Session C23: Acoustic, Thermal, and Photonic Metamaterial Concepts

Photonic hypercrystals

We introduce a new “universality class” of artificial optical media - the photonic hypercrystals. These hyperbolic metamaterials with a periodic spatial variation of dielectric permittivity on a subwavelength scale, combine the features of optical metamaterials and photonics crystals within the same medium.   

A Metasurface Anti-reflection Coating for Enhancing Surface Plasmon-Polariton of Metallic Hole Array.

We demonstrate a metasurface made of metallic disk resonator array as an anti-reflection (AR) coating to enhance (reduce) the transmission (reflection) through metal hole array (MHA). Our result show that the simulated (measured) transmission at the first order surface plasmon-polariton (SPP) resonance is increased up to 82 {\%}( 88{\%}) compared to uncoated MHA. The electric field of the surface wave is also enhanced by 33{\%}. Using an effective medium theory, we show that the metasurface operates at off-resonance wavelengths and can be understood as a thin film that exhibits high effective permittivity (\textasciitilde 30) with very low loss (loss tangent \textasciitilde 0.005). Thus we reveal the mechanism of the metasurface AR coating as the traditional thin film AR coating. With tunable effective permittivity, our structure provides great flexibility to achieve AR coating for general substance at any wavelength.   

Hybrid graphene/dielectric metasurfaces for enhanced transmission modulation

All-dielectric silicon based metasurfaces are powerful platforms to enhance light-matter interactions at nanoscale regions. Their low-loss nature, CMOS processing compatibility and increased damage threshold promise to outperform the functionalities of the recently established plasmonic metallic metasurfaces. In our talk, we will demonstrate ways to hybridize all-dielectric metasurfaces with graphene in order to obtain new electro-optical devices. In particular, a hybrid graphene/dielectric metasurface design will be presented to achieve tunable and modulated transmission at near-infrared (near-IR) frequencies (C. Argyropoulos, Optics Express, vol. 23, No. 18, pp. 23787-23797, 2015). The proposed all-dielectric metasurface is composed of periodically arranged pairs of asymmetric silicon nanobars, which can sustain trapped magnetic resonances with a sharp Fano-type transmission signature. One-atom-thick graphene is placed over this dielectric metasurface and strong transmission modulation is obtained at near-IR telecom wavelengths as the doping level of graphene is increased. The enhanced in-plane fields along the all-dielectric metasurface strongly interact with the tunable properties of graphene. This leads to strong coupling between the incoming radiation and graphene. Several new integrated nanophotonic components are envisioned based on the proposed device, such as efficient electro-optical transmission modulators.   

Nonlinear light-matter interactions in engineered optical media

In this talk, we consider fundamental optical phenomena at the interface of nonlinear and singular optics in artificial media, including theoretical and experimental studies of linear and nonlinear light-matter interactions of vector and singular optical beams in metamaterials. We show that unique optical properties of metamaterials open unlimited prospects to ``engineer'' light itself. Thanks to their ability to manipulate both electric and magnetic field components, metamaterials open new degrees of freedom for tailoring complex polarization states and orbital angular momentum (OAM) of light. We will discuss several approaches to structured light manipulation on the nanoscale using metal-dielectric, all-dielectric and hyperbolic metamaterials. These new functionalities, including polarization and OAM conversion, beam magnification and de-magnification, and sub-wavelength imaging using novel non-resonant hyperlens are likely to enable a new generation of on-chip or all-fiber structured light applications. The emergence of metamaterials also has a strong potential to enable a plethora of novel nonlinear light-matter interactions and even new nonlinear materials. In particular, nonlinear focusing and defocusing effects are of paramount importance for manipulation of the minimum focusing spot size of structured light beams necessary for nanoscale trapping, manipulation, and fundamental spectroscopic studies. Colloidal suspensions offer as a promising platform for engineering polarizibilities and realization of large and tunable nonlinearities. We will present our recent studies of the phenomenon of spatial modulational instability leading to laser beam filamentation in an engineered soft-matter nonlinear medium. Finally, we introduce so-called virtual hyperbolic metamaterials formed by an array of plasma channels in air as a result of self-focusing of an intense laser pulse, and show that such structure can be used to manipulate microwave beams in a free space.   

Broadband non-unity magnetic permeability in planar hyperbolic metamaterials

Metal/dielectric heterostructures with extreme anisotropy and topologically nontrivial dispersion are of fundamental and applied interest due to unique optical and opto-electronic properties. Here we demonstrate that, surprisingly, such systems exhibit a broadband non-unity magnetic response. Typically the electromagnetic properties of such metal-dielectric stacks are deduced from effective medium theories for unbounded, i.e., infinite in size periodic arrangements (c.f., Maxwell-Garnett approximation). In this talk, we show that this description is incomplete for metamaterials with finite number of layers. We demonstrate that a few-layer metal-dielectric metamaterial exhibits a non-unity magnetic permeability across the whole visible spectrum. The response can be diamagnetic or paramagnetic depending on the type of the terminating layers: metallic or dielectric, with non-resonant magnetic permeability that can be engineered to attain values as low as -2 or as high as 2. We have developed a theoretical model that explains the underlying mechanism. We further experimentally validate non-unity effective permeability in the optical range of frequencies. Ag/SiO$_{\mathrm{2\thinspace }}$and Ge-based metamaterials fabricated with electron beam evaporation are characterized by ellipsometric measurements and also phase and amplitude of transmittance/reflectance. These results open pathways for creating broadband subwavelength magnetic structures in the visible regime.   

