Interconnection between ground state and excited state gain in InAs/GaAs quantum dot semiconductor optical amplifiers

Different energy levels are generally assumed to be associated with ground state (GS) and excited state (ES) gain in semiconductor quantum dot lasers and amplifiers. We present calculations based on an 8 band k.P Hamiltonian which show that this is not the case. Two distinct absorption bands are calculated due to transitions between the GS electrons and all dot hole states. In addition to the peak due to transitions between GS electron and GS hole levels, a higher energy band is also calculated due to transitions between GS electrons and highly excited dot hole states. We demonstrate that the removal of electron-hole pairs through stimulated emission at the ES gain maximum can result in an amplitude reduction for a probe pulse tuned to the GS gain maximum, due to the existence of these higher energy transitions, and also due to the effects of two photon absorption. We also find that the instantaneous phase change at the ES gain maximum due to dot to dot interband transitions is small compared to the measured value.