Long lifetime of the E1u in-plane infrared-active modes of h -BN

We present an infrared reflectivity study of the ${E}_1u$ in-plane phonons of hexagonal BN as a function of temperature in the 40--680 K range. The infrared reflectance spectra of high-quality lamellar single crystals are accurately fitted using Lowndes' factorized form of the dielectric response, where the longitudinal-optic (LO) frequency is an independent adjustable parameter. From this analysis we obtain reliable values for the phonon damping of the IR-active ${E}_1u$ phonons which couple to light and give rise to the phonon-polariton excitations in this hyperbolic material. Anharmonic coupling potentials are estimated from the temperature dependence of the damping parameters. The ${E}_1u(\text{LO})$ mode exhibits a substantially longer lifetime than its transverse-optic counterpart, as it is basically unaffected by isotopic-disorder scattering owing to the low density of phonon states around the ${E}_1u(\text{LO})$ frequency.