Patent ID: 8514903

Claim:
A quantum cascade laser comprising: a semiconductor substrate, and an active layer provided on the semiconductor substrate and having a cascade structure formed by alternately laminating quantum well emission layers and injection layers by multistage-laminating unit laminate structures each including the quantum well emission layer and the injection layer, wherein the unit laminate structure has, in its subband level structure, a first emission upper level, a second emission upper level of an energy higher than the first emission upper level, and a plurality of emission lower levels of energies lower than the first emission upper level, light is generated by intersubband transitions of electrons from the first emission upper level and the second emission upper level to the plurality of emission lower levels in the quantum well emission layer, and electrons after the intersubband transitions are injected into the quantum well emission layer of the unit laminate structure of the subsequent stage via the level in the injection layer, the quantum well emission layer includes n, is an integer of 2 or more, well layers, one of the first emission upper level and the second emission upper level is a level arising from a ground level in a first well layer closest to the injection layer side of the preceding stage, and the other is a level arising from an excitation level in a well layer except for the first well layer, and an energy interval ΔE 43 between the first emission upper level and the second emission upper level is set to be smaller than the energy of a longitudinal optical phonon, and for a higher energy level on the higher energy side adjacent to the second emission upper level, an energy interval ΔE 54 between the second emission upper level and the higher energy level is set to be larger than the energy of a longitudinal optical phonon.