The ripples and nanoparticles on silicon irradiated by femtosecond laser

The surface configuration evolvement of silicon with the increasing pulse number irradiated by femtosecond laser has been studied. The ripples and nanoparticles are formed in the irradiated region. The period of the ripples decreases with the increasing pulse number, while the diameter of the visible structured region increases with the increasing pulse number. The interference of laser with surface plasmons is proposed to explain the formation of the ripple. The decreasing of the spatial period for surface ripples is due to the decrease of the surface plasmon wavelength with the decreasing electron number density in the plasma.