Self-mode-locking of quantum cascade lasers with giant ultrafast optical nonlinearities

We report on the generation of picosecond self-mode-locked pulses from midi nfrared quantum cascade Lasers, at wavelengths within the important molecul ar fingerprint region. These devices are based on intersubband electron tra nsitions in semiconductor nanostructures, which are characterized by some o f the largest optical nonlinearities observed in nature and by picosecond r elaxation Lifetimes. Our results are interpreted with a model;in which one of these nonlinearities, the intensity-dependent refractive index of the La sing transition, creates a nonlinear waveguide where the optical Losses dec rease with increasing intensity. This favors the generation of ultrashort p ulses, because of their Larger instantaneous intensity relative to continuo us-wave emission.