Femtosecond Optical Breakdown in Dielectrics

We report measurements of the optical breakdown threshold and ablation depth in dielectrics with different band gaps for laser pulse durations ranging from 5 ps to 5 fs at a carrier wavelength of 780 nm. For $\ensuremath{\tau}<100\mathrm{fs}$, the dominant channel for free electron generation is found to be either impact or multiphoton ionization (MPI) depending on the size of the band gap. The observed MPI rates are substantially lower than those predicted by the Keldysh theory. We demonstrate that sub-10-fs laser pulses open up the way to reversible nonperturbative nonlinear optics (at intensities greater than ${10}^{14}\mathrm{W}/{\mathrm{cm}}^{2}$ slightly below damage threshold) and to nanometer-precision laser ablation (slightly above threshold) in dielectric materials.