Abstract
This paper utilizes the time dependent orientation change of the laser-induced periodic subwavelength structures to capture the nonequilibrium states on copper metal surface.
Highlights
Understanding the dynamic processes of metal surfaces induced by femtosecond laser irradiation is of importance in both fundamental science and applications, because it is the basis for effective manipulation of material properties at a microscopic level [1,2]
laser-induced periodic surface structures (LIPSSs), associated with a slantwise orientation angle (SOA) of α ≈ 26◦ relative to the vertical direction. (Because the single-laser irradiation S2 at the higher energy would like to bring the LIPSSs with the vertical orientation, this direction will be considered as a reference for the rest of the paper.)
Such structural orientation is totally different from those formed under two individual laser irradiations with the higher energy fluence (F = 0.28 J/cm2), in which each laser irradiation usually interacts with the equilibrium states of the metal surface, resulting in the LIPSS orientation perpendicular to the direction of the laser polarization, as shown in the right panel of Fig. 2(a).The measured SOA approximates one half of the intersection angle θ = 45◦ between two laser polarizations
Summary
Understanding the dynamic processes of metal surfaces induced by femtosecond laser irradiation is of importance in both fundamental science and applications, because it is the basis for effective manipulation of material properties at a microscopic level [1,2] The ultrafast dynamics, such as electron thermalization and the acoustic phonons, generally have been studied using optical transient reflectivity measurements, where the very weak detection is considered from the temporal evolution of the refractive index of materials [3,4,5,6,7,8]; achieving high signal-to-noise-ratio is often needed with sensitive and sophisticated instruments.
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