Study On Phase Transformation In Glasses Induced By High Repetition Rate Femtosecond Laser

Femtosecond laser has ultrashort pulse and ultrahigh peak power, which can inducemulti-photon absorption, tunneling ionization and avalanche ionization when it is coupled intotransparent materials. The nonlinear behavior of the absorption confines the energy depositedin the focal volume. It provides a unique tool for realization of space-selective modificationsin various transparent materials due to multi-photon processes. Furthermore, integrate3Doptical or photonic devices inside transparent materials can be fabricated by femtosecondlaser. Various new phenomena with regard to femtosecond laser-material interaction havebeen observed in recent years, which shed new lights on various fundamental sciences such asmaterials science, nonlinear optics, high-field physics, and plasma physics etc.In this thesis, phase transformations in several glasses induced by1030nm,500kHz,femtosecond laser are systematically studied. Here, the phase transformations includenanoparticles precipitation and phase separation. Meanwhile, migration of ions, coordinationtransformation and luminescence change induced by high repetition rate fs laser are alsoinvestigated. Detailed researches are described as followed:Bismuth nanoparticles are directly precipitated inside a Bi2O3-GeO2glass by irradiationwith a1030nm,500kHz femtosecond laser, which are confirmed by means of X-raydiffraction, transmission electron microscope and Raman spectra. In addition, the results ofelectron probe microanalyzer indicate that the ions distribution (Bi3+, Ge4+and O2-) aroundthe focal point is induced simultaneously. We suggest that the precipitation of Bi nanoparticlesand redistribution of ions with different diffusion coefficient should mainly be ascribed to theheat accumulation effect and thermal diffusion. The results provide a novel approach toproduce Bi nanoparticles inside glass and have promising applications for the fabrication ofphotonic devices.Microstructure modification and local luminescence change in oxyfluoride telluriteglasses are induced by a1030nm,500kHz femtosecond laser. Tellurium nanoparticles areprecipitated in the laser-modified region based on X-ray diffraction analysis and transmissionelectron microscopy observation. Micro-Raman spectra and energy dispersive spectroscopyshow that migration of ions (Te4+, Zn2+and O2-) and coordination state transformation of Te4+ions occurred after the irradiation. The luminescence property of Er3+-doped glass alsochanges in the irradiated zone. The mechanisms of above phenomena are interpreted. Duringthe500kHz femtosecond laser irradiation, photo-ionization process plays a significant role in the precipitation of Te crystalline phase. The creation of sharp temperature gradient results inmodification of element distribution, which leads to the coordination change of Te4+ions andluminescence change of Er3+ions. These results are of reference value to fabricate integratedoptical waveguide amplifiers and lasers in oxyfluoride tellurite glasses.Localized phase separation is induced inside a Na2O-B2O3-SiO2glass by changing theglass composition locally from a miscible composition to an immiscible one via femtosecondlaser irradiation at a high repetition rate. The structural change is confirmed by analyzing thecomposition of the irradiated area with confocal Raman spectroscopy and electron probemicroanalyzer. The formation of a nanoscale discontinuous porous structure due to phaseseparation is observed by field emission scanning electron microscopy after heat treatment,polishing until the inside of laser-modified area is exposed, and subsequent etching. Thecompositional change seems to be related to thermomigration, which is the migration of ions(Na+, B3+, Si4+and O2-) by temperature gradient, because the sharp temperature gradient iscaused with a high-repetition femtosecond laser. And ions migration results in thecoordination transformation of B3+ions. With this method, we can obtain space-selectivenanoscale porous structure, which would have high surface area, inside the glass. Moreover,we consider that a co-continuous structure may be obtained by controlling heat treatmentwhile avoiding phase separation in the entire glass because the composition of thenon-modified region is not immiscible. The above results are useful for the fabrication oflocal catalyst supports, modification of micro flow devices, and so on.