Void-Nanograting Transition by Ultrashort Laser Pulse Irradiation in Silica Glass

The structural evolution from void modification to self-assembled nanogratings in fused silica is observed for moderate (NA > 0.4) focusing conditions. Void formation, appears before the geometrical focus after the initial few pulses and after subsequent irradiation, nanogratings gradually occur at the top of the induced structures. Nonlinear Schrödinger equation based simulations are conducted to simulate the laser fluence, intensity and electron concentration in the regions of modification. Comparing the experiment with simulations, the voids form due to cavitation in the regions where electron concentration exceeds 1020 cm-3 but is below critical. In this scenario, the energy absorption is insufficient to reach the critical electron concentration that was once assumed to occur in the regime of void formation and nanogratings, shedding light on the potential formation mechanism of nanogratings.