Anisotropic interference of degenerate four-wave mixing in crystalline silicon

Degenerate-four-wave-mixing experiments have been performed on crystalline silicon at 620 nm using 100-fs pulses. Symmetry analysis has been applied to monitor the fourfold (bulk) symmetry of the [100] direction. Deviations from orthogonal polarization of the pump-and-probe beam with respect to the diffracted-signal beam have been studied in detail. The rotation patterns exhibit the transformation behavior of the relevant ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ tensor. Preliminary time-resolved measurements reveal distinct variations of the signal occurring on a time scale of 10 fs upon excitation with 100-fs pulses, which cannot be detected by standard correlation techniques.