The ultrafast dynamics of periodic ripples are studied during their formation on the surface of a gold film with a prefabricated nanogroove. These transient ripples are induced by a single 800-nm, 50-fs laser pulse and are observed by a collinear pump-probe imaging method. When the laser polarization is parallel to the nanogroove, transient ripples begin appearing after an elapsed time of 25–80 ps, and become clear and regular at 400–600 ps. The ripple period increases from 685 to 770 nm as the laser fluence $F$ increases from 0.73 to $3.42\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$. The evolution of temperature and lattice temperature are theoretically studied using the two-temperature model. When the laser fluence $F$ is above $0.73\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$, the electron temperature rises to several ${10}^4\mathrm{K}$, and the collision frequency rises above ${10}^{16}/\mathrm{s}$, which further causes the localization of hot electrons. Moreover, the $d$-band electrons can be excited through two-photon absorption and become free electrons. Using the dielectric constant of the excited states, which includes the effects of hot-electron localization and $d$-band transitions, the period predicted by the surface-plasmon-polarization (SPP) model accords well with experimental results. Both theory and experiment give support to SPP excitations playing a prominent role in the formation of periodic ripples induced by femtosecond laser pulses.

• Received 23 July 2018
• Revised 24 October 2018

DOI:https://doi.org/10.1103/PhysRevB.98.184106