Study On Formation Mechanism And Application Of Femtosecond Laser Induced Periodic Ripples

In this paper,the periodic ripples induced by femtosecond laser on the surface and inside of different materials are studied.Maxwell equations are solved numerically with the finite-difference Time-domain（FDTD）method,the formation principle is analyzed and the application of periodic ripples is explored.In this paper,femtosecond laser is used to induce different periods of fringe on Ga P crystal surface,and the electromagnetic principle and evolution process of femtosecond laser induced periodic fringe on semiconductor are studied by FDTD numerical analysis.Secondly,based on the Sub-SPP theory,a dual interface model is established to discuss the relationship between the periodic characteristics of ripples and the excitation efficiency of SPP and Sub-SPP.Finally,a femtosecond laser is used to induce periodic nanostructures inside the quartz glass to form five-dimensional storage data points,which realizes the high-dimensional storage technology with a single point of 5-bit density.This paper has obtained the following research results:1.The surface morphology changes of Ga P crystal irradiated by femtosecond laser under different pulse energies and pulses are experimentally studied.It is found that the transition from high spatial frequency LIPSS（HSFL）to low spatial frequency LIPSS（LSFL）on the crystal surface occurs with the increase of the number of irradiated pulses.The formation and evolution of LIPSS under multiple pulses were simulated by the finite difference time domain method and holographic ablation model.The simulation results are in good agreement with the experimental results.The results show that the electromagnetic origin of HSFL and LSFL is the interference of the incident electric field and the scattered electric field under multiple pulses.The effect of initial surface roughness on the formation of LIPSS was also analyzed.2.The dual interface model of air-plasma and plasma-substrate combined with FDTD numerical simulation was used to analyze the role of surface plasmon polaritons（SPP）in the formation of LIPSS.The results show that the SPP excited at two different interfaces have different spatial frequencies,and there is a competitive relationship.The final LIPSS period depends on the period of the dominant SPP.The thickness change of the intermediate plasma layer caused by laser energy and the concentration of free electrons play an important role in this process.3.For the new five-dimensional optical storage technology,FDTD-HAM method is extended to three dimensions to simulate the generation process of internal periodic structures,and the structure generated by simulation is in good agreement with the existing research.In the experiment,optical storage is achieved by inducing periodic structure Voxel in Si O₂ using femtosecond laser and relying on the birefringence effect of this structure.The memory density of 32 states（5-bit）of a single voxel was achieved by adjusting the polarization and laser energy,and the memory data was decoded by training the deep neural network decoder and building the birefringence measuring equipment.