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Controlling the permittivity of materials enables control over the amplitude, phase and polarization of light interacting with them. Tailorable and tunable transparent conducting oxides have applications in optical switching, beam steering, imaging, sensing, and spectroscopy. In this work, we experimentally demonstrate wide tailoring and tuning of the optical properties of oxides to achieve fast switching with large modulation depths. In cadmium oxide, the permittivity and the epsilon-near-zero points can be tailored via yttrium doping to achieve large, ENZ-enhanced mid-IR reflectance modulation. In zinc oxide, the permittivity is tuned by interband pumping, achieving large reflectance modulation in the telecom regime. With aluminum-doped zinc oxide, we demonstrate tailorable Berreman-type absorbers that can achieve ultrafast switching in the telecom frequencies. Our work will pave the way to practical optical switching spanning the telecom to the mid-infrared wavelength regimes.

Three-dimensional (3D) metafilms composed of periodic arrays containing single and multiple micrometer-scale vertical split ring resonators per unit cell were fabricated using membrane projection lithography. In contrast to planar and stacked planar structures such as cut wire pairs and fishnet structures, these 3D metafilms have a thickness t ~λd/4, allowing for classical thin film effects in the long wavelength limit. The infrared specular far-field scattering response was measured for metafilms containing one and two resonators per unit cell, and compared to numerical simulations. Excellent agreement in the frequency region below the onset of diffractive scattering was obtained. The metafilms demonstrate strong bi-anisotropic polarization dependence. Further, we show that for 3D metafilms, just as in solids, complex unit cells with multiple atoms (inclusions) per unit cell possess a richer set of excitation mechanisms. The highlight of these new coupling mechanisms is the excitation of the 3D analog to the 2D cut-wire-pair magnetic response.

Coupling of metasurfaces to intersubband transitions (ISTs) in semiconductor quantum wells (QWs) has been extensively studied for various applications ranging from generating giant nonlinear optical response to designing tunable metasurfaces for applications such as ultrafast spatial optical modulators and voltage tunable filters. In this work, we experimentally demonstrate a fundamentally new approach of actively controlling the coupling of ISTs in QWs to a metasurface for voltage tuning its optical response. Unlike previous approaches, we use voltage-controlled quantum tunneling to control the carrier concentration in the QWs for turning on/off the ISTs. We design a multi-quantum well structure consisting of four undoped InGaAs wells with AlInAs barriers grown on top of a highly doped InGaAs layer that acts as an electron reservoir. The heterostructure is optimized such that the first IST in all the wells is at 11µm. A complementary gold metasurface with dipole resonances at 11µm is fabricated on top of the QW structure. We designed the heterostructure such that by applying a bias of 1V, the energy bands of all the QWs get aligned simultaneously, leading to the occupation of the ground state of all the QWs via quantum tunneling of the electrons from the electron reservoir. The ISTs which were turned off due to negligible electron density gets turned on at 1V, and this leads to coupling between the ISTs and the dipoles resonances of the metasurface. The voltage induced coupling leads to reflectance modulation which we confirmed experimentally by rapid scan double modulation FTIR measurements.

Metasurfaces have been investigated for various applications ranging from beam steering, focusing, to polarization conversion. Along with passive metasurfaces, significant efforts are also being made to design metasurfaces with tunable optical response. Among various approaches, voltage tuning is of particular interest because it creates the possibility of integration with electronics. In this work, we demonstrate voltage tuning of reflectance from a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode in an ultrathin semiconductor layer. Our approach involves electrically controlling the carrier concentration of the ENZ layer to modulate the polaritonic coupling between the dipole resonances of the metasurface and the ENZ mode for modulating the reflectance of the metasurface. The hybrid structure we fabricate is similar to MOSCAP configuration where the complementary metasurface offers a continuous gold top layer for biasing and positive/negative bias to the metasurface leads to accumulation/depletion of carriers in the ENZ layer beneath it. We optimized our structure by using InGaAs as the ENZ material because of its high mobility and low effective mass. This allowed us to reduce the doping requirement and thereby reduce the ionized impurity scattering as well as the reverse bias required to deplete the ENZ layer. For low leakage and efficient modulation of carrier density, we used Hafnia as the gate dielectric. We further added a reflecting backplane below the ENZ layer to enhance the interaction and by applying bias, we achieved spectral shifts of 500 nm and amplitude modulation of 11% of one of the polariton branches at 14 µm.

Thin layers of semiconductors where the permittivity crosses zero, support a particular polariton mode called epsilon-near-zero (ENZ) mode. This zero crossing can be obtained near optical phonon resonances in dielectrics or the plasma frequency in doped semiconductors. The coupling of metamaterial resonators to these ENZ modes leads to particularly large Rabi splittings. ENZ layers can be added to metamaterial-based strongly coupled systems to increase this coupling even further. I will discuss several examples of these coupled systems that include metasurfaces, phonons, intersubband transitions and ENZ modes.

We analyze the theoretical and physical properties of a CMOS compatible optical leaky wave antenna (OLWA) integrated into a Fabry-Pérot resonator (FPR) at 193.4 THz (wavelength ?0 = 1550 nm). The presented OLWA design is composed of a silicon (Si) dielectric waveguide sandwiched between two silica glass (SiO2) domains, and it comprises periodic perturbations (cavities of vacuum). We first describe the radiation of the isolated OLWA whose radiation pattern is due to the excitation of a leaky wave, slowly decaying while traveling. The perturbations are indeed designed to obtain a leaky wave harmonic with very low attenuation and phase constants. Then, we integrate the same OLWA into a FPR where two leaky waves with the same wavenumber are travelling in opposite directions inside the resonator. We show that the radiation level at the broadside direction can be effectively controlled by modifying the optical properties of the Si waveguide through electron-hole excess carrier generation (found to be highly enhanced when it is integrated into a FPR). The design of the integrated OLWA is properly set to guarantee the constructive interference of the two radiated beams provided by the two leaky waves in the FPR. The modal propagation constant in the integrated OLWA can be then altered through excess carrier generation in Si, thus the antenna can be tuned in and out of the resonance thanks to the high FPR quality factor, and the LW modal dispersion relation. This allows for enhanced radiation level control at broadside, and preliminary results show up to 13 dB beam modulation. © 2012 SPIE.

Composite materials based on plasmonic nanoparticles allow building metamaterials with very large effective permittivity (positive or negative) or ε-near-zero; moreover, if clustered or combined with other nanoparticles, it is possible to generate also effective magnetic permeability (positive or negative), and an ad-hoc design would result in the generation of double negative materials, and therefore backward wave propagation. However, losses are usually significant and affect the metamaterial performance. In this work, we report on the possibility of adopting fluorescent dye molecules or quantum dots, optically pumped, embedded into the dielectric cores of the employed nanoshell particles, and provide loss-compensation in ordered 3D periodic arrays at optical frequencies. Each spherical nanoshell is modeled as an electric dipole. We consider nanoparticles with gold and silver shells. We then find the modes with complex wavenumber in the metamaterial, and describe the composite material in terms of homogenized effective material parameters (refractive index and permittivity). Furthermore, in case of loss-compensation, we compare the results obtained from modal analysis with the ones computed by using two different homogenization methods: (i) Maxwell Garnett homogenization theory and (ii) Nicholson-Ross-Weir retrieval method. We show the design of two ε-near-zero metamaterials with low losses by simulating gain material made of dyes or quantum dots with realistic parameters. A brief discussion about the employment of the two kinds of active gain materials adopted here is given in the end. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).