偏振对飞秒激光辐照LiF晶体的影响

Influence of polarization on irradiating LiF crystal by femtosecond laser

飞秒激光聚焦到LiF晶体内部,晶体的加工形貌随偏振改变.实验表明,偏振方向平行于〈110〉晶向时,加工起点到表面的距离是〈100〉偏振下的1.08倍;而〈110〉偏振下加工终点到表面的距离是〈100〉偏振下的1.01倍.为了解释加工形貌的偏振依赖,建立了逆韧致辐射、雪崩电离和无辐射跃迁的模型,首先,价带电子通过强场电离和雪崩电离,从激光中吸收能量跃迁到导带,该过程用电子密度演化方程和傍轴非线性薛定谔方程描述,求解方程得到导带电子密度;其次,导带电子通过无辐射跃迁过程释放能量给晶格,由能量守恒计算出晶格温度沿激光传播方向的分布;最后,晶格温度超过熔点以上的区域被加工.模拟结果显示,〈110〉偏振下加工起点到表面的距离是〈100〉偏振下的1.03倍,而〈110〉偏振下加工终点到表面的距离是〈100〉偏振下的0.981倍,与实验结果基本一致.虽然Z扫描技术测量的非线性折射率随偏振方向变化,但是非线性折射率的变化趋势与实验结果相反.模拟和实验证明逆韧致辐射导致加工形貌随偏振变化.

The processing morphology of cubic crystal LiF irradiated by femtosecond laser varies with the polarization direction.When the polarization direction is parallel to the crystal orientation 〈110〉,the distance between the starting point and the surface is 1.08 times that along（100） polarization,and the distance between the end point and the surface is1.01 times.While the cubic crystal is irradiated by a femtosecond laser,self-focusing and inverse bremsstrahlung are two probable mechanisms dependent on polarization.In order to investigate the relation between the self-focusing and polarization,in this paper we report the nonlinear refractive index n_2 of LiF crystal which is linear with respect to selffocusing coefficient.The Z-scan technique is used to measure the nonlinear refractive indexes at different polarizations.As the polarization direction is rotated from 〈110〉 to 〈100〉,the nonlinear refractive index decreases,and the self-focusing effect becomes weaker.If self-focusing leads to the dependence of morphology on polarization,the distance between the starting point and the surface for 〈100〉 polarization should be longer than that for 〈110〉 polarization.However,the experiment exhibits an opposite result that the distance between starting point and the surface for 〈100〉 polarization is shorter than that for 〈110〉 polarization.Therefore,the processing morphology which changes with polarization is not a consequence of the self-focusing.So in order to understand why the processing morphology varies with polarization,in this paper we present a model which combines inverse bremsstrahlung,avalanche ionization and radiationless transition.We believe that the recombination due to radiationless transition has a great effect on laser machining.The inverse bremsstrahlung coefficient of 〈110〉 polarization is less than that of 〈100〉 polarization,as a result,the density of free electrons which are produced by inverse bremsstrahlung and avalanche ionization at 〈110〉 polarization is less than that at〈100〉 polarization.At first,the laser energy is transferred to the free electrons by inverse bremsstrahlung and avalanche ionization,which is described by the paraxial nonlinear Schrodinger equation and evolution equation of electron density.The density of free electrons is obtained by solving the equations.Then free electrons transfer the energy to the crystal lattice in the process of recombination through radiationless transition,which is depicted by energy conservation and gives the distribution of lattice temperature along the propagation direction.Finally,the area in LiF crystal of which the lattice temperature climbs up to above the melting point is processed.According to the simulation,the distance between the starting point and the surface at 〈110〉 polarization is 1.03 times that at 〈100〉 polarization,and the distance between the end point and the surface at 〈110〉 polarization is 0.981 times that at 〈100〉 polarization.These are consistent with the experimental results.Simulation and experimental results demonstrate that the inverse bremsstrahlung,which is dependent on polarization,is the main reason for morphology changing with the polarization of femtosecond laser.These research results may contribute to inducing microstructure in transparent dielectrics through femtosecond laser.