Femtosecond laser induced ripples and nanohole arrays on silicon

We investigate the formation of ripple and nanohole induced by femtosecond laser pulses on the surface of silicon. Periodic ripples aligned perpendicular to the direction of laser polarization has been observed. The period of the periodic ripples decreases with the increasing pulse number. Particularly aperiodic ripples with orientation parallel to the laser polarization are formed depend on the number of laser pulses and energy. The nanohole arrays are formed on the overlapped areas of periodic and aperiodic ripples. The interference between the surface scattered or excited wave and the laser itself is proposed to explain the decrease of ripple period.