High-Quality Periodic Surface Stuctures Induced By Temporally Shaped Femtosecond Laser Pulse

Ultrafast laser ablation and micro-nano processing is a complex process,which involves various interactions between light and matter,such as laser induced carrier excited,carrier heating,lattice heating,heat conduction,melting of material and structural processes.Laser induced periodic surface structure(LIPSS)is a phenomenon with important scientific significance and application value in the field of laser ablation and micro-nano processing.The uniformity,depth,and orientation of LIPSS have a significant influence on their functions.Three major challenges exist in the fabrication of regular and uniform LIPSS:enhancing the periodic energy deposition,reducing the residual heat,and avoiding the deposited debris.Temporally shaped femtosecond laser pulse is an effective method to control the ultrafast dynamics of the interaction between laser and materials.In this thesis,based on 4f zero dispersion pulse shaping system,the method and mechanism of the fabrication of the regular LIPSS on silicon by a temporally shaped pulse were studied.The main research results are as follows:1.The formation of low spatial frequency LIPSS(LSFL)on a silicon surface induced by a single-shaped 800 nm femtosecond laser pulse was studied.A single Gaussian pulse(that is,Fourier Transform Limit pulse)is difficult to form LSFL at the center of the ablation area.A 4f zero dispersion pulse shaping system with a focal length of 200 mm is constructed.Using periodic π-phase step modulation on laser spectral component,a Fourier transform limit(FTL)pulse is shaped into a temporally shaped pulse with varying subpulse intervals in the range of 0.1-7.2 ps.The single shaped pulse is applied to a silicon surface to produce LSFL.The results show that when the interval between two subpulses is greater than 0.9 ps,periodic ripples can be observed at the center of the ablation crater.Increasing the subpulse interval can effectively reinforce the formation of periodic ripples.The two-temperature model and Drude model(TTM-Drude model)are applied to theoretically study the excitation of electrons on Si surface after irradiation by a single shaped pulse,and the evolution of the electron density,electron temperature,lattice temperature.The results show that the temporally shaped femtosecond laser pulse can enhance the excitation of surface plasmon polaritons(SPP)and the periodic energy deposition while reducing residual thermal effects on the silicon surface,eventually resulting in periodic ripples at the center of the ablation area.2.A large-area extremely regular LSFL on a silicon surface is fabricated by a temporally shaped femtosecond laser.The cylindrical lens in the 4f configuration zero-dispersion pulse shaping system are replaced by a pair of cylindrical lens with a focal length of 400 mm,thus the modulate resolution of spectral is increased,the subpulse interval of a temporally shaped pulse is adjustable in the range of 0.25-16.2 ps.Under the irradiation of the shaped pulse with an interval of 16.2 ps,the scan velocity for fabricating regular LSFL is 2.3 times faster,while the LSFL depth is two times deeper,and the diffraction efficiency is three times higher than those of LSFL using the FTL pulse.The formation mechanisms of regular LSFL have been studied experimentally and theoretically.The results show that the temporally shaped pulse enhances the excitation of surface plasmon polaritons and the periodic energy deposition,while reducing the residual thermal effects and avoiding the deposition of the ejected debris,eventually resulting in regular and deeper LSFL on the silicon surface.Large-area LSFL are efficiently fabricated with a shaped pulse of 16.2 ps on Si surface by laser direct writing method.The fabricated pattern of "Chinese knot"filled with regular LSFL shows a vivid structural color from blue to red.