Laser fluence dependence of periodic grating structures formed on metal surfaces under femtosecond laser pulse irradiation

Periodic structures self-formed on the surface of several metals under femtosecond laser-pulse irradiation are investigated by electron microscopy. For the self-formation of periodic gratings on metal surfaces, the interspaces of the periodic structures depend on laser fluence. This dependence is the same for all metals, although the range of laser fluence in which the structures are formed differs between metals. The laser fluence dependence can be explained by the generation of a plasma wave through the parametric decay of laser light [S. Sakabe, M. Hashida, S. Tokita, S. Namba, and K. Okamuro, Phys. Rev. B 79, 033409 (2009)]. This indicates that the formation of periodic structures depends not only on metal properties but also on the electron density of plasma produced on a surface by femtosecond laser pulses.