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 ${\text{SiO}}_2$ in $<300\text{fs}$ following free-carrier injection and decay biexponentially with lifetimes of $34\pm 8$ and $338\pm 67\text{ps}$ 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.

• Received 31 May 2010

DOI:https://doi.org/10.1103/PhysRevB.81.212301