Self-Diffraction from Laser-Induced Orientational Gratings in Semiconductors

Abstract

We have resolved new ultrafast structure in the picosecond excitation-probe response of thin semiconductor wafers. This structure, located near zero delay and observed only at the very highest excitation intensities (~10 GW/cm²), can be understood only in terms of self-diffraction from a transient orientational grating produced by anisotropic state-filling. This anisotropic state-filling, as opposed to band-filling, arises from a δ-function-like spike in energy and a directional dependence in momentum of the carrier distribution function, caused by the nearly monochromatic polarized nature of the exciting radiation. This is the first observation of anisotropic state-filling of which we are aware.