Thickness-dependent phonon polaritons in exfoliated γ-InSe

Light-matter coupling in anisotropic two-dimensional (2D) materials can result in polaritonic effects that depend on the thickness and direction of propagation within the 2D material. Here, we investigate directional phonon polaritons in mechanically exfoliated γ-InSe using resonance Raman spectroscopy. Bulk γ-InSe exhibits two sets of Raman- and IR-active transverse and longitudinal optical (TO and LO, respectively) polar phonons (with in-plane and out-of-plane vibrations) owing to its highly ionic nature; our measurements revealed that one TO phonon (with out of plane A₂ symmetry) couples to the incident excitation very effectively, resulting in a large phonon polariton dispersion. We measured this dispersion by varying the tilt angle between the exfoliated γ-InSe flakes (with thicknesses ranging from 10 – 1210 nm) and the incident normal resonant laser excitation (2.41 eV), allowing us to access the phonon dispersion at wavevectors away from the center of the Brillouin zone. The dispersion depends strongly on flake thickness and we explain our results in terms of confinement-induced exciton/electron-phonon coupling with the guided phonon mode within the ultrathin γ-InSe flakes.