Overtones of interlayer shear modes in the phonon-assisted emission spectrum of hexagonal boron nitride

We address the intrinsic optical properties of hexagonal boron nitride in the deep ultraviolet. Weshow that the fine structure of the phonon replicas arises from overtones involving up to six low-energy interlayer shear modes. These lattice vibrations are specific to layered compounds since theycorrespond to the shear rigid motion between adjacent layers, with a characteristic energy of about6-7 meV. We obtain a quantitative interpretation of the multiplet observed in each phonon replicaunder the assumption of a cumulative Gaussian broadening as a function of the overtone index,and with a phenomenological line-broadening taken identical for all phonon types. We show fromour quantitative interpretation of the full emission spectrum above 5.7 eV that the energy of theinvolved phonon mode is 6.8±0.5 meV, in excellent agreement with temperature-dependent Ramanmeasurements of the low-energy interlayer shear mode in hexagonal boron nitride. We highlightthe unusual properties of this material where the optical response is tailored by the phonon groupvelocities in the middle of the Brillouin zone.