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.

This paper reports the ultrafast imaging on the formation of periodic surface ripples induced by a single 800 nm, 50 fs laser pulse. The evolution process is observed on a Si surface with a prefabricated nanogroove. The ripples emerge very quickly, only 3 ps after the laser pulse with a fluence of 0.18 J/cm² irradiating on the surface, and last for several hundreds of picoseconds. The ultrafast dynamics of laser-matter interaction, such as free carrier excitation, carrier and lattice heating, surface plasmon polariton (SPP) excitation, etc, are studied theoretically. The theoretical and experimental results support that the periodic ripples are caused by the periodic energy deposition due to SPP excitation. The emerge time could identify the surface melting causing the formation of periodic ripples, and exclude the other thermal effects, for example, hydrodynamics.

We present a controllable fabrication of nanogratings and nanosquares on the surface of ZnO crystal in water based on femtosecond laser-induced periodic surface structures (LIPSS). The formation of nanogrooves depends on both laser fluence and writing speed. A single groove with width less than 40 nm and double grooves with distance of 150 nm have been produced by manipulating 800 nm femtosecond laser fluence. Nanogratings with period of 150 nm, 300 nm and 1000 nm, and nanosquares with dimensions of 150 × 150 nm² were fabricated by using this direct femtosecond laser writing technique.

The electron energy band structures and optical properties of silver and nickel are notably different, and they are identified as representative of noble and common metals, respectively. The ultrafast dynamics of the formation of periodic ripples on the surface of silver and nickel films induced by a single 800 nm, 50 fs laser pulse was investigated using the collinear pump-probe imaging method. The dependence of the transient ripple periods on the laser fluences was also studied. A surface plasmon polariton (SPP) model was developed to explain the formation of periodic ripples on noble and common metals induced by a femtosecond laser pulse. The evolution of the electron and the lattice temperature was investigated by numerically solving the equations of the two-temperature model. In addition, the effects of electron collision, localization of hot electrons, and inter-band transition on the dielectric constant in the excited states are discussed in detail as functions of laser fluence. The ripple periods obtained using the developed SPP model agree well with the experimental results, which supports the view that the SPP excitation plays an important role in the formation of periodic ripples. This model is expected to be a general approach for understanding the formation of subwavelength periodic ripples on metals induced by a femtosecond laser pulse.

We report the period-increasing effect of low spatial frequency ripples on a GaP crystal irradiated by 1 kHz, 50 fs, 800 nm femtosecond laser pulses. Massive free electrons are excited by a two-photon absorption process and surface plasmon is excited. The Drude model is used to estimate the changing of the dielectric constant of the GaP crystal. The period-increasing effects of low spatial frequency laser-induced ripples are theoretically predicted in air/water, and the experimental results agree well. The experimental and theoretical results indicate that surface plasmon excited by two-photon absorption plays a key role in the formation of low spatial frequency ripples.