Time-resolved soft x-ray absorption spectroscopy using femtosecond laser plasma x-rays

摘要

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 show the spectral and temporal properties of broadband soft x-rays emitted from a femtosecond laser-produced on a metal surface. The minimum pulse duration was < 5 ps. We achieved an approximately 20-fold enhancement of the soft x-ray emission by fabricating