Patent ID: 8068528

Claim:
A quantum cascade laser comprising: a semiconductor substrate; and an active layer which is provided on the semiconductor substrate, and in which unit laminate structures having quantum well emission layers and injection layers are laminated in multiple stages to form a cascade structure in which the quantum well emission layers and the injection layers are alternately laminated, wherein each of the unit laminate structures included in the active layer has an emission upper level, an emission lower level, and a relaxation miniband including energy levels lower than the emission lower level to function as relaxation levels in a subband level structure in the quantum well emission layer, the relaxation miniband is set such that an energy difference between the emission lower level and the relaxation miniband corresponds to energy of longitudinal optical phonon, and light is generated by an intersubband transition of electrons from the emission upper level to the emission lower level in the quantum well emission layer, and the electrons after the intersubband transition are relaxed from the emission lower level to the relaxation miniband through longitudinal optical phonon scattering, to be injected to the quantum well emission layer in the latter stage unit laminate structure from the relaxation miniband in the quantum well emission layer via the injection layer.