using physical vapor deposition and electron beam lithography on a sapphire substrate. Emitter materials studied include gold, platinum, and iridium. Here we report on the emission spectra of the selective emitters and the post-heating structural integrity.   

Enhancing harmonic generation using nonlinear Metamaterials

In this work, we demonstrate the double-resonator meta-atom design in a nonlinear metamaterial can significantly enhance harmonics in microwave frequency regime. Nonlinearity in the structure is introduced by adding a varactor diode in the common slit of the double split ring resonator (DSRR) design. By engineering the structure such that inner ring resonance frequency of the DSRR is twice as the outer ring resonance frequency, we have demonstrated that the second harmonic of the outer ring can be enhanced by factor of 70 compared to a conventional SRR structure. Furthermore, the second harmonic of the periodic arrays can be further improved by carefully positioning the unit cells. In addition, with the enhancement of the second harmonic, other higher order harmonics can be enhanced.   

Controlling the Polarization State of Light with a Dispersion-Free Metastructure

By combining the advantages of both a metallic metamaterial and a dielectric interlayer, we demonstrate the general mechanism to construct the dispersion-free metastructure, in which the intrinsic dispersion of the metallic structures is perfectly cancelled out by the thickness-dependent dispersion of the dielectric spacing layer. As examples to apply this concept, a broadband quarter-wave plate and a half-wave plate are demonstrated. By selecting the structural parameters, the polarization state of light can be freely tuned across a broad frequency range, and all of the polarization states on the Poincare sphere can be realized dispersion free.   

Photon-Spin Drag on a Plasmonic Metasurface

Classical polarized light carries spin and reveals spin-orbit coupling when propagating along a curved trajectory, however this interaction typically is very weak. Utilizing a metasurface can greatly enhance this interaction because it can bend light abruptly within an extremely small thickness due to the large induced momentum. This strong photonic spin-orbit coupling on a metasurface could drive the electrons collectively and lead to direct electric currents flowing transversely to the light-bending direction, even at normal incidence and without external magnetic fields. Such a photon-spin drag effect has never until now been demonstrated. Here we report the first direct observation of this effect on a metallic metasurface consisting of complementary nanoantennas. By inputting opposite photonic spins, we directly detect the changes inversion of the transverse current direction. This effect enables an electrical detection of photon spin-orbit interactions and provides a viable route to directly integrate modern electronic chips with the additional spin degree of freedom of light for future information processing and communication applications.   

Creation of optical near-field orbital angular momentum in a gold metasurface

Nanocavities in a gold thin film is optimized and arranged to form a metasurface. We demonstrate both numerically and experimentally that surface plasmon vortex carrying optical orbital angular momentum can be generated using linearly-polarized optical excitation.   

Anisotropic Fabry Perot resonances in Metal-Dielectric Meta-Nano-Layer

We theoretically propose a new type of mode which exists in a metal-dielectric metamaterial layer of nanoscopic width. These modes exhibit anisotropic dispersion and can be used for strongly directional emission as well as ultra-compact 90 degree polarization rotation. The anisotropic Fabry-Perot (FP) modes appear from regular FP resonances when metal strips are introduced into a dielectric layer forming a metalayer. Each regular FP resonance splits into two anisotropic FP resonances in this situation. In the metalayer structures under consideration with metal fraction of about 0.2 the lower-energy mode of a higher FP mode becomes degenerate with the higher-energy mode of a lower FR resonance and their interference produces transmission of TE polarized waves upon TM incidence from another side, promising 90 degree polarization rotators with dimensions on the order of 100 nm, an order of magnitude smaller than recently proposed by other authors. Although the described behavior is due to the metamaterial-like response of the metal-dielectric strip array the non-local effects in those structures will be discussed.   

Meta-Optics with Nanowire Grid Arrays: Hyperbolic Fabry-Perot Modes and Hyperbolic Tamm Plasmons

In this talk we introduce a new class of structures -- cavities formed by metal-dielectric metasurfaces. These cavities support a zoo of various resonances, including hyperbolic Tamm plasmons and hyperbolic Fabry-Perot modes, which feature anisotropic clover-leaf dispersion parallel to the metasurface and strong coupling between TM and TE polarizations in the modes. The properties and spectrum of the modes are highly tunable by the dimensional and material parameters of the structure and can be used for directional emission, modification of radiation produced by electric dipole emitters into magnetic dipole radiation as well as 90 degree polarization rotators and polarization rotation mirrors.