Study On Periodic Structures Of SiC Surface Induced By Femtosecond Laser Based On Electron Dynamics Control

With the development of micro-nano structure processing technology,micro/nano structure surfaces with functional properties induced by laser processing has gradually become a research hotspot for most scholars.Due to interaction time between femtosecond laser with high peak power and material is very short.Therefore,the regulation of the femtosecond laser-induced surface periodic ripple structure and the study of the ultrafast dynamic formation mechanism of the surface periodic ripple structures have very important practical value and scientific significance.In this paper,we first used a double pulse train with an energy ratio of 1:1 to linearly scan the 6H-SiC crystal experimentally,and studied the effects of the interpulse delay and polarization direction on LIPSS(laser-induced periodic surface structure).The SEM characterization and spectrum analysis of the ablation zones show that the period of LSFL(low spatial frequency ripples)decreases sharply with the double-pulse delay and then slowly decreases until it stabilizes.Meanwhile,the period of LSFL increases with the increase of polarization angle under the same interpulse delay,showing a polarization-dependent anisotropy.The electron density level excited by the laser is calculated by the electron density rate equation,and the interactions between incident light and transient SiC surface with different electron density levels are simulated based on the fnite-difference time-domain method(FDTD)method.The simulation results show that there is a periodic light field enhancement on the surface of 6H-SiC,which can be controlled by the transient electron density level.The simulation results explain the phenomenon that the period decreases sharply with increasing interpulse delay at the first beginning.The polarization-dependent periods of LSFL may be related to the relative orientation of the laser pulse front tilt(PFT)with respect to different laser polarizations.Then,the electron density rate equation,the Drude model and the two-temperature model are utilized to simulate theoretically the ablation of 6H-SiC crystals by double pulse ultrashort lasers with different interpulse delays.The transient optical parameters and thermal parameters regulated dynamically by electron density and their effects on ablation depth and width of LIPSS were studied.(1)When the double-pulse delay is relatively short,the optical properties of the material are converted from semiconductor to metal-like state and then restored to semiconductor state.The duration of the metal state depends on the interpulse delay;When the interpulse delay is relatively long,the optical properties of SiC change from semiconductor state,metal state,semiconductor state,metal state and finally to semiconductor state due to the excitation of free electrons.(2)The ablation depth and ablation width of LIPSS both decrease sharply and then slowly increase with the increase of interpulse delay.When the double-pulse delay is relatively short,the total absorption coefficientαof 6H-SiC determines that the ablation depth decreases with interpulse.The laser thermal density Q determines that the ablation width decreases with the increase of the interpulse delay.When the double-pulse delay is relatively long,the ablation depth and ablation width increase with the increase of the double-pulse delay,mainly due to the influence of lattice thermal diffusion K_L.Our experiments and simulation results show that the dynamic control of the electronic density on SiC surface can be realized by adjusting the interpulse delay and polarization of the double pulse,and then the period and depth and width of the LIPSS can be regulated.