Time-resolved differential reflectivity as a probe of on-resonance exciton dynamics in quantum wells

We investigate the use of time-resolved differential reflectivity for the study of the ultrafast and recombination dynamics of resonantly excited quantum-well excitons. We illustrate the technique with extensive time-resolved measurements using femtosecond laser pulses on a 4-nm In0.1Ga0.9As|GaAs quantum well. Simple considerations applicable to quantum-well systems allow us to relate a generic change in reflectivity to a change in transition oscillator strength, which in turn is proportional to the exciton density (population inversion) created by an intense pump beam. Having as a starting point the temporal evolution of a two-level system under resonant excitation, we interpret differential reflectivity measurements in terms of dynamical processes that range from polarization dephasing (hundreds of femtoseconds) to radiative and nonradiative recombination (nanoseconds). These various dynamical processes span over four orders of magnitude in the time domain and may be extracted from a single time-resolved differential reflectivity experiment. We also find that the observed temporal decays are quite sensitive to the initial (and cold) photogenerated population. We show that a power dependence of the differential reflectivity signal can be utilized to obtain estimates of the exciton saturation density as well as to study the role of exciton-exciton scattering and thermalization on the total recombination time. Under low-power excitation conditions, we relate the differential reflectivity decay to the total exciton recombination lifetime. Lifetime measurements as a function of temperature confirm this assignment as they show an initial and quasilinear rise at low temperatures (T<70 K) followed by a decrease at higher temperatures owing to the dominance of nonradiative recombination channels.