Patent ID: 7158545

Claim:
A quantum cascade laser, comprising a semiconductor heterostructure providing a plurality of lasing modules connected in series, each lasing module comprising a plurality of quantum well structures collectively generating at least an upper lasing state, a lower lasing state, and a relaxation state such that said upper and lower lasing states are separated by an energy corresponding to an optical frequency in a range of about 1 to about 10 Terahertz, and such that a radiative lasing transition between said upper lasing state and said lower state is spatially vertical, and wherein electrons populating said lower lasing state exhibit a non-radiative relaxation via resonant emission of LO-phonon into said relaxation state and wherein said resonant LO-phonon emission selectively depopulates the lower lasing state such that a ratio of a lifetime of said upper lasing state relative to a lifetime of said lower lasing state is at least sbout 5.