Durability of the refractive index change induced by a single femtosecond laser pulse in glass

Ultrafast-laser inscribed optical memories have been considered as a high-density low-energy-consumption alternative to the magnetization-based memories. The optical memories are based on laser-induced material modifications resulting in the refractive index change. The long-term stability of such modifications has been indicated by subjecting them to the accelerated aging by annealing at elevated temperatures. Here, the first direct evidence is provided of the durability of the type II refractive index change in BK7 glass. The investigation was performed for over 27 months at room temperature. The results show the existence of the laser pulse intensity threshold above which the magnitude of the index change does not deteriorate with time and, hence, is suitable for optical memory, photonic crystal and fibre-grating writing.