Avalanche ionization and dielectric breakdown in silicon with ultrafast laser pulses

Experimental evidence is presented demonstrating avalanche ionization as the dominant mechanism for dielectric breakdown in silicon with ultrafast laser pulses at above-gap photon energies. Data are presented for pulses between 80 fs and 9 ns at 786 nm and 1.06 mu m. Associated electric fields range from 0.3 to 40 MV/cm. Avalanche ionization coefficients range from 10(10) to 10(14) s(-1) and are discussed in relation to semiempirical de ionization theory and recent ac Monte Carlo calculations. Correlation is obtained between electron collision times and associated ionization rates.