Novel type-II quantum cascade lasers

A new class of quantum cascade lasers based on type-II quantum wells is analyzed. In these novel mid- and long-wavelength IR lasers, not only can a population inversion be easily created with a nearly 100% current injection efficiency, but also the nonradiative loss from the optical phonon scattering can be greatly suppressed. A general description of how the lasing threshold current depends on the injection, radiative, and inversion efficiencies is formulated to illustrate the expected improvements over the recently reported quantum cascade laser. Also, the features that distinguish quantum cascade lasers from traditional bipolar lasers are discussed in the context of the carrier transport in equivalent circuit models to illustrate the advantages of quantum cascade lasers for high power mid- and long-wavelength IR source applications.