Multifunctional Metasurface Tuning by Liquid Crystals in Three Dimensions.

Optical metasurfaces present remarkable opportunities for manipulating wave propagation in unconventional ways, surpassing the capabilities of traditional optical devices. In this work, we introduce and demonstrate a multifunctional dynamic tuning of dielectric metasurfaces containing liquid crystals (LCs) through an effective three-dimensional (3D) control of the molecular orientation. We theoretically and experimentally study the spectral tuning of the electric and magnetic resonances of dielectric metasurfaces, which was enabled by rotating an external magnetic field in 3D. Our approach allows for the independent control of the electric and magnetic resonances of a metasurface, enabling multifunctional operation. The magnetic field tuning approach eliminates the need for the pre-alignment of LCs and is not limited by a finite set of directions in which the LC molecules can be oriented. Our results open new pathways for realizing dynamically reconfigurable metadevices and observing novel physical effects without the usual limitations imposed by the boundary conditions of LC cells and the external voltage.

Infrared all-dielectric Kerker metasurfaces.

The unidirectional scattering of electromagnetic waves in the backward and forward direction, termed Kerkers' first and second conditions, respectively, is a prominent feature of sub-wavelength particles, which also has been found recently in all-dielectric metasurfaces. Here we formulate the exact polarizability requirements necessary to achieve both Kerker conditions simultaneously with dipole terms only and demonstrate its equivalence to so-called "invisible metasurfaces". We further describe the perfect absorption mechanism in all-dielectric metasurfaces through development of an extended Kerker formalism. The phenomena of both invisibility and perfect absorption is shown in a 2D hexagonal array of cylindrical resonators, where only the resonator height is modified to switch between the two states. The developed framework provides critical insight into the range of scattering response possible with all-dielectric metasurfaces, providing a methodology for studying exotic electromagnetic phenomena.

Engineering scattering patterns with asymmetric dielectric nanorods.

By controlling interference of Mie resonance modes of various nanostructures, we can achieve a large number of nontrivial effects in nanophotonics. In this work, we propose a cylindrical structure in which the spectral overlap of the Mie-type modes can be controlled by drilling a hole parallel to the axis, thus changing unidirectional scattering. We further demonstrate that the scattering patterns can be tailored by rotating the structure to achieve almost arbitrary scattered wave direction.

Experimental realization of a terahertz all-dielectric metasurface absorber.

Metamaterial absorbers consisting of metal, metal-dielectric, or dielectric materials have been realized across much of the electromagnetic spectrum and have demonstrated novel properties and applications. However, most absorbers utilize metals and thus are limited in applicability due to their low melting point, high Ohmic loss and high thermal conductivity. Other approaches rely on large dielectric structures and / or a supporting dielectric substrate as a loss mechanism, thereby realizing large absorption volumes. Here we present a terahertz (THz) all dielectric metasurface absorber based on hybrid dielectric waveguide resonances. We tune the metasurface geometry in order to overlap electric and magnetic dipole resonances at the same frequency, thus achieving an experimental absorption of 97.5%. A simulated dielectric metasurface achieves a total absorption coefficient enhancement factor of FT=140, with a small absorption volume. Our experimental results are well described by theory and simulations and not limited to the THz range, but may be extended to microwave, infrared and optical frequencies. The concept of an all-dielectric metasurface absorber offers a new route for control of the emission and absorption of electromagnetic radiation from surfaces with potential applications in energy harvesting, imaging, and sensing.

Ultrathin tunable terahertz absorber based on MEMS-driven metamaterial.

The realization of high-performance tunable absorbers for terahertz frequencies is crucial for advancing applications such as single-pixel imaging and spectroscopy. Based on the strong position sensitivity of metamaterials' electromagnetic response, we combine meta-atoms that support strongly localized modes with suspended flat membranes that can be driven electrostatically. This design maximizes the tunability range for small mechanical displacements of the membranes. We employ a micro-electro-mechanical system technology and successfully fabricate the devices. Our prototype devices are among the best-performing tunable THz absorbers demonstrated to date, with an ultrathin device thickness (~1/50 of the working wavelength), absorption varying between 60% and 80% in the initial state when the membranes remain suspended, and fast switching speed (~27 µs). The absorption is tuned by an applied voltage, with the most marked results achieved when the structure reaches the snap-down state. In this case, the resonance shifts by >200% of the linewidth (14% of the initial resonance frequency), and the absolute absorption modulation measured at the initial resonance can reach 65%. The demonstrated approach can be further optimized and extended to benefit numerous applications in THz technology.

