Population Inversion Induced by Resonant States in Semiconductors

We present a theoretical prediction of a new mechanism for carrier population inversion in semiconductors under an applied electric field with suitable field strength. The mechanism is originated from a coherent capture-emission-type inelastic scattering of resonant states. We support our theory with concrete calculations for shallow acceptor resonant states in strained $p$-Ge where a lasing in THz frequency region has been recently observed.