Ablation And Micromachining Of Dielectris With A Femtosecond Laser

In this dissertation, theoretical and experimental studies on the femtosecond laser ablation of dielectrics and fabrication of micro-optics devices based on femtosecond laser micromachining are presented. The main contents are classified as follows.(1) The ionization processes of multi-photon ionization and avalanche ionization in laser-induced damage are studied. The changes of avalanche ionization rate and multi-photon ionization rate with the changes of laser intensity are investigated by taking the silica glass as an example. The relation between damage threshold and laser pulse duration, photon energy is studied.(2) Microstructural modifications of MgAl₂O₄ transparent ceramic induced by femtosecond laser pulse with a wavelength of 800 nm have been investigated by experiments. The ablation thresholds of MgAl₂O₄ transparent ceramic for single-pulse and multiple-pulse are studied. It was found that the periodic ripples about 200 nm in width and 300 nm in neighbouring ripples distance are formed on the bottom surfaces of ablation holes. The ablated spot under optimized energy pulses (near the damage threshold energy) can improve the IR transmission from 82% to 86% in the band of 2500-7000 cm-1 for MgAl₂O₄ transparent ceramic. When the pulses energy exceeds the threshold energy, the transmission of MATC decreases rapidly.(3) The morphology of structural changes of lithium niobate (LiNbO₃) single crystal ablated by a femtosecond laser pulse has been investigated. Sub-wavelength spots in LiNbO₃ crystal ablated by femtosecond laser focused with 20×(NA=0.5) microscope objective have been achieved. The spot-size is about 400 nm and 800 nm with 170 nJ single-pulse and 100 nJ, 17 pulses ablation, respectively. Raman analysis indicates some loss of crystallinity of the material after ablation with femtosecond laser. This indicates that LiNbO₃ suffers some change in chemical structure at the superficial layer, but remains largely unchanged in the bulk.(4) Diffraction gratings were written at the surface of LiNbO₃ single crystal under irradiation with femtosecond laser pulses. The experimental and theoretical results show that the diffraction efficiencies of gratings can be improved by increase of ablation speed and grating constant or decrease of pulse energy. The waveguide structure was fabricated with different parameters by vertical illumination method in LiNbO₃ single crystal. The influences of refractive index on laser intensity and ablation rate are investigated by experiments.