The impact ionization coefficient in dielectric materials revisited

Avalanche ionization plays a crucial role in the photoionization of dielectric materials with respect to optical damage. Although it has been investigated closely during the last years, its significance on the ultrashort time scale remains a contentious issue. Here we present UV-pump IR-probe experiment on the fs time scale that allowed us to isolate the avalanche ionization from other major ionization processes, especially multiphoton ionization, electron tunneling, and relaxation into traps and their re-excitation. We show that the assumption of an intensity independent impact ionization factor a cannot explain the results. Application of a simple avalanche ionization model within the flux-doubling approximation requires an intensity dependent coefficient a(I) to explain the data.