Impact of nonlinear carrier-carrier scattering on gain dynamics and nonlinear optical properties of quantum dot semiconductor optical amplifiers

Quantum dot semiconductor optical amplifiers (QD-SOAs) are investigated in terms of their gain recovery performance and wavelength conversion efficiency using four wave mixing (FWM). Fast carrier-carrier scattering and a low amplitude phase coupling (alpha factor) in semiconductor QD devices make them promising candidates for ultrafast gain recovery dynamics and efficient wavelength conversion. There is an ongoing discussion in the literature whether direct capture or cascading relaxation processes dominate the gain recovery dynamics of QD SOAs. Using a microscopic approach to carrier-carrier scattering between the extended QW states and the confined QD ground and excited states we are able to quantify the strength of the different scattering contributions and thus to answer the above question. In nondegenerate FWM the beating of two injected optical signals with different carrier frequencies, i.e. a pump and a probe signal, modulates the carrier densities of the QDs and creates a conjugate optical signal. The intrinsic carrier-carrier scattering strongly determines this FWM dynamics which we investigate in dependence of the frequency detuning of the optical input signals and the injected current density. We find that fast carrier-carrier scattering leads to enhanced modulation bandwidth in QD based devices compared to conventional QW based devices.