Ultrafast nonlinear phonon response of few-layer hexagonal boron nitride

Abstract

Ultrafast optical-phonon dynamics and the related nonlinear lattice response of few-layer hexagonal boron nitride ($h$-BN) are elucidated in a combined experimental and theoretical study. The lifetimes of transverse optical (TO) phonons and interlayer modes are 1.2 and 20 ps. In-plane TO phonon displacements are shown to couple to interlayer shear and breathing motions. In the nonlinear phonon response, a transient reduction of the TO phonon frequency arises from this anharmonic coupling, which is a particular property of few-layer $h$-BN.