Transient reflection of light near excitonic resonances

The authors present an experimental analysis of the temporal structure of picosecond light pulses reflected from GaAs crystals near the lowest excitonic resonance at normal and Brewster angle incidence. The reflected pulses exhibit nonexponentially decaying tails, measured over five orders of magnitude, which follow the dynamics of the coherent polarization in the reflecting layer. Model calculations, including spatial dispersion effects and dead layer effects, provide a general understanding of the data. The numerical fit, however, fails to describe the fast initial decay observed experimentally. The latter can be explained by taking into account an inhomogeneous transition layer with a depth-dependent damping constant. Strong intensity dependence of the transients is observed.>