Ultrafast dynamics after optical pulse excitation in semiconductor waveguide structures

The ultrafast carrier dynamics of semiconductor optoelectronic devices set some of the fundamental limits in their practical use. The nonlinear gain and refractive index changes encountered under high-speed modulation of semiconductor optical amplifiers can be investigated using ultrafast optical techniques. We employ in particular two different techniques: single pulse propagation and pump-probe investigations.
In the single pulse propagation experiment, an ultrafast optical pulse is launched into the semiconductor amplifier, which is at normal operating conditions. The output pulse is the characterized with respect to pulse energy, temporal profile, chirp, spectrum, and spectral phase. We have used the technique to characterize a commercial InGaAsP-bulk amplifier for long-haul optical communication. We have demonstrated that gain saturation and non-linear gain compression limits the performance of the device for operation above 20 GHz. Strong non-linear effects giving rise to pulse break-up for high input pulse energies are demonstrated. These effects are related to self-phase modulation occurring in the strong saturation regime. The invesigations have been extended to multiple-quantum well (MQW) devices from the SCOOP program and British Telecom within a COST program. The non-linear gain compressioin coefficient and the ultrafast carrier dynamics of the optical amplifier can be measured directly using ultrafast pump-probe techniques with a heterodyne detection scheme. The technique allows for nonlinear experiments in waveguide geometry and has been demonstrated on the commercial devices. We have extended the experiments to MQW and quantum dots (QD) based optical amplifiers. The QD devices show new and interesting results on the ultrafast carrier dynamics such as a substantially decreased carrier relaxation time of the inverted material. We have also been able, for the first time, to measure the optical dephasing time of InAs QDÆs at room temperature.