Time-resolved intersubband optical transitions in resonantly optically pumped semiconductor lasers

Using picosecond visible-pump mid-infrared-probe technique, we directly measured the photoexcited electron dynamics in the first conduction subband of a GaAs/AlGaAs quantum well laser, below and above its threshold. Our results clearly show that even at low temperatures, under optical excitation into the lowest heavy hole exciton resonance, GaAs/GaAlAs quantum well lasers lase in accordance with the common two-plasma model and not from an excitonic phase.