Electron relaxation in SiO₂ under strong laser field using high order harmonics generation

Time-resolved photoemission spectroscopy using high order harmonics is used to measure the energy relaxation rate of hot electrons in α-SiO₂ with sub-picosecond time resolution. Our results indicate that electrons of a few band gap energy relax at a rate which is at least two orders of magnitude lower than the one of photo-excited carriers of a few eV. As a result, we give insight in the relaxation process of hot electrons and show that impact ionization probability per unit time is only of the order of 1/40 ps^-1, in very strong contrast with the much higher value generally assumed in models of optical breakdown.