Exciton-seeded multiphoton ionization in bulk SiO2

In a femtosecond pump-probe experiment the pump pulse injects a modest density of free carriers by multiphoton absorption. Measuring the nonlinear absorption of the probe pulse we observe exciton-seeded multiphoton ionization. The excitons self-trap in SiO2 in <300 fs following free-carrier injection and decay biexponentially with lifetimes of 34 +/- 8 and 338 +/- 67 ps at room temperature. The extent of the probe-pulse absorption provides a model-independent demonstration that avalanche ionization plays a significant role in free-carrier generation by laser pulses as short as 45 fs. Free-carriers injected into a dielectric from extreme-ultraviolet (XUV) sources created by high harmonic or attosecond pulse generation and then avalanched with a perfectly synchronized infrared (fundamental) probe pulse, can provide a route to nanoscale laser machining.