The Research On The Mechanism Of Subwavelength Stripes Induced By Nanosecond Laser Based On Laser Field

The period stripe structure can be observed in the laser irradiation experiments, and a sub-wavelength periodic stripe with similar grating structure whose period is breakthrough the diffraction limit has attracted widely interest of researchers. Currently this stripes can only been observed in femtosecond and barely picosecond laser ablation experiments. In the nanosecond laser irradiation experiments, the thermal effect is the main mechanism, we generally think that we cannot get the sub-wavelength periodic structure stripes in the nanosecond laser experiment. But a sub-wavelength stripes structure by nanosecond laser irradiation has been observed on the film packed with the silica particle structure by sol-gel method. The process will extend sub-wavelength laser etched stripes to nanosecond laser range, and will promote the industrialization of sub-wavelength periodic stripes laser etching process.Based on the experimental results, this paper has established a theoretical model of the nanosecond laser etching sub-wavelength stripes by combining the nanosecond laser damage theory with the femtosecond laser etching theory. According to the classical Maxwell equations, we have analyzed the laser field during damaging process; especially the damage morphology and size of the micro-pit affect the laser field distribution. By using the finite element analysis software COMSOL, we have simulated the thin film laser field distribution during the sub-wavelength stripes formation process in different structures and different laser wavelength. Finally, we have preliminarily explored the mechanism of the nanosecond laser which can etch the sub-wavelength periodic stripes.In the beginning of nanoseconds stripes formation, the structure of the particles has played an important role. Under irradiation, the dielectric constant of the particles will change, and induce a periodic field distribution on the substrate. This periodic field distribution will etch the glass substrate to form a periodic groove on the glass substrate. When the grooves formed, the SP-laser in grating interference is the dominant mechanism. The period of the stripes depends on the dielectric constant of the materials and the wavelength of the laser in the process. The simulation results show that1) in the early laser irradiation, the porous structure of the SiO2has played an important role which can form a strong local field.2) in the early formation of the strip structure,the size of the particle has a little effect on the laser field distribution.3) the stripes structure in nanosecond laser are uniform and perpendicular to the polarization direction, and the distance between the strips have a direction connection with the wavelength.4)the distance between the strips has a connection with the dielectric constant which has a connection with the ionization of the material, that is to say, the period of the stripe structure depends on the energy density of the laser.The results of this study will certainly promote the development of the high power laser materials science; meanwhile will guide the process parameters of nanosecond laser etched sub-wavelength stripes structures in the actual production process, such as the laser etching process parameters and the film structure.