Interband coherence and inversionless bistability in semiconductor lasers

We study how interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection curr ent is the only source to establish the interband coherence in a semiconduc tor laser system. It shows that a strongly coupled high-Q microcavity has a low lasing threshold value, which corresponds to the threshold intensity b eing unity. When an external pump-field serving as another mechanism to cre ate the interband coherence is applied, the threshold value of the injectio n current can be lowered and it vanishes for sufficiently strong field. Whe n the pump-field exceeds a threshold value, it is possible to achieve a bis tability in the inversionless region.