Nonlinear coupling in graphene-coated nanowires.

We propose and analyze nonlinear coupler based on a pair of single mode graphene-coated nanowires. Nonlinear wave interactions in such structure are analyzed by the coupled mode equations derived from the unconjugated Lorentz reciprocity theorem. We show that the routing of plasmons in the proposed structure can be controlled by the input power due to the third order nonlinear response of graphene layer. Our findings show that graphene nonlinearity can be exploited in tunable nanoplasmonic circuits based on low-loss, edgeless cylindrical graphene waveguides.

Strong terahertz absorption in all-dielectric Huygens' metasurfaces.

We propose an all dielectric metamaterial that acts as a perfect terahertz absorber without a ground plane. The unit cell consists of a dielectric cylinder embedded in a low index material. In order to achieve near-perfect terahertz absorption (99.5%) we employ impedance matching of the electric and magnetic resonances within the cylinders of the Huygens' metasurface. The impedance matching is controlled by changing the aspect ratio between the height and diameter of the cylinder. We show that the absorption resonance can be tuned to particular frequencies from 0.3 to 1.9 THz via changing the geometry of the structure while keeping a nearly constant aspect ratio of the cylinders.

Second harmonic generation in graphene-coated nanowires.

We study second harmonic generation in a pair of graphene-coated nanowires. We show that the phase matching condition for harmonic generation can be engineered in a wide range of frequencies by tuning the spacing between graphene nanowires. We derive coupled mode equations describing the process of second harmonic generation using an unconjugated Lorentz reciprocity theorem. We show that the highest harmonic generation efficiency can be achieved by phase matching the fundamental mode with the two lowest order symmetric modes at the second harmonic frequency. Despite losses in graphene, we predict that the efficiency can be further enhanced by reducing the radius of nanowires due to larger mode overlap and lower propagation loss.

Elastic metamaterials for tuning circular polarization of electromagnetic waves.

Electromagnetic resonators are integrated with advanced elastic material to develop a new type of tunable metamaterial. An electromagnetic-elastic metamaterial able to switch on and off its electromagnetic chiral response is experimentally demonstrated. Such tunability is attained by harnessing the unique buckling properties of auxetic elastic materials (buckliballs) with embedded electromagnetic resonators. In these structures, simple uniaxial compression results in a complex but controlled pattern of deformation, resulting in a shift of its electromagnetic resonance, and in the structure transforming to a chiral state. The concept can be extended to the tuning of three-dimensional materials constructed from the meta-molecules, since all the components twist and deform into the same chiral configuration when compressed.

Electrically tunable terahertz metamaterials with embedded large-area transparent thin-film transistor arrays.

Engineering metamaterials with tunable resonances are of great importance for improving the functionality and flexibility of terahertz (THz) systems. An ongoing challenge in THz science and technology is to create large-area active metamaterials as building blocks to enable efficient and precise control of THz signals. Here, an active metamaterial device based on enhancement-mode transparent amorphous oxide thin-film transistor arrays for THz modulation is demonstrated. Analytical modelling based on full-wave techniques and multipole theory exhibits excellent consistent with the experimental observations and reveals that the intrinsic resonance mode at 0.75 THz is dominated by an electric response. The resonant behavior can be effectively tuned by controlling the channel conductivity through an external bias. Such metal/oxide thin-film transistor based controllable metamaterials are energy saving, low cost, large area and ready for mass-production, which are expected to be widely used in future THz imaging, sensing, communications and other applications.

Tunable Meta-Liquid Crystals.

Meta-liquid crystals, a novel form of tunable 3D metamaterials, are proposed and experimentally demonstrated in the terahertz frequency regime. A morphology change under a bias electric field and a strong modulation of the transmission are observed. In comparison to conventional liquid crystals, there is considerable freedom to prescribe the electromagnetic properties through the judicious design of the meta-atom geometry.

Optical Metacages.

We suggest a novel strategy for spectrally selective optical shielding of arbitrary shaped volumes by arranging specifically designed two- or three-layer nanowires around an area that needs to be protected. We show that such nanowire shields preserve their functionality for almost arbitrary geometry, and we term such structures optical metacages. We analyze several designs of such optical metacages made from either metallic or dielectric materials with experimentally measured parameters. We employ a semianalytical approach and also verify our results by numerical simulations. We further study optical properties of the introduced metacages in both near- and far-field regions, as well as analyze their frequency selectivity and the vanishing backscattering regime.

