Femtosecond Laser Induced Ions Migration In Glasses

Femtosecond laser can be used for spatially selective micromodification of transparent materials due to its ultrashort pulse duration and ultrahigh energy density. The most important application of this microfabrication technique is its ability to integrate3D optical or photonic devices inside transparent materials. 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, nonlinear optics, high-field physics, plasma physics and etc.In this thesis, micro structures in various glasses induced with250kHz femtosecond lasers were studied. We focused on the following issues:1. migration of ions in glasses induced by high repetition femtosecond laser pulses, and2. femtosecond laser induced-change of ions’ valence state in glasses, and3. micro-modificaion of luminescence in glass using femtoscecond laser irradiation.A heat accumulation effect is proposed as an important contribution to ions migration in the irradiated regions. The mechanism of the ions migration 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 high repetition rate femtosecond laser is focused inside glass, the temperature at the focal spot can reach more than3000K, localized melting of the glass will happen at the high temperature, and all the bonds linking the network modifiers or the network formers will be broken in the glass melts. Then under a sharp temperature distribution, various kinds of ions would diffuse away from the focal point to the cooler periphery region around the focal point. We successfully realized micr-modification of residual stress and elemental distribution in a bismuth germanate glass by using the250kHz high repetition rate femtosecond laser irradiation. The residual thermal stress increases from the unmodified region to the center of the laser modified region, while Bi is enriched at the boundary area of the inner structure of the laser modified region relative to Ge. Femtosecond laser-induced space selective aggregation of ions and local residual stress may change the refractive index distribution, thus is promising for fabrication of three dimensional optical waveguide by direct femtosecond laser writing or using intensity distribution-modified laser beam.We also successfully realized photoreduction of Ag+to Ag0and micromodification of luminescence property of Ag+-doped phosphate glass with high repetition rate femtosecond laser pulses. The luminescence intensities due to (Ag2)+and Ag0reduced by femtosecond laser changed with the distance from the center of the laser-affected zone. The short range order shows a complicated change behavior with increasing from the center of the laser-affected zone resulted from the redistribution of the Na and Al ions in the inner structure of the laser-affected zone. This technique based on femtosecond laser induced valence variation and migration of ions may find some promising application in the fabrication of3D multicolored industrial art object, optical memory with high storage density and integrated waveguide all-optical switches with ultrafast nonlinear reponse.The last topic is the migration of ions in different alkali metal oxide doped silicate glasses induced with high repetition rate femtosecond laser pulses irradiation. After analysis, we found that the diameter of the laser-affected zone depends on the glass-transition temperature of the glass sample. The higher the glass-transition temperature is, the bigger the diameter of the laser-affected zone is. Experimental results by EPMA demonstrated that the relative concentration of K is higher at the margin of the inner structure and lower in the central area, while that of Cs which also is the network modifier has an opposite tendency. These results provide some new insights about control of the femtosecond laser-induced microstructure and ions migration in glasses.