Direct observation of the reversible and irreversible processes in femtosecond laser-irradiated fused silica at the near-damage-threshold fluence

For fused silica irradiated by near-100-fs, 795-nm laser pulses with fluence approaching the damage threshold, the transient transmission spectroscopy based on a wavelength-degenerate pump-probe technique clearly presents two dynamic processes corresponding to the instantaneous effects of laser optical field and the delayed effects of free electron dynamics, respectively. The reversible, instantaneous process originates in third-order nonlinear optical responses (in particular the optical Kerr effect) ascribed to virtual optical-field ionization (VOFI) that significantly contributes to the nonlinear optical polarization with energy exchange recoverability. Whereas, the irreversible, delayed process originates in the effects of free electron plasma generated by initial real optical-field ionization (ROFI) and subsequent impact ionization (II), being responsible for the energy dissipation and optical breakdown. In general, the femtosecond wavelength-degenerate pump-probe spectroscopy can detect VOFI, ROFI, and II simultaneously in strong-field nonlinear polarization and ionization of fused silica, and offer flexible ways to distinguish the different mechanisms. For the near-100-fs pulses, our results confirm that II provided with a typical delay time about 300 fs is responsible for the optical breakdown of fused silica.