Optic Phonon Confinement for Modifying the Infrared Dielectric Function

<b>In recent years, polar dielectrics have become a principal material class of interest for infrared nanophotonics. In such materials, the dielectric function is largely driven by the optic phonons, which dictate the material’s ability to host phonon-polaritons, quasi-particles that enable strong light-matter interactions, deeply sub-diffractional confinement of light, and a variety of exotic nanophotonic behavior. The optic phonons also play a critical role in many electrical and thermal processes through scattering with free carriers and acoustic phonons. In general, optic phonons are somewhat fixed for a given material, limiting the toolbox for engineered materials and resultant devices. However, it has been shown that for atomically thin layers, optic phonons become confined, resulting in a spectral shift in the primary phonon modal frequencies and introducing additional higher order phonon modes leading to a new material dielectric function that cannot be described using the properties of the bulk constituents. This phonon confinement has been reasonably well modeled in Raman studies, however the link to the infrared optical response has not been clearly defined. Here, we explore such phonon confinement within near-lattice-matched superlattices of GaSb, AlSb, and InAs, correlating the bulk phonon dispersion and Raman confined modes to the IR dielectric function in order to determine the impact that such phonon confinement plays in this regard. Through this, we gained insights into the impact of atomic-scale phonon confinement on the optical response, and provide a predictive, generalized view of phonon confinement as a pathway towards engineering the infrared dielectric permittivity. We also explore the off-gamma phonon dispersion through inelastic x-ray scattering to better understand the mechanism of phonon confinement, providing the first direct measurements of the impact of the phonon confinement in superlattices on the optic phonon dispersion. </b>