Microscopic model of the nonlocal response of metamaterial plasmonic structures

Nonlocal effects are generally omitted in typical approaches to calculating the electromagnetic response of the metamaterial plasmonic structures. In some situations, however, where the electron momenta far exceed those of photons, nonlocal corrections are essential. In this work, we investigate simple models of the nonlocal dielectric functions, based on the d-function formalism of Feibelman [1,2], and assess their validity by comparing with experiments. We show, that the applicability of the commonly used hydrodynamic approximation is very limited, since it often strongly overestimates the nonlocal response. [1] Feibelman, Prog. Surf. Sci. 12, 287 (1982); Phys. Rev. B 40, 2752 (1989) [2] Liebsch, Phys. Rev. B 48, 15 (1993)   

Semiconductor Hyperbolic Metamaterials for the Mid-Infrared

Hyperbolic metamaterials have shown great promise for controlling light in the visible spectral range. However, moving metamaterials to the infrared is not just a matter of scaling geometries, but also of choosing new materials with appropriate optical properties. We demonstrate infrared hyperbolic metamaterials with optical properties tunable across the mid-infrared created from semiconductor superlattices grown by molecular beam epitaxy. The metamaterials are made of alternating subwavelength layers of metal (doped semiconductor) and dielectric (undoped semiconductor). By tuning the doping density, layer thicknesses, and metal:dielectric thickness ratio, we can control the onset and bandwidth of metamaterial behavior across the infrared. Our materials exhibit low optical losses as well as high sample uniformity and sharp interfaces. Transmission and reflection properties of the samples are studied by Fourier transform infrared spectroscopy and modeled with effective medium theory. We will also show the results from a beam optics experiment which demonstrates that our materials exhibit negative refraction.   

Semiconductor-based mid-IR metamaterials: experimental and theoretical studies

All-semiconductor (III-V) metamaterials (MTM) for the infrared (IR) can be applied to superlensing and optical cloaking [1]. 1-D metallic-semiconductor superlattices can be designed to have hyperbolic dispersion due to the choice of their effective permittivity tensor components. In this paper we go beyond the effective-medium theories and provide a detailed analysis of how the choice of doping levels and layer thicknesses in the InAs - InAs:Si will affect the reflectance of the MTM superlattice in the IR. In order to do that, four metamaterial samples with various doping profiles were grown by MBE and characterized using FTIR. For the same samples we performed full-wave calculations of the wavelength- and angle-resolved reflectance. Our numerical model is suitable for 1-D inhomogeneous lossy dispersive media and is capable of accounting for an arbitrary doping profile and the quantum mechanical tunneling of electrons in the heterostructure. Experimental and theoretical results for the reflectance of IR metamaterial structures are compared. [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.   

Manipulate acoustic waves by impedance matched acoustic metasurfaces

We design a type of acoustic metasurface, which is composed of carefully designed slits in a rigid thin plate. The effective refractive indices of different slits are different but the impedances are kept the same as that of the host medium. Numerical simulations show that such a metasurface can redirect or reflect a normally incident wave at different frequencies, even though it is impedance matched to the host medium. We show that the underlying mechanisms can be understood by using the generalized Snell's law, and a unified analytic model based on mode-coupling theory. We demonstrate some simple realization of such acoustic metasurface with real materials. The principle is also extended to the design of planar acoustic lens which can focus acoustic waves.   

Simulation and Experimental Realization of a Nano-scale Thermal Cloak.

Manipulation of heat flow at microstructures plays an important role in modern industry, especially for electronic and optoelectronic devices, for their performance and reliability are highly temperature dependent. Analogous to the invisible cloak in transformation optics, the thermal cloak can hide objects from heat and realize isothermal region in transformation thermodynamics. However, due to the macro-scale thermal properties may not be suitable for nano-materials, the realization of the nano-scale thermal cloak highly relies on the thermal transport in nanostructures. Here, we report our recent work of the realization of nano-scale thermal cloak based on the thermal property study of nano- materials via a spatially resolved thermal resistance measurement technique. The simulation and experiment verified its maintenance of isothermal region and heat protection capabilities. This work may provide a new way to manipulate heat transport in nano-scale devices..   

Microscopic Model of the Nonlocal Response of Metamaterial Plasmonic Structures II

Nonlocal effects are generally omitted in typical approaches to calculating the electromagnetic response of metamaterial plasmonic structures. In some situations, however, where the electron momenta far exceed those of photons, nonlocal corrections are essential. In this work, we calculate the nonlocal plasmonic response of a microscopic model of a metamaterial plasmonic structure by employing the random phase approximation, and the self-consistent ground Lang-Kohn states. We compare our results with experiment, and various simple models, including the hydrodynamic approximation.