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∼ √τ_p, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO₂ is measured up to ∼3×10⁸ V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.