Résumé
I will discuss our recent experiments by two-photon spectroscopy demonstrating that hBN is an indirect bandgap material. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions involving either virtual or real excitonic states. I 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. I will also show that transverse optical phonons at the K point of the Brillouin zone assist intervalley scattering, which becomes observable because stacking faults in bulk hexagonal boron nitride provide a density of final electronic states.