Theory of phonon-assisted luminescence in solids: Application to hexagonal boron nitride

We study the luminescence of hexagonal boron nitride (h-BN) by means of nonequilibrium Green's functions plus finite-difference electron-phonon coupling. We derive a formula for light emission in solids in the limit of a weak excitation that includes perturbatively the contribution of electron-phonon coupling at the first order. This formula is applied to study luminescence in bulk h-BN. This material has attracted interest due to its strong luminescence in the ultraviolet region of the electromagnetic spectrum [K. Watanabe et al., Nat. Mater. 3, 404 (2004)]. The origin of this intense luminescence signal has been widely discussed, but only recently a clear signature of phonon-mediated light emission started emerging from the experiments [G. Cassabois et al., Nat. Photonies 10, 262 (2016)]. By means of our theoretical framework, we provide a clear and full explanation of light emission in h-BN.