Laser-induced plasma formation and subsequent relaxation in dielectric solids is the precursor to structural modifications serving as the basis for direct laser writing of functional optical micro- and nanostructures. Based on an experimental arrangement combining a time-resolved transmission measurement with a cross-phase modulation measurement, we isolate the plasma formation and relaxation dynamics in the bulk of amorphous fused silica excited by femtosecond short-wavelength infrared (λ= 2100 nm) laser pulses. Whereas the relaxation time of the generated subcritical electron-hole plasma was so far assumed to be constant, our findings indicate an intensity-dependent relaxation time. We attribute this intensity dependence to vibrational activation of the medium, leading to detrapping of trapped carriers and a reduced trapping probability.
