Void recording in silica

We report on the formation of voids of 200-300 nm in diameter by single femtosecond pulses (800 nm/150 fs/ < 100 nJ) inside silica glass. The mechanism of void formation can be understood in terms of dielectric breakdown and shock wave formation. The voids were recorded below the threshold of self-focusing. Diffraction efficiency of gratings recorded by arranging voids into a pattern of open core channels have shown an interesting tendency of an increasing efficiency for the first five diffraction orders. It is demonstrated by numerical simulation using the finite difference time domain (FDTD) algorithm that such anomalous diffraction is indeed caused by empty core channels surrounded by densified shells inside silica.