Mechanism Of Preparing Micro-nano Periodic Structures On The Surface Of Fused Silica By Femtosecond Laser

The physical processes of femtosecond(fs)laser induced micro-nano periodic structures on the surface of fused silica and K9 glass were studied.Firstly,the influence of various laser parameters and the environments on the periodic structure was studied via experiments.Secondly,the finite element analysis method was used to analyze and calculate the physical process of fs laser ablation of fused silica.Thirdly,the multiphysics simulation software was used to simulate the carrier-lattice temperature evolution,temperature distribution on fused silica.The influence of interference of the incident laser with surface plasmon polaritons(SPPs)on the ripple period under single pulse irradiation was also investigated.Finally,the simulation results were compared with the experimental data.The specific work and results are as follows:(1)The relationship between laser-induced periodic surface structures(LIPSS)and experimental environment,pulse duration,laser fluence,pulse number and scanning speed was studied.For comparison,the K9 glass with lower band gap than fused silica was also explored.Under a fixed number of pulses,the period of LIPSS increasesd with the increase of laser fluence.The critical fluences of fused silica transiting from a high spatial frequency LIPSS(HSFL)to a low spatial frequency LIPSS(LSFL)were 2.5 J/cm2 and 2.0 J/cm2 under 35 fs in air and 260 fs in vacuum,respectively.The super-wavelength periodic structures(SWPSS)were easier to be formed on materials with small band gap of K9 glass.The SWPSS on K9 glass were also formed under a wider range of laser fluence irradiation.At high or low fluence,the experimental environment was more likely to affect the period of LIPSS.The LIPSS period and K9 glass increased with the increase of pulse number.Furthermore,the LIPSS period on fused silica increased slowly and slightly with the scanning speed increasing(the equivalent pulse number decreased accordingly).(2)The finite element method was used to analyze the temperature field during the fs laser irradiation,the interference of incident laser with SPPs,and the fluid dynamics of the molten material.Carriers and lattices inside fused silica have have undergone nonequilibrium heat transfer process.The two-temperature method of the electron-lattice was used to calculate the lattice temperature.The LIPSS period is essentially determined by the instantaneous density of the free electrons.The main driving force of the fluid dynamics of molten materials is the Marangoni effect.The temperature gradient is the main cause of the surface tension gradient.(3)Using simulation software,the temperature distribution of the fused silica,the carrier-lattice temperature evolution,and the ripple period determined by the instantaneous density of free electrons were calculated.The calculation results showed that after the single-pulse fs laser irradiation,on the surface of the fused silica,the nonequilibrium heat transfer process of the internal carriers and lattices maintained about 5 ps.The peak temperature of the carrier on fused silica surface increased with the increase of laser fluence.With a fixed pulse duration,as the fluence increased,the ablation area and depth increased,as well as the peak of the carrier density.The simulation results of the ripple period were comparable to the experimental data in the LSFL section,but far from the experimental results in the SWPSS section.The interference model of incident laser with the SPPs was the formation mechanism of near-wavelength periodic ripples.The studies of micro/nano structures formation on the fused silica and K9 glass may be helpful of the development and application of the laser technology of preparing LIPSS on the surface of transparent dielectric materials.