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/cm(2), while the surface ablation threshold is 2.5 J/cm(2) 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 x 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 light-induced 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