LASER-INDUCED BREAKDOWN BY IMPACT IONIZATION IN SIO2 WITH PULSE WIDTHS FROM 7 NS TO 150 FS

Results of laser-induced breakdown experiments in fused silica (SiO2) employing 150 fs-7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse-width range. Fluence breakdown threshold does not follow the scaling of F(th) is similar to square-root tau(p), when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to approximately 3 X 10(8) V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.