Laser-based nonlinear optical probes can be used to study the dynamics of surface reactions and phase transitions on timescales as short as a few femtoseconds. Here we report the results of an experiment in which second harmonic generation is used to follow the change in symmetry of crystalline silicon as it evolves, after the silicon has been electronically excited with a 100 femtosecond optical pulse. One finds that the electronic structure in the top 75 - 130 Å of the Si surface loses cubic order only 150 fsec after excitation. This suggests that the atomic disorder is induced directly by electronic excitation and occurs before the material becomes vibrationally excited. In contrast, the electronic properties of the equilibrium molten phase are not obtained for several hundreds of fsec.
H. W.K Tom, H. W.K Tom,
"Femtosecond Time-Resolved Nonlinear Optics On Semiconductor Surfaces", Proc. SPIE 1056, Photochemistry in Thin Films, (15 August 1989); doi: 10.1117/12.951619; https://doi.org/10.1117/12.951619