An experimental and theoretical comparative analysis of the output characteristics of λ ≈
9m GaAs/Al0.45Ga0.55As quantum cascade lasers based on single and double phonon resonance
depopulation mechanisms were presented. The layer structures were grown with solid source
molecular beam epitaxy and consist of 48 or 36 active stages embedded in a symmetrical plasmon
enhanced waveguide. From the wafers, ridge waveguide lasers were fabricated by optical
lithography and dry etching. The theoretical model is based on a fully non-equilibrium Schrödinger-
Poisson self-consistent analysis of the coupled scattering rate and single-temperature energy
balance equations, taking all relevant electron-LO phonon, electron-electron and electron-ionised
impurity scattering processes into account. Single phonon resonance devices exhibit clear current
saturation, simultaneously with a decrease of the optical power. In the moderate doping regime, a
quasi-linear dependence of both the threshold and saturation current densities on injector doping,
were measured, in a very good agreement with theoretical predictions. Double phonon resonance
lasers exhibit ‘saturation’ mechanism evident from their decrease in optical power, but without
pronounced current saturation. Previously reported saturation of the ‘maximal’ current under higher
injector doping in single phonon resonance lasers, is also observed in the double phonon resonance
structure for injector sheet doping above 8x1011cm-2.
Wavelength selection and spectral narrowing of distributed Bragg reflector quantum cascade lasers up to peak optical power