An Overview of the Thermal Erasure Mechanisms of Femtosecond Laser-Induced Nanogratings in Silica Glass

Type II modifications induced by infrared (IR) femtosecond (fs) lasers are used in many optical devices due to their excellent thermal stability at high temperatures (typically >800 °C). The characteristic feature of type II modifications is the formation of nanogratings, which are easily detected through birefringence measurements. However, the measured birefringence is an aggregate value of multiple contributions including form birefringence, stress-induced birefringence due to permanent volume changes, and point defects. This work investigates the thermal erasure kinetics of each one of these contributions in silica glass. Firstly, samples are irradiated with a fs-laser using different conditions (polarization, energy). Secondly, accelerated aging experiments are conducted to evaluate the stability of the laser-induced modifications, including defects, densification, stress field, and porous nanogratings. Finally, the aforementioned contributions to the thermal stability of the nanogratings are identified using spectroscopic techniques (Raman, Rayleigh scattering, UV–vis absorption) and electron microscopy. Moreover, porous nanogratings erasure kinetics are simulated using the Rayleigh–Plesset equation. Herein, a valuable framework in the realization of silica glass-based optical devices operating at high temperatures (≫800 °) is provided by 1) evidencing the effect of annealing on each erasure mechanism and 2) providing information on the optical response (mainly birefringence) upon annealing.