In the femtosecond laser ablation experiment for silicon in water, we observed formation of two kinds of nanostructure with a fine period of ~ 150 nm and a coarse one of ~ 400 nm. It is found that the fine structure can be formed in a restricted range of low fluence of 60 – 70 mJ/cm2, and its formation is preceded with the surface corrugation due to the coarse structure. In addition, the period sizes in fine and coarse nanostructures were insensitive to the superimposed shot number of laser pulses, while those are very sensitive to the fluence. The formation process of fine nanostructure is illustrated and analyzed with our model of the periodically enhanced near-field through the excitation of surface plasmon polaritons. The calculated near-field period is in good agreement with that observed in the experiment.
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