Microscopic theory of gain, absorption, and refractive index in semiconductor laser materials influence of conduction-band nonparabolicity and Coulomb-induced intersubband coupling

The influence of the conduction-band nonparabolicity and Coulomb coupling b etween different electronic subbands and different hole subbands on gain, a bsorption, and refractive index in semiconductor heterostructures is invest igated. We implement these features into a fully microscopic approach. At l ow carrier densities, the nonparabolicity leads to a steeper increase of th e absorption for increasing transition energy. In this regime, the Coulomb subband coupling allows for a shift of oscillator strength to energetically lower transitions. In the gain regime, the conduction-band nonparabolicity is shown to reduce the gain width for a given carrier density and to stron gly modify the corresponding refractive index. The Coulomb coupling is espe cially important to determine the correct energetic position and density de pendence of the gain maximum. In addition, it leads to a steeper transition from the gain to the absorptive region.