Quantum cascade laser

Abstract

A quantum cascade laser includes a semiconductor substrate, and an active layer which is provided on the semiconductor substrate, and has a cascade structure in which unit laminate structureshaving quantum well emission layersand injection layersare laminated in multiple stages. Further, the quantum cascade laser is configured such that the unit laminate structurehas an emission upper level L, an emission lower level L, and a relaxation miniband MB including an energy level lower than the emission lower level in its subband level structure, and light is generated by an intersubband transition of electrons from the upper level to the lower level, and the electrons after the intersubband transition are relaxed from the lower level Lto the miniband MB through LO phonon scattering, to be injected from the injection layerto the latter stage emission layer via the miniband MB. Thereby, the quantum cascade laser which is capable of efficiently forming an inverted population in the quantum well emission layer, to improve its laser operation performance, is realized.