Investigation of phase transition thresholds by nonlinear transient laser gratings on silicon surfaces

Nonlinear transient laser gratings were excited by crossing two 180 ps Nd:Y AG laser pulses (355 nm) at the silicon surface. The launched counterpropag ating surface acoustic wave trains were detected outside the source with a cw laser probe-beam-deflection setup. Phase transitions occurring within th e Gaussian fluence distribution of the grating induced characteristic chang es in the amplitude of the corresponding grating oscillations involved and higher harmonics were found in the frequency spectrum. In the molten surfac e region, the amplitude decreased compared to the thermoelastic excitation, and second-harmonic generation was observed, whereas ablation generated an asymmetric profile with higher amplitudes and even the third harmonic coul d be detected. An accurate method is introduced to determine the ablation t hreshold (340 mJ/cm(2) for silicon) from the transition of linear to nonlin ear behavior in the distorted non-Gaussian amplitude distribution. (C) 2000 American Institute of Physics. [S0021-8979(00)01507-3].