Résumé
In this paper, we discuss our recent experiments by two-photon spectroscopy demonstrating that hBN is an indirect bandgap material. We will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions involving either virtual or real excitonic states. We will present our measurements of the exciton binding energy by two-photon excitation, leading to the estimation of 6.08 eV for the single-particle bandgap in hBN. We will also discuss our experimental evidence that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering in hexagonal boron nitride.