Transient light absorption induced in glassby femtosecond laser pulses

The dynamics of the transient light absorption induced in K8 optical glass by filamented femtosecond laser pulses have been studied using time-resolved transmitted-light microscopy at wavelengths from 450 to 700 nm. The transient absorption measured as a function of probe beam wavelength is compared to that predicted by the Drude plasma model. We conclude that, just 450 fs after a pump pulse, the transient absorption is dominated by transient electronic states, presumably, self-trapped excitons, with an excitation energy of 2.6 — 2.7 eV. These states are filled with free-carriers from a long-lived plasma, which acts as a "carrier reservoir". The relaxation of transient absorption has two components. The slow component, with τ₁ ~17-17.5 ps, is governed by the plasma thermalisation time, whereas the second, with τ₁ ≫ 300 ps, is determined by the plasma lifetime.