2, U.S. Naval Research Laboratory, Washington, District of Columbia, United States
Hyperbolic metamaterials (HMMs) are artificial materials with an engineered subwavelength structure. The permittivities of HMMs in the plane versus out of the plane are of opposite sign, resulting in an open hyperbolic isofrequency surface. HMMs possess novel properties, like negative refraction and an enhanced Purcell effect, which are difficult to find in natural materials. One simple way to create HMMs is by growing a multilayer structure with alternating metal and dielectric layers. Previously, researchers have used traditional metals (silver, gold) and dielectrics (silica) to create HMMs for the ultra-violet and visible spectral ranges. We are interested in working in the infrared, so we choose to use semiconductor building blocks. It has been demonstrated that doped semiconductors grown by molecular beam epitaxy act as infrared plasmonic metals with optical properties tunable across the infrared and low optical losses . We have demonstrated the designer infrared HMMs comprising Si:InAs (metal) and intrinsic InAs (dielectric) . Using Fourier transform infrared spectroscopy, we observed discontinuity of the Brewster angle and negative refraction for our samples, both hallmarks of HMM behavior. Another interesting property of HMMs is that they can support the propagation of light with large wavevectors (volume plasmon polariton, or VPP, modes) which are not allowed in normal materials. These collective modes in the HMM arise from the coupling of surface plasmon polaritons at each metal/dielectric interface. We investigated the VPP modes in Si:InAs/InAs HMM and Si:InGaAs/InAlAs HMM by depositing gold gratings with different periods on top of their surface. We found that the detailed distribution of electrons at metal/dielectric interface strongly affects the signal of the collective modes. Conversely, the strength and full width-half maximum of these features indicates the quality of the interface . Studying the details of the VPP mode shape and dispersion gives important information about the interface quality and subwavelength structure with in an HMM. This information cannot be obtained any other way and is necessary for the design of devices using these collective modes. The study of the novel optical properties of HMM and their collective modes will lay the foundation for the applications such as enhanced infrared detectors, superlens, hyperlens and other optical devices.
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 D. Wei, C. Harris, and S. Law, Opt. Mater. Express 7, 2672 (2017).