Picosecond transient orientational and concentration gratings in germanium

We present and discuss the results of picosecond transient grating self-diffraction measurements in germanium that can be understood in terms of an orientational grating produced by anisotropic (in space) state-filling. Although there have been predictions and indirect experimental evidence for isotropic state-filling in germanium, this is the first direct experimental indication of anisotropic state-filling in a semiconductor. We compare the self-diffracted signals from orientational gratings with those from band-filling induced concentration gratings and find several distinctions, all of which can be explained in terms of the theory developed in the preceding paper. In addition, we have performed three-pulse transient grating lifetime measurements which indicate that the concentration grating decay is tens of picoseconds, while the orientational grating decay is too rapid to resolve with our 8 ps pulses.