Study On Ablation And Domain Inversion Induced By Femtosecond Laser In Metal And Ferroelectric Crystal

In contrast with traditional laser, Femtosecond laser has some incomparable advantages in microfabrication due to its ultra-short duration and extremely high peak power. This thesis studied the femtosecond laser interaction with metal by ablation and drilling experiments as well as the two-temperature model stimulation which also serve as theoretical basis for micromachining. Besides, surface ablation experiments of ferroelectric crystals were conducted and introduced in this paper to discuss and explain the influence of ablation threshold fluence by doping. At last, another experiment on domain inversion induced by femtosecond laser was done. The effect of doping is also demonstrated. These will provide some new ideas for the fabrication of small-period domain reversal structure with a certain thickness.In the metal ablation experiment, the threshold ablated fluence with different pulse number was calculated. And the influence of ablated spot size and depth by pulse number as well as pulse energy was also analyzed qualitatively. In the experimental investigation of drilling of hole with large aspect ratios in mental, the distortion of exit caused by the polarization direction of incident beam was shown and interpreted. An optimization method was proposed after that and achieved a good result.Based on the experimental fact and the sample parameter, the electron and lattice temperature distributions were stimulated by one-dimension two-temperature model. The physical picture described by modeling agreed well with theoretical mechanism. Moreover, taking the Gaussian distributions both in time-domain and in space into consideration, 2D two-temperature model was established to calculate the relationship between ablated spot size and injected pulse energy. The numerical results showed a good match with experimental results. Furthermore, under some assumptions, the role of multi-pulses was included in the model creatively to predict the number of pulses to puncture the sample as well as to demonstrate the evolution of depth with pulse number.The single-pulse and multi-pulse ablated threshold fluence of undoped and Mg-doped lithium crystal, which was calculated after surface ablation experiments, showed the fact that the doped sample always had higher ablated threshold fluence than the undoped one regardless of pulse number. A reasonable explanation was raised according to color centre theory and intrinsic defect models of pure lithium niobate crystal.Finally, the multi-ring structure observed on the lithium niobate sample where domain reversal structures were induced by femtosecond laser and the calculation of corresponding fluence for every inversed ring were consistent with"domain inversion window"theory supposed by modeling. An indirect measurement method was applied to gain the thickness of inversed area, which should be around10μm. The comparative experiments of undoped and Mg-doped sample indicated that the doped one was easier to be induced by an ultrashort pulse. .