Theory of intracavity nonlinear processes in quantum cascade lasers

We overview coherent nonlinear optical phenomena that occur in active quantum-cascade structures which support both laser action and, at the same time, nonlinear self-conversion of laser light into coherent radiation at different frequencies. In other words, the laser field serves as an intracavity optical pump for the desired nonlinear optical process. In such systems, resonant absorption of the pump field is overcome by laser gain, and huge resonant nonlinearities of the intersubband transitions can be fully exploited. The proposed approach holds promise to extend the operating wavelength of QC lasers to the spectral regions where they become less efficient, constrained by material limitations, or operate only at cryogenic temperatures. It may also lead to the development of broadly tunable injection-pumped sources.