Abstract
This study demonstrates a non-degenerate pump-probe spectroscopy with a white light beam probe based on a regenerative, amplified, mode-locked, Ti:sapphire laser. This white light beam probe is produced by supercontinuum generation of sapphire crystal after ultra-short pulse excitation. To implement the pump-probe experimental operation, the ablation dynamics with and without fresh spot measurements in fused silica samples are demonstrated. Combining the time-resolved differential reflection profiles in the white light range and X-ray photoelectron spectroscopy spectra of fused silica, the following ablation dynamics processes can be observed: Without fresh spot measurements, once carriers are excited, first, the three absorption bands of the intrinsic defect sites are observed within 750 fs. Then, a fast recovery is observed. This recovery comes from defect-trapped carriers excited to conduction bands through hot-carrier-phonon interactions. In the final step, a rapidly rising signal is observed after 800 fs. This signal rise comes from the creation of free-electron plasma, the density of which increases with increasing excitation energy accumulation. With fresh spot measurements, time delay of carrier dynamics among the three bands can be identified clearly within 750 fs. The intrinsic defect sites of fused silica play the key role during the ultrafast laser ablation process. (C) 2011 Optical Society of America