Directional Control of Infrared Emission in Tunable III-V Semiconductor Platforms

We investigate the design of infrared metamaterials with directionally dependent emissivity and absorptivity. We envision an electrically tunable material for which the applied voltage perturbs the refractive index in the insulator layer of a metal-insulator-metal (MIM) metamaterial. We examine the options for tuning the angular dependence of emissivity for both confined MIM modes and extended Bloch modes. In the case of the MIM modes, we introduce a scheme for breaking the mirror symmetry of the unit cell via voltage perturbation. We show that the broken symmetry can be used to couple a previously hidden, or symmetry-forbidden “dark” mode to radiation. This creates a mechanism for switching on and off emissive spectral features in the infrared spectrum at normal incidence. In the case of extended Bloch modes, we consider the use of the index perturbation to double the period of the material. In an appropriately designed structure, we show that period doubling opens a band gap for off-angle radiation, splitting the emissivity features in the spectrum. We further consider possibilities for creating asymmetric emissivity as a function of angle. We show that when a Bloch mode metamaterial is designed to support diffraction orders, we can switch from a more isotropic emissivity to a more one-sided, directional emissivity. We present experimental results on the fabrication and characterization of the basic materials platform needed to achieve these theoretical effects. The platform includes a transferred GaAs layer containing a vertical p-i-n junction for tuning. We further characterize the measured spectral shift as a function of applied voltage. From the results, we can extract the typical index shifts that can be achieved in experiment. The results point to the potential for control over spectral and directional emissivity characteristics in electrically tunable semiconductor platforms.