Influence of polarization on irradiating LiF crystal by femtosecond laser

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 is 1.01 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 n2 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.