In this work, the laser-induced periodical surface structure (LIPSS) on silicon is generated by 532nm nanosecond laser with pulse duration of 10ns and repetition frequency of 10Hz. The formation of LIPSS is observed under laser fluence of 260mJ/cm2 and pulse number of 600 shots with p- and s-polarized laser. For p- polarized laser, when the incident angle is 10°, 20°, 30°, 45°, and 60°, the LIPSS period is 400nm, 743nm, 902nm, 1148nm and 2525nm correspondingly. Besides, the orientation of LIPSS is always perpendicular to the polarization direction of the incident laser. For s- polarized laser, the LIPSS is perpendicular to the polarization direction and the period increases with the incident angle when it is less than 42°. The period is 457nm, 515nm, 549nm and 610nm at incident angle of 10°, 20°, 35°, and 41° correspondingly. when the incident angle exceeds 42°, a set of crossed LIPSSs emerge which are symmetric about the polarization direction. Meanwhile, the period reduces a lot which is around 400nm.
The thermodynamic properties of silicon plasma generated by picosecond laser in vacuum were studied by using spatiotemporally resolved emission spectroscopy technique. Temporal and spatial evolution behavior about plasma has been analyzed. Meanwhile, the temporal and spatial dependence of silicon ions with different charges were examined. Finally, the validity of assuming a local thermodynamic equilibrium (LTE) in the silicon plasma expansion was checked, and the deviation degree of LTE was measured. The results indicate that the assumption was valid only at longer (< 180 ns) delay times in the area z < 4 mm away from the target surface. At the initial stage of plasma expansion, the plasma deviates from LTE due to the sharp decrease of plasma temperature and electron density.
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