Periodic Nanostructure Formation on Silicon Irradiated with Multiple Low-fluence Femtosecond Laser Pulses in Water

It is shown that superimposed multiple shots of linearly polarized 800-nm, 100-fs laser pulses at lower fluence than the single-pulse ablation threshold produce periodic nanostructures with almost constant periods of 150 nm and 400 nm on silicon surface immersed in water. The nanostructure formation and its characteristic properties observed are illustrated well with the excitation of surface plasmon polaritons in the newly created surface layer. Pump and probe measurements of surface reflectivity during the ultrafast interaction demonstrate that multiple shots of low-fluence fs pulses are crucial to the accumulation of non-thermal bonding structure change and subsequent ablation for the periodic nanostructure formation.