Ultrafast dynamics of excited carriers in GaAs by a time-resolved differential reflectivity

Abstract

In this thesis, the dynamics of the photoexcited carriers in the bulk GaAs are investigated by a time-resolved differential reflectance method. The setup is mounted in a pump-probe configuration, with tunable wavelength and 100 fs pulse duration. Three bulk GaAs samples, each with different doping levels are considered in this work. The obtained data from the pump-probe spectroscopy reveal that the relaxation time of the excited electrons in the conduction band is highly dependent on the energy of the electrons. A relaxation time around 4 ps is measured for the electrons that are excited by 720 nm while for 800 nm excitation 12 ps relaxation time is recorded for the electrons. The behavior is attributed to different mechanisms of the electron-phonon interaction such that at the low excitation energy Frohlich interaction is dominant while in the higher energy excitation the intervalley scattering plays the principal role in the energy relaxation of the electrons. Besides, GaAs samples with different doping levels are considered to observe the effect of the doping in the dynamics of the electron, particularly the energy relaxation. The data show no considerable changes in the relaxation time of the electrons for different doping levels. However, the influences of the excited carrier concentration are considerable in the relaxation time and the recovery of the induced optical properties changes. The high concentration of the photoexcited carriers gives rise to a faster energy relaxation of the electrons and the recovery of nonlinear optical effects. Moreover, the effects of carrier diffusion are studied by performing simulations based on an ambipolar model. The simulation results show that the diffusion effects in GaAs are more pronounced at shorter wavelength in a degenerate pump-probe setup.