Interband Optical Absorption in Semiconductors

This lecture provides the theoretical framework behind the interband optical absorption process in direct bandgap semiconductors. It starts by deriving the transition rate between valence and conduction band states using Fermi's Golden Rule, ensuring energy and momentum conservation. The lecture then discusses the link between the linear optical susceptibility's imaginary part and the absorption coefficient. It further explores a general expression for the absorption coefficient, valid out of equilibrium, depending on the occupancy of interconnected bands. This leads to the introduction of optical gain and the Bernard-Duraffourg condition specific to semiconductors.