Experimental Research Of Femtosecond Laser Pulse Induced Periodic Surface Structures On Metals

Over the last few years, femtosecond laser has proven to be a very promising tool forprocessing solid materials because of its specially machining advantages, especially formetals, dielectrics and semiconductors. The amount of thermal diffusion can be limitedbecause of the very shot heating duration and rapid solid-to vapor phase transition. Thus, itcan lead to a high quality, high precision and high efficiency micromachining process. Thispaper is based on the thought of electron dynamics control, we use a femtosecond lasersingle pulse and pulse train micromachining the metals of aluminum and copper toinvestigate the relationship between the quality of micro/nano structures and laserparameters. And we also compared the differences between femtosecond laser single pulseand pulse train in controlling the quality and morphologies of metal micro/nanostructures.The innovative points and main research results of this paper are as follows:(1) Using a femtosecond laser single pulse to induce periodic surface structures on thesurface of Al, we found that the morphology becomes clear and the quality gets good withthe increasing of laser pulse number, but the ripple’s period almost remains unchanged. Theablation size obviously increases as the increasing of laser energy fluence,and the ripple’speriod increases with the increasing of laser energy fluence, we thought it contributes to theincreasing of density of surface free electrons.(2)One can effectively control the ablation size by designing proper pulse train tomodulate the electron state of electron excitation, electron density, electron temperature,electron recombination and electron ionization. Moreover, we can achieve high quality,high uniformity and high clearness ripple morphology by using a pulse train scanning onthe surface of Cu. We conclude that the pulse train is more effective compared with singlepulse in controlling ripple’s shape and quality by adjusting sub-pulse separation.(3) With a femtosecond laser single pulse ablation of Cu, we found the ablation sizegets large as the increasing of laser pulse number when the laser energy fluence isdetermined and this trend is also available to the laser energy fluence. The depth of heataffected zone on Cu increases with the increasing of laser pulse number and decreases asthe increasing of laser energy fluence.However, we found some interlaced ripple structuresformed on the wall of ablation crater with a single pulse ablation of Cu, which attributes tothe incident laser light completely or partially overlay on the ripple induced by the previous pulses. We also concluded that the morphology of induced ripple structures becomeambiguous as the increasing of scan velocity by using a femtosecond laser single pulsescanning on the surface of Cu, which implies that the morphology and quality of ripplestructure are closely related to the laser scan velocity.