Phonon polariton in thin β-Ga2O3 crystal

Abstract

The thickness constraint of thin crystals imposes spatial limitations on phonon polaritons (PhPs) and thus significantly affects their properties. In this work, we theoretically study the PhPs in different type hyperbolic bands of thin β-gallium oxide (β-Ga₂O₃) crystals and find that the volume-confined phonon polaritons (v-PhPs) can be launched. The v-PhPs excited by the light frequency corresponding to type Ⅰ_┴ hyperbolic bands can propagate along in-plane direction which is impossible in bulk crystal. Furthermore, the modes coupling results in a “petal-like” isofrequency curve for the v-PhPs excited by the light frequency of type Ⅱ_‖ hyperbolic bands in thin β-Ga₂O₃, enabling the polariton waves to propagate along mutually orthogonal directions. This feature renders the thin crystal as a dynamic beamsplitter that can be activated by selecting an appropriate incident wavelength, thereby facilitating the manipulation of light or heat propagation. These findings provide a promising avenue for applications in nanophotonic and heat conduction applications.