Theory of interband coherence and inversionless bistability in semiconductor lasers

Coherent transitions of charge carriers between the conduction and valence bands of a semiconductor medium are essential for the operation of a semico nductor laser. In this paper, we study how such interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection current is the only source to establish the interband coherence in a semiconductor laser system. A laser threshold is o btained, which shows that a strongly coupled high-Q microcavity has a low t hreshold value. However, when an external pump-field serving as another mec hanism to create the interband coherence is applied, the threshold value of the injection current can be lowered and it vanishes for sufficiently stro ng field. Besides, if the pump-field exceeds a threshold value, it is even possible to achieve a bistability in the inversionless region. Some fundame ntal macroscopic properties, including polarization, absorption and dispers ion for the semicoductor system, are also obtained analytically.