Interaction Of Femtosecond Laser Pulses With Transparent Materials

Femtosecond laser has two characteristics: ultrashort pulse length and ultrahigh electric field intensity . They are widely used in the analysis of dynamic processes of various physiochemical reactions and fine processing with minimal thermal shock. In the process of micro-machining , femtosecond laser pulse shows excellent advantages over picosecond and nanosecond laser pulse, such as scarcely thermal effect ,breakthrough of optical diffraction limit and good space-selective property ,which enables femtosecond laser pulse be used in many fields ,such as ultra-precision micro-machining , the fabrication of micro-photonic device , nano-operation in bioengineering and biology and three dimensional optical storage. This thesis is based on the study of above-mentioned fields. Particularly ,the preliminary study of interaction between femtosecond laser pulse and transparent material is conducted ..1. Fused silica is irradiated by laser with various pulse duration of magnitudes of ps and fs, wavelength of 800nm,repetition of 1kz. Optical microscope and image sensor are used for observation of morphology and shape of damage zone in the fused silica. The mechanism contributed to separate damage construction shaped along incidence direction after laser irradiation is analyzed. In addition, The influence of focus conditions on the threshold energy of transparent material is observed ,The result shows threshold energy decrease with the increase of NA..2. Different damage shape of sapphire is formed and investigated after laser irradiation ,with which laser pulse is ns, ps and fs respectively. Structure looks like triangle are found in crystal irradiated by ns laser pulse , structure of cross is made by ps laser pulse ,and structure of stroke is made by fs laser pulse. The experiment results show polarization has no effect on damage shape, and laser irradiation with different pulse duration has different thermal shock in the crystal.3. Color centers are induced in high-purity silica glass after infrared fs laser exposure. The analysis shows the energy collection in focus area and the self-traped excitons are ascribed to the main reason of SiE'color centers formation when fs laser pulse intensity is lower than damage threshold of material, which belongs to nature alteration of glass network structure.4. lithium silicate photosensitive glasses doped silver were fabricated by the method of high temperature melt in silica crucible. After irradiation of Ti-sapphire femtosecond laser followed by a thermo development which is photo-thermo-refractive (PTR)process. Li2O ? A12O^ ? 3SiO2 micro crystalline phase structure was observed & confirmed in the glass matrix with the help of optical microscope and X-ray diffraction. And this micro crystal is white and hexagonal structure. Variations of a refractive index in exposed parts of glasses were induced while almost no apparent variations occurred in unexposed parts. So this is a process of structure change with spatial selectivity .