Microfabrication by a high fluence femtosecond exposure: Mechanism and applications

We report the observation of 3/2-frequency generation during an Optically-induced failure of silica under femtosecond laser pulse irradiation. The origin of 3/2-frequency generation is due to a two-plasmon decay instability, which occurs at the quarter critical density of free charge carriers. We observed this emission during the optical damaging of glasses by tightly focused (numerical aperture of the objective lens was 0.5-1.35) femtosecond laser pulses. The pulse duration at the irradiation spot was about 0.35 ps, the energy 25-250 nJ, and the damage was recorded in a single shot event inside the glass. The emission at about 530 nm was only present in the spectra measured during an optical damage by 795 nm irradiation with the pulse energy 9 times and more higher than the threshold. We observed a new phenomenon applicable for microstructuring of glass. The high energy fs pulses (50-200 μJ) were focused by a piano-convex lens (focal length 2-10 cm) on the exit surface of a glass plate. The surface was ablated and the ablation was transferred into a volume of glass by translation of a "plasma spark". The length of such a channels can by up to few-cm and with a diameter of tens-of-micrometers. The mechanism and application of high-fluence fs fabrication in dielectrics is discussed.