We present an infrared reflectivity study of the E1u 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 E1u 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 E1u(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 E1u(LO) frequency.

This work has been supported by the Spanish MINECO/FEDER under Contracts No. MAT2015-71035-R and No. MAT2016-75586-C4-1-P. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant No. JPMXP0112101001, JPS, KAKENHI Grant No. JP20H00354, and the CREST (JPMJCR15F3), JST.

Peer reviewed