Study On Crystal Formation And Its Mechanism Induced By Femtosecond Laser

Recently, with the development of the scientific research, much attention has been paid on the microstructures induced by femtosecond laser. Femtosecond laser can be used to induce microstructures in glass and crystals to find new properties and functions of the optical materials, thus the applications of the microstructural changes in glass and crystals induced by femtosecond laser have been a worldwide hot topic in high dense optical storage, optical waveguide, photonic crystals, three-dimensional display devices, three- dimensional optical devices and so on.In this paper, crystals generation in transparent glass and crystals phase transitions induced by femtosecond laser are analyzed experimentally and theoretically.These studies begin with the research about the generation of barium metaborate crystals, lithium niobate crystals, anatase crystals and Ba₂TiO₄ crystals induced by femtosecond laser, the mechanism of which is studied systematically.High temperature phase barium metaborate crystals are generated in the borate glass irradiated by femtosecond laser. Accordingly, the behavior of the transformation from borate glass is studied, and the microstructural characters of the irradiated area in the borate glass after femtosecond laser irradiation are discussed. Stratified spherical structures about low temperature phase barium metaborate crystals due to blast wave effect is detected the first time, and a void is observed in the borate glass as a consequence of microexplosion accompanied by the generation of crystals.Anatase crystals and Ba₂TiO₄ crystals are generated in different femtosecond laser irradiation conditions. Raman spectroscopy is employed to analyze the titanate glass in depth and radial direction. Consequently, the relations between the growth of anatase and Ba₂TiO₄ crystals and irradiation time and other irradiation conditions are obtained, and anatase and Ba₂TiO₄ crystals coexistence can be obtained in the focal area.Microstructural character of the niobate glass is also investigated, and lithium niobate crystals are produced in three-dimensional space by controlling femtosecond laser power and irradiation time.Secondly, crystals phase transition of barium metaborate crystals and lithium niobate crystals is induced by femtosecond laser.Crystals phase transition is induced in barium metaborate crystals, and low temperature phase barium metaborate crystals are produced in high temperature phase barium metaborate crystals via femtosecond laser induction. For the lithium niobate crystals, other crystal phases are also generated by femtosecond laser irradiating the surface of the lithium niobate crystals.Finally, the mechanism of the crystals formation and crystal phase transition induced by femtosecond laser is studied by taking the borate glass as an example. The photo ionization and avalanche ionization, free carrier-absorption, microexplosion, dynamic equilibrium, group reunion and heat diffusion are emphasized in this study; accordingly, the origin of the crystals formation and phase transition is analyzed systematically. When femtosecond laser irradiates the glass, the laser energy deposit begins due to multiphoton and avalanche ionization. Thus the laser energy is absorbed intensely in local area when the density of the plasma is high enough. And microexplosion takes place when the plasma is extremely high in density, temperature and pressure. Therefore, a void in borate glass can be generated during long time femtosecond laser irradiation, and high temperature gradient is formed the material nearby. So bonds breaking and groups' reunion occur in high temperature and pressure conditions. When the laser is turned off, high temperature phase and low temperature phase barium metaborate crystals are generated in different place as a result of the nonuniformity of the cooling process in the borate glass. Moreover, another key condition to form crystals is the high repetition rate of the femtosecond laser.Recombination of the microstructural motif in solid material is achieved by using femtosecond laser technique, and new performance of the material is obtained in this paper. Therefore, a new approach is offered to discover the rule of the formation of tautomerism, and to study the structure and character of glass and crystals. Based on the structure motif of the glass and crystals, the transformation of B₃O₆ rings, NbO₆ octahedra, TiO₆ octahedra and other microstructural units are studied, and the microstructural character of glass and crystals are analyzed experimentally and theoretically in this paper, which can guide the practice and advance the theory of synthetic crystal growth.Also, photonic microstructures integrated in glass and crystals induced by femtosecond laser are of vital importance in research and fabrication of the photonic devices. Hence, the way to form crystals in glass substrate by femtosecond laser provides a brand new method in integrated optics technique.