Abstract
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.
Highlights
H-BN has emerged as a revolutionary material in the field of optical [1,2] and nanophotonics [3,4] applications because of its unique properties [5]
Experimental difficulties in measuring the absolute reflectance in the extremely small h-BN samples, the IR reflectance spectra are given in arbitrary units, since we are mainly interested in the accurate determination of the phonon parameters from the shape of the reflectance spectra
The frequencies of the IR-active modes of h-BN display a temperature dependence very similar to that previously reported for the E2hgigh Raman-active mode
Summary
H-BN has emerged as a revolutionary material in the field of optical [1,2] and nanophotonics [3,4] applications because of its unique properties [5]. Polariton dispersion in guided surface hyperbolic polariton experiments has been simulated using a generic value of 2 cm−1 for the phonon damping [11], and, in some instances, the Raman linewidth of the E2g mode has been taken as a first approximation for the IR-active phonon lifetime [9], on the assumption that the phonon lifetimes are roughly equivalent for E2g and E1u phonons. We extensively studied in detail the different mechanisms that limit the optical phonon lifetime by means of temperature-dependent Raman scattering measurements [15,16,17,18,19]. Those studies addressed only the Raman-active E2g mode. We present experimental spectroscopic data on the IR-phonon lifetimes that are relevant for phonon-polariton applications and give further insight into the mechanisms that limit phonon lifetimes in h-BN
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