Study On Nanosphere-assisted Femtosecond Laser Beyond The Diffraction Limit Processing

At present,the miniaturization of devices is an inevitable trend in the development of biomedicine,mechatronics,and national defense technology.How to realize the preparation of nanostructures that exceed the laser optical diffraction limit is one of the main research directions at present.Nanosphere-assisted femtosecond laser processing has the unique advantages of simple operation,high processing accuracy,and large-area processing potential.Now it has become one of the important methods of femtosecond laser micro-nano processing.The principle is that femtosecond lasers can induce enhanced local near-fields on the substrate surface with nanometer-sized spatial dimensions through nanospheres,reach the ablation threshold of the substrate material in a very small space,and achieve accurate material removal to form the surface structure.This paper uses a combination of numerical simulation and experimental research to systematically study the processing beyond the diffraction limit with nanosphere-assisted femtosecond lasers on the surface of silica substrates.The main contents are as follows:For nanosphere-assisted single-pulse femtosecond laser processing,numerical simulation by using Finite Difference Time Domain method,the variation of the electric field on the surface of the silica substrate induced by silica spheres and gold spheres with different diameters under different incident wavelength conditions was obtained.Based on the simulation results,with the help of silica spheres with diameters of 200 nm and 500 nm and gold spheres with a diameter of 200 nm,femtosecond lasers with a wavelength of 800 nm were used to successfully prepare the nanostructures beyond the diffraction limit on the surface of silica substrate.The structure morphology is in good agreement with the simulated electric field distribution on the substrate surface.Furthermore,aiming at the weakness of near-field enhancement ability of dielectric nanospheres,based on the electronic dynamics of the interaction between femtosecond lasers and matter,the first proposed and verified dual-pulse femtosecond laser electronic control to optimize near-field nanomachining a new method — regulating the dielectric environment of the nanosphere and substrate through the first femtosecond laser pulse,changing the physical mechanism of near-field enhancement,and realizing a substantial increase in near-field enhancement capabilities.A numerical simulation model of double pulse machining was established,and the simulation results of the near-field distribution of the processed substrate surface under different conditions of the energy of the first pulsed laser and the double-pulse time delay were obtained.The transition of the near-field enhancement mode from the lens effect to the electric dipole resonance mode was found,and Continuous regulation of the enhancement factor can be achieved by changing the energy of the first pulse and the time delay of the double pulse.Furthermore,with the aid of a silica ball with a diameter of 500 nm,a double-pulse femtosecond laser with a wavelength of 800 nm was used for experimental research.With a combination of double-pulse energy far below the single-pulse laser hole ablation threshold,an elliptical hole with a long axis of about 200 nm was successfully prepared on the surface of the substrate,and its variation law was consistent with the variation law of the substrate surface enhancement factor in the simulation results,which proved the correctness of the numerical simulation model.Finally,combined with the energy transfer process of the femtosecond laser acting on the material,an in-depth theoretical analysis of the formation of the ablation structure on the substrate surface was carried out.The development and verification of this method provides a new idea for femtosecond laser micro-nano processing,and it is of great significance for the development of surface nanostructure processing technology beyond the diffraction limit.