Time-Resolved Absorption Spectroscopy Using Femtosecond Laser-Produced Plasma X-rays.

Abstract

High-density plasmas created near a solid surface by a femtosecond laser pulse emit ultrashort x-ray pulses that are synchronized with the laser pulse. We achieved more than 30-fold enhancement of the soft x-ray emission by fabricating an array of nanoholes on an alumina surface. We measured the duration of the soft x-ray emitted from the laser plasma by the cross-correlation method using an optical field-induced ionization process in Kr gas. Utilizing a 10-ps soft x-ray pulse, we measured the time-resolved absorption of optically excited silicon near its LII,III edge. We found that laser-pulse irradiation caused a more than 10 % increase in the soft x-ray absorption near the edge, which recovered within 20 ps. We assume the origin of this absorption change to be the bandgap renormalization of Si. We also employed picosecond soft x-rays to measure the ablated particles in Al plasma created by a 100-fs laser pulse.