  Laser-induced plasma formation and subsequent relaxation in solid dielectrics
is the precursor to structural modifications that are accompanied by a
permanent alteration of material properties. The decay of the electron-hole
plasma through distinct relaxation channels determines the properties of the
resulting modification. 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
($\lambda =\,2100\,$nm) laser pulses. Whereas the relaxation time of the
generated 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 detrapping of self-trapped excitons.
