Time resolved study of femtosecond microfabrication in silica glass

We report investigation of light-induced damage threshold (LIDT) in purified silica (transmission band down to 160 nm) by 350 fs laser pulses at the wavelength of 795 nm and 498 nm. Focusing a single pulse by a high numeric aperture NA = 1.35 microscope objective lens results in one of the lowest single-shot bulk LIDT values reported so far, 5 J/cm², while the surface ablation threshold is 2.5J/cm² with both values being well below the critical self-focusing power in silica. Furthermore, we report the peculiarities of damage by two-pulse irradiation (duration of each pulse is 440 fs), where both pulses have energies at the level of 0.5 × LIDT. Comparison between the experimental data and numeric simulation, which takes into account optical free-carrier generation and relaxation, demonstrates that these processes can explain the measured self-focusing, super-continuum generation, and lightinduced damage threshold values. We argue that use of high numeric aperture objective, despite substantial temporal pulse stretching, results in tight focusing which is capable of overcoming the beam self-focusing, and the resulting fabrication quality is comparable to that obtained using shorter pulses.