Femtosecond Laser-Induced Crystallization in Glasses: Growth Dynamics for Orientable Nanostructure and Nanocrystallization

Laser-induced crystallization in glasses is of great interest because of its significant applications in optics. However, the mechanisms involved are not yet concluded, and this paper is aimed at making progress on this problem. Major aspects of laser-induced crystallization are the nanostructure formation, textured crystallization, and growth dynamics. Lithium niobium silicate glasses were used as a "glass model" to investigate the nanostructure formation and crystallization by femtosecond (fs) laser irradiation at a high repetition rate (100-500 kHz). Three crystallization regimes can be classified with the increase of laser power. In addition, the boundary between neighboring regimes can be adjusted by changing the writing parameters. Regime 1: modifications of the amorphous structure. Regime 2: appearance of textured nanocrystals embedded in amorphous lamellas. Polar axes of nanocrystals are perpendicular to laser polarization direction. Remarkably, this hierarchical micro-/nanostructure can induce an orientation tunable second harmonic generation and form birefringence. Regime 3: crystallization morphology is sensitive to the angle between laser writing and laser polarization direction. With parallel configuration, crystal grains are at the micron scale. With perpendicular configuration, crystal morphologies resemble regime 2, but they are not textured. A crystallization dynamic model is proposed to explain the various nano-/microcrystals formation and their morphology induced by fs laser irradiation. This model allows thinking that it is possible to extend conclusions to other noncongruent glasses. This provides guidelines for manufacturing multifunctional optical devices.