Surface modification of crystalline Si irradiated by femtosecond laser pulses

Surface modification of crystalline Si by a femtosecond laser at room temperature in air was studied. The femtosecond laser pulses (800 nm wavelength, 100 fs pulse duration, 1 kHz repetition rate) were focused on the crystalline Si targets (p type, 99.999% purity, (100) direction) by a plano-concave lens with a focal length of 100 mm. It was found that there are four threshold fluences which characterize this structural modification