Ultrafast nonlinear phonon response of few-layer hexagonal boron nitride

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