Patent ID: 7843981

Claim:
A quantum cascade laser comprising: a semiconductor substrate; and an active layer which is provided on the semiconductor substrate and has a cascade structure formed by alternately laminating quantum well light emitting layers and injection layers by multistage-laminating unit laminate structures each of which includes the quantum well light emitting layer and the injection layer, wherein each of the plurality of unit laminate structures included in the active layer has an emission upper level, an emission lower level, and an injection level as an energy level higher than the emission upper level in its subband level structure, light is generated by intersubband emission transition of electrons from the emission upper level to the emission lower level in the quantum well light emitting layer, electrons through the intersubband emission transition transit from the emission lower level to a level in the injection layer, and are injected from the level in the injection layer into the injection level in the quantum well light emitting layer of the subsequent stage due to a resonant tunneling effect via an injection barrier layer provided between the injection layer and the quantum well light emitting layer of the subsequent stage, and electrons injected into the injection level through the injection barrier layer are supplied from the injection level to the emission upper level by means of longitudinal optical phonon scattering.