A new technique to study dielectric breakdown with picosecond laser pulses

With ultrashort light pulses at high intensity, a sharp increase of second harmonic generation is observed in various media presenting direct evidence for a large carrier concentration produced by dialectric breakdown. In water, signals up to 10 ⁸ photons at the second harmonic frequency are measured and compared with model calculations, which suggest an electron density of ∼ 10 ²⁰ cm ^-3. Breakdown is found to occur at power levels notably higher than inferred from the ocular observation of broad band luminescence.