Sub-picosecond optical damaging of silica: Time resolved measurements of light induced damage threshold

We investigate damage of purified silica (transmission band down to 160 nm) by sub-ps light pulses having a wave-length of 795 nm. Illumination by 350 fs duration pulses focused by a high numerical aperture NA = 1.35 microscope objective results in one of the lowest reported values for the single-shot bulk light-induced damage threshold (LIDT) of 5 J/cm², well below the critical self-focusing power in silica. We have also investigated peculiarities of damage by two coincident laser pulses (duration 440 fs) having power of about 0.5 × LIDT, and linearly cross-polarized to avoid interference effects. The reduction of LIDT in silica is demonstrated for an elevated lattice temperature T=400 K, at which the thermal linear/volume expansion coefficient has its maximum. Comparison between the LIDT values obtained from the numeric simulation and experiments demonstrates, that the critical density of optically generated free carriers corresponding to LIDT n_cr≃10²¹ cm^-3 is reached during the first half time of the laser pulse illumination(0.2 ps).