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. 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. 
M.A.Odnoblyudov (1 and 2), I.N.Yassievich (1 and 2), M.S.Kagan (3),   Yu.M.Galperin (4), X.H.Wang (1) and K.A.Chao (1) ((1) ...