Strong Coupling in Semiconductor Hyperbolic Metamaterials.

Semiconductor-based layered hyperbolic metamaterials (HMMs) house high-wavevector volume plasmon polariton (VPP) modes in the infrared spectral range. VPP modes have successfully been exploited in the weak-coupling regime through the enhanced Purcell effect. In this paper, we experimentally demonstrate strong coupling between the VPP modes in a semiconductor HMM and the intersubband transition of epitaxially embedded quantum wells. We observe clear anticrossings in the dispersion curves for the zeroth-, first-, second-, and third-order VPP modes, resulting in upper and lower polariton branches for each mode. This demonstration sets the stage for the creation of novel infrared optoelectronic structures combining HMMs with embedded epitaxial emitter or detector structures.

Far-field thermal emission from a semiconductor hyperbolic metamaterial.

This paper investigates the far-field thermal emission of a Si:InAs/AlSb semiconductor hyperbolic metamaterial (HMM). Understanding the thermal emission of HMMs could result in advancements in thermophotovoltaics and thermal emission management. In recent years, there has been controversy about whether the large wavevector volume plasmon polariton (VPP) modes that exist in HMMs could be outcoupled into the far field using a grating and give rise to super-Planckian thermal emission. In this experiment, gold gratings with varying periods were applied to the surface of the HMM to outcouple the VPP modes into free space. The sample was heated to 185°C, and the polarized thermal emission was measured using Fourier transform infrared spectroscopy. The HMM showed minimal angle sensitivity up to 50°. The sample showed multiple peaks in emission which corresponded to the resonant wavelengths of the VPP modes, demonstrating outcoupling from these modes. Although these modes showed increases in the emissivity, super-Planckian emission was not observed. Emission from VPP modes could be leveraged to create materials with designer emissivity profiles.

The Role of Optical Phonon Confinement in the Infrared Dielectric Response of III-V Superlattices.

Polar dielectrics are key materials of interest for infrared (IR) nanophotonic applications due to their ability to host phonon-polaritons that allow for low-loss, subdiffractional control of light. The properties of phonon-polaritons are limited by the characteristics of optical phonons, which are nominally fixed for most "bulk" materials. Superlattices composed of alternating atomically thin materials offer control over crystal anisotropy through changes in composition, optical phonon confinement, and the emergence of new modes. In particular, the modified optical phonons in superlattices offer the potential for so-called crystalline hybrids whose IR properties cannot be described as a simple mixture of the bulk constituents. To date, however, studies have primarily focused on identifying the presence of new or modified optical phonon modes rather than assessing their impact on the IR response. This study focuses on assessing the impact of confined optical phonon modes on the hybrid IR dielectric function in superlattices of GaSb and AlSb. Using a combination of first principles theory, Raman, FTIR, and spectroscopic ellipsometry, the hybrid dielectric function is found to track the confinement of optical phonons, leading to optical phonon spectral shifts of up to 20 cm-1 . These results provide an alternative pathway toward designer IR optical materials.