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For fused silica irradiated by near-100-fs, 795-nm laser pulses with fluenceapproaching the damage threshold, the transient transmission spectroscopy basedon a wavelength-degenerate pump-probe technique clearly presents two dynamicprocesses corresponding to the instantaneous effects of laser optical field andthe delayed effects of free electron dynamics, respectively. The reversible,instantaneous process originates in third-order nonlinear optical responses (inparticular the optical Kerr effect) ascribed to virtual optical-fieldionization (VOFI) that significantly contributes to the nonlinear opticalpolarization with energy exchange recoverability. Whereas, the irreversible,delayed process originates in the effects of free electron plasma generated byinitial real optical-field ionization (ROFI) and subsequent impact ionization(II), being responsible for the energy dissipation and optical breakdown. Ingeneral, the femtosecond wavelength-degenerate pump-probe spectroscopy candetect VOFI, ROFI, and II simultaneously in strong-field nonlinear polarizationand ionization of fused silica, and offer flexible ways to distinguish thedifferent mechanisms. For the near-100-fs pulses, our results confirm that IIprovided with a typical delay time about 300 fs is responsible for the opticalbreakdown of fused silica.

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