InGaAs quantum dots coupled to a reservoir of nonequilibrium free carriers

We discuss the impact of a 2-D-charged carrier reservoir for high-speed optical amplification and modulated lasing in quantum dot (QD)-based devices by testing the amplification of short trains of high power, femtosecond optical pulses in an InGaAs QD-in-a-well-based semiconductor optical amplifier (SOA). We adapt a laser-like rate equation model to describe heterodyne pump-and-probe experiments. After an optically induced perturbation, we identify the gain recovery process as a forced steady-state situation which can be consistently described within rate-equation based laser theory. The model is systematically applied to analyze the experimental amplification and the overall SOA dynamics as a function of injected current. We conclude that, under conditions of high optical pump power close to the device saturation regime, the ultrafast SOA dynamics is governed by the overall injection current. The carrier relaxation pathway of a direct capture from the 2-D reservoir to the QD ground state is needed to explain the observed pulse train amplification.