Ultrafast imaging analysis of femtosecond laser induced periodic nanoripples on Al film

The formation dynamics of periodic ripples induced by two 800 nm femtosecond laser pulses on Al film are studied using a pump-probe imaging technique. The results shows that the first femtosecond laser pulse only induced random nanostructures on the surface of the Al film. Periodic ripples appear near these nanostructures after the second femtosecond pulse irradiation. The surface-plasmon-polariton (SPP) model combined with the Drude-Lorentz model is used to explain the formation of periodic ripples structure on the surface of the smooth Al film after the second femtosecond laser irradiation. The dynamics of electron and lattice temperatures after femtosecond laser irradiation on the Al surface is calculated by a two-temperature model, and the influences of the electron and lattice temperatures are taken account on electron scattering frequencies. Single-photon absorption and the localization of hot electrons are also considered in the Drude-Lorentz model. The simulated results show that the period of the periodic ripples predicted by the SPP model accords well with experimental results. It demonstrates that the SPP are excited by the second femtosecond laser pulse irradiating the nanostructure formed by the first femtosecond laser pulse, are the main reason for the formation of the periodic ripples.