This paper reviews recent activities of quantum cascade lasers in research of mid and far infrared (>μm) semiconductor lasers. The quantum cascade laser has unique characteristics, such as tailored lasing wavelength from mid to far infrared regions and apparent quantum efficiency exceeding 100%, due to the light emitting mechanism based on the sequential steps of the transitions between artificially created subband levels in semiconductor quantum wells. We discuss the lasing mechanism, the specific performance, and the future potentiality of this new type of mid and far infrared semiconductor lasers.
