Formation Mechanisms Of The Transient Subwavelength Periodic Ripples On Metal Surface Induced By Femtosecond Laser

Femtosecond lasers inducing subwavelength and deep subwavelength periodic ripples on metals,semiconductors,and dielectric surfaces are universal phenomena.It is important in many advanced application field such as surface structural colors,modification of wetting properties,absorption enhancement.The formation mechanism of subwavelength periodic ripples is always a research hotspot.At present,the scattering light model and surface plasmon polariton（SPP）model are generally used for explaining the formation mechanism of subwavelength periodic surface structures.The SPP model well explains the formation of ripples on semiconductors such as silicon and gallium phosphide,but it cannot explain the subwavelength periodic ripples on metal surface,especially the experimental results on the surface of strong SPP materials such as gold and silver,which is a main challenge for the SPP model.In this paper,the formation of transient ripples on thin films of gold,silver,and nickel are studied by the pump-probe imaging technology.The SPP model is developed to reveal the mechanism of subwavelength periodic ripple formation on metal surfaces.The main results are as follows:（1）The transient ripples on the surface of a gold film with a prefabricated nanogroove induced by a single 800 nm,50 fs laser pulse are observed by the collinear pump-probe imaging method.When the laser polarization direction is perpendicular to the nanogroove,the transient ripples begin appearing after an elapsed time of 25-80 ps,and become clear and regular during 400-600 ps.The ripple period increases from 685 to 770 nm as the laser fluence F increases from 0.73 to 3.42 J/cm².The evolution of electron temperature and lattice temperature are theoretically studied by using the two-temperature model.When the laser fluence F is above 0.73J/cm²,the electron temperature rises to several 10⁴ K,and the collision frequency rises above 10¹66 s^-1,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 include the effects of hot-electron localization and d-band electron transitions,the period predicted by the SPP model accords well with the experimental results.Both theory and experiment support to SPP excitations playing a prominent role in the formation of periodic ripples induced by femtosecond laser pulse.（2）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 are investigated by using the collinear pump-probe imaging method.The dependence of the transient ripple periods on the laser fluences is also studied.The evolution of the electron and the lattice temperature are investigated numerically by using 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 by using the developed SPP model agree well with the experimental results,which supports that the SPP excitation plays an important role in the formation of periodic ripples.The developed SPP model is extended from gold to other precious metals represented by silver and then further to common metals represented by nickel and aluminum.The developed SPP model is expected to be a general one for understanding the formation of subwavelength periodic ripples on metals induced by a femtosecond laser pulse.