Dispersive gain and loss in midinfrared quantum cascade laser

We report the measurements of dispersive gain (simultaneous coexistence of gain and losses on a single intersubband transition) in a quantum cascade laser. Broadband transmission spectra through the waveguide of a λ∼4.7μm In0.53Ga0.47As∕AlAs0.56Sb0.44∕InP quantum cascade laser have been studied at a bias below laser threshold and at different temperatures. For a certain range of current, and at temperatures higher than about 150K, the transmission spectra show clear dispersive gain/loss behavior with the possibility for intersubband gain to be observed even without global population inversion between laser levels.