We show that mid-infrared transmission spectroscopy of a quantum cascade laser provides clear-cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for a ϳ 7.4 m InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in the active region and the states in the injector goes hand in hand with a decrease of discrete intersubband absorption peaks and an increase of broad, high-energy absorption toward the continuum delocalized states above the barriers.
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 Lambda ~4.7 µm 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 150 K, 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
A broadband midinfrared laser operating up to 340K in pulsed mode is presented. It was realized by incorporating seven different quantum cascade active regions with 0.2μm increments to emit at different wavelengths. Despite the relatively large wavelength increments, a continuous lasing spectrum is obtained from 7.7to8.4μm, with significant improvements in threshold current and temperature performance compared with previously reported broadband quantum cascade lasers. A full temperature dependent characterization was performed, and the threshold current density at 300K was 6.5kA∕cm2, with peak output power of over 100mW. A characteristic temperature of T0=192K was obtained.
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