Influence of the electron-hole density profile on the reflectivity of laser irradiated silicon

We study the influence of the spatial extension of the electron-hole plasma created by a pump pulse on the reflectivity of a probe pulse. We show that the density deduced from reflectivity measurements is the surface density value with a very good accuracy, except very close to the plasma resonance. We also show that the resonance broadening due to the spatial inhomogeneity can be larger than the one due to free carriers absorption and has to be included in the usual experimental determination of the plasma relaxation time.