High Repetition Rate Femtosecond Laser Induced Functional Microstructures In Glasses

The interactions between femtosecond laser and various transparent materials are usually based on nonlinear processes due to its ultrashort pulse duration and ultrahigh energy density. Various new phenomena with regard to femtosecond laser-mater interaction have been observed in recent years, which shed new lights on various fundamental sciences such as material science, plasma physics, high-field physics, nonlinear optics and etc.In this thesis, microstructures in various glasses induced with 250 kHz femtosecond lasers were studied. We focused on the following issues: 1. the space-selective precipitation of nonlinear optical functional crystals induced by high repetition femtosecond laser pulses in glasses and 2. migration of ions in glasses induced by high repetition femtosecond laser pulses.Nonlinear optical crystals such as TiO₂ and CaF₂ has been induced in three dimensions by 250 kHz femtosecond laser inside transparent glasses. A heat accumulation effect is proposed as an important contribution to crystal growth in the irradiated regions. The mechanism of the crystal formation is as follows: The glass will transform to plasma state via multiphoton ionization due to the ultrahigh light intensity of femtosecond laser, the dense plasma will also absorb the laser energy effectively. So there exists a temperature gradient in the heat-affected zone. When the glass temperature exceeds the crystallization temperature T_c, the phase transformation from glass to crystalline phase occurs. We successfully induced three-dimensional crystallization of rutile TiO₂ with a high refractive index inside a glass. This result may find important applications in fabrication of optical devices such as gratings and photonic crystals. We also used 800 nm, 250 kHz femtosecond laser pulses to precipitate Er³⁺ doped CaF₂ crystals inside oxyfluoride glass, which led to greatly enhanced upconversion luminescence intensity in comparison with unmodified glass in the laser modified region. We demonstrate the possibility of three-dimensional optical data storage in the glass by the use of the confocal upconversion luminescence imaging.The other topic is the migration of ions in glasses induced with high repletion rate femtosecond laser pulses irradiation. Raman spectra indicate that coordination transformation of B³⁺ ions in borate glass was induced with 250 kHz infrared femtosecond laser irradiation. EDX spectra show that a portion of Na⁺ and O^2- ions migrate from the vicinity of focal point after the femtoseond laser irradiation. A possible mechanism based on thermal driven diffusion of ions is proposed to explain the observed phenomena. We also used EPMA to investigate the element distribution near the focal point in silicate glasses induced with 250 kHz femtosecond laser pulses irradiation. Element redistribution occurred near the laser focal point due to migration of ions. The relative concentration of network modifier ions such as Ca²⁺, Ba²⁺ increased in a ring-shaped region around the focal point while decreased in near the focal point. Futhermore, we achieved micro-modification of Eu element distribution in a silicate glass with femtosecond laser irradiation. We successfully applied the femtosecond laser induced migration of ions technique to micro-modification of fluorescence properties of the Eu³⁺ doped silicate glass. The femtosecond laser induced migration of ions technique is opening up new prospects for fabricating integrated optical devices such as waveguide lasers and optical memory devices inside glasses.