Periodic ordering of random surface nanostructures induced by femtosecond laser pulses on metals

In this paper, we performed a detailed study of the formation of femtosecond laser-induced periodic surface structures (LIPSSs) on platinum and gold at near-damage threshold fluences. We find a unique type of LIPSS entirely covered with nanostructures. A distinctive feature of the nanostructure-covered LIPSS is that its period is appreciably less than that of the regular LIPSS. We show that the reduced period is caused by an increase of the real part of the effective refractive index of the air-metal interface when nanostructures develop and affect the propagation of surface plasmons.