Micromachining Of Dielectris And Fabricating Of Micro-optics Devices With Femtosecond Laser Pulses

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) A theoretical model based on solid state energy band theory and energy conservation is developed which can describe temporal and spatial distribution of carriers in dielectric materials during laser-induced damage. The relation between damage threshold and laser pulse duration, photon energy is studied by this model. The respective roles of multi-photon ionization, tunnel ionization, and avalanche ionization in laser-induced damage are examined.(2) Femtosecond laser ablation of Si wafer and fused silica are studied by experiments. It was found that avalanche ionization is seeded by electrons due to defects, which is the main process during laser-induced damage in Si wafer and that avalanche ionization is seeded by electrons excited by multi-photon ionization, which is the main process during laser-induced damage in fused silica. It was found that the energy deposition is initiated by multiphoton ionization rather than having to rely on impurities or defects to start an electron avalanche in transparent materials. Based on the ablation and cutting of Si wafer, a micro-mould for MEMS application is fabricated with an accuracy of ~1μm.(3) The morphology of structural changes in KTP crystal induced by single femtosecond laser pulse has been investigated. The structurally changed region is depressed at energies close to the threshold for producing a structural change and melting ablation morphologies are observed as pulse energy is increased. Furthermore, periodic nanostructures are formed around the edge of the laser-induced craters. The crater shape and the periodic nanostructures of femtosecond laser ablation of dielectrics are analyzed. And the femtosecond laser ablation technique was then employed for micromachining of mutilayer optical waveguides.(4) Surface relief diffraction gratings were written at the entrance surface of BBO crystal under irradiation with femtosecond laser pulses. Probe-beam diffraction and atomic force microscopy (AFM) were employed to characterize the diffraction properties and the microstructures of gratings. This periodic spatial modulation of the material surface induces noncollinear propagation of the input fundamental signal in the crystal. By slightly changing the angle of the incident beam, collinear and noncollinear phase matching can be achieved between different diffraction orders, in this way allowing the efficient generation of several second-harmonic beams.