Superabsorption of light by nanoparticles.

Nanoparticles have a fundamental limit as to how much light they can absorb. This limit is based on the finite number of modes excited in the nanoparticle at a given wavelength and maximum absorption capacity per mode. The enhanced absorption can be achieved when each mode supported by the nanoparticle absorbs light up to the maximum capacity. Using a stochastic optimization algorithm, we design multilayer nanoparticles, in which we can make several resonant modes overlap at the same frequency resulting in superabsorption. We further introduce the efficiency of the absorption for a nanoparticle, which is the absorption normalized by the physical size of the particle, and show that efficient absorbers do not always operate in the superabsorption regime.

Superabsorption of light by multilayer nanowires.

We suggest a new strategy for tailoring and enhancing the absorption of light by multilayered nanowires. We use the multipole expansion method and experimental data for dielectric and plasmonic materials and demonstrate that the absorption for one of the polarizations can be substantially enhanced due to an overlap of different resonant modes in nanowires. We show that our approach can be employed for a design of multiband tunable optical absorption across a wide spectral range for both TE and TM polarizations.

All-dielectric multilayer cylindrical structures for invisibility cloaking.

We study optical response of all-dielectric multilayer structures and demonstrate that the total scattering of such structures can be suppressed leading to optimal invisibility cloaking. We use experimental material data and a genetic algorithm to reduce the total scattering by adjusting the material and thickness of various layers for several types of dielectric cores at telecommunication wavelengths. Our approach demonstrates 80-fold suppression of the total scattering cross-section by employing just a few dielectric layers.

Spontaneous chiral symmetry breaking in metamaterials.

Spontaneous chiral symmetry breaking underpins a variety of areas such as subatomic physics and biochemistry, and leads to an impressive range of fundamental phenomena. Here we show that this prominent effect is now available in artificial electromagnetic systems, enabled by the advent of magnetoelastic metamaterials where a mechanical degree of freedom leads to a rich variety of strong nonlinear effects such as bistability and self-oscillations. We report spontaneous symmetry breaking in torsional chiral magnetoelastic structures where two or more meta-molecules with opposite handedness are electromagnetically coupled, modifying the system stability. Importantly, we show that chiral symmetry breaking can be found in the stationary response of the system, and the effect is successfully demonstrated in a microwave pump-probe experiment. Such symmetry breaking can lead to a giant nonlinear polarization change, energy localization and mode splitting, which provides a new possibility for creating an artificial phase transition in metamaterials, analogous to that in ferrimagnetic domains.

Cloaking and enhanced scattering of core-shell plasmonic nanowires.

We study scattering of light from multi-layer plasmonic nanowires and reveal that such structures can demonstrate both enhanced and suppressed scattering regimes. We employ the mode-expansion method and experimental data for material parameters and introduce an optimized core-shell nanowire design which exhibits simultaneously superscattering and cloaking properties at different wavelengths in the visible spectrum.

Flexible helices for nonlinear metamaterials.

The successful fabrication and experimental verification of a novel metamaterial based on flexible metallic helices is reported. The helices undergo compression under the influence of incident radiation, demonstrating a nonlinear chiral electromagnetic response, associated with the power-dependent change in the helix pitch. This design is promising for application to power-dependent polarization rotation of propagating waves.

Self-focusing of femtosecond surface plasmon polaritons.

We study the propagation of femtosecond pulses in nonlinear metal-dielectric plasmonic waveguiding structures by employing the finite-difference time-domain numerical method. Self-focusing of plasmon pulses is observed for defocusing Kerr-like nonlinearity of the dielectric medium due to normal dispersion. We compare the nonlinear propagation of plasmon pulses along a single metal-dielectric interface with the propagation within a metal-dielectric-metal slot waveguide and observe that nonlinear effects are more pronounced for the single surface where longer propagation length may compensate for lower field confinement.

Metamaterials controlled with light.

We suggest and verify experimentally the concept of functional metamaterials whose properties are remotely controlled by illuminating the metamaterial with a pattern of visible light. In such metamaterials arbitrary gradients of the effective material parameters can be achieved simply by adjusting the profile of illumination. We fabricate such light-tunable microwave metamaterials and demonstrate their unique functionalities for reflection, shaping, and focusing of electromagnetic waves.