Formation of high spatial frequency ripples in stainless steel irradiated by femtosecond laser pulses in water

We report the formation of high spatial frequency ripples (HSFRs) in stainless steel irradiated by 50 fs, 800 nm, 1 kHz femtosecond laser pulses in water. The period of the HSFRs, Λ, is less than 0.2λ, where λ is the laser wavelength. We further conduct theoretical calculations to study the ultrafast dynamics, and find that double splitting of the low spatial frequency ripples (LSFRs, Λ > 0.45λ) plays a decisive role in the formation of HSFRs. Similar experiments are conducted in copper, however, no splitting of LSFRs is observed. The different experimental results on stainless steel and copper conducted in water and in air are also discussed.