Dynamic interplay between femtosecond laser ionization mechanisms in solid dielectrics

Abstract

Dominating ionization mechanisms are revealed for bulk dielectrics irradiated by ultrashort (femtosecond) laser pulses, analyzing reported in literature well-established and consistent experimental data dealing with dependences of electron-hole plasma density on laser intensity, damage thresholds and bulk damage (surface crater) size on dielectric bandgaps. Multi-photon ionization was found to be predominant at low laser intensities (I las I break - ionization processes are strongly damped by unknown non-linear mechanisms, one of those could be Auger recombination, and are followed by microscopic damage of dielectric materials. In the latter range of I las the most accurate source of experimental information on ionization processes is scaling relationships between damage thresholds for corresponding dielectrics and their dielectric bandgaps, fs-laser pulsewidth, wavelength and polarization, as well as spatial size of damage (surface crater) on bandgap width or laser pulse energy. Theoretical analysis of these experimental data and some experimental scaling relationships provides an important insight into interplay of various ionization mechanisms and Auger recombination, supported by results of our numerical calculations of EHP density vs. I las , and enables to explain other separate important experimental facts on high-intensity ionization in bulk dielectrics.