Control of element distribution in glass with femtosecond laser

Element migration in multicomponent glass is a phenomenon induced by high-repetition femtosecond laser irradiation and enables spatially selective modification of glass composition. Since the composition of a glass affects its material properties such as refractive index, luminescence, etching rate, viscosity, crystallization temperature, and phase-separation property, element migration is of great interest for practical applications. However, the mechanisms underlying migration have not been elucidated. In this study, we succeeded in identifying its driving force. In an experimental study, we simultaneously focused two beams of femtosecond laser pulses into two spatially-separated spots inside silicate glass. We observed the formation of characteristically shaped element distributions by electron probe microanalysis. In addition, we performed numerical simulations in which we considered concentration- and temperature-gradient-driven diffusions. The simulation results were in excellent qualitative agreement with the experimental results, indicating that element migration can be explained by thermodiffusion and that the driving force is the temperature gradient. These results constitute an important advance for three-dimensional control of glass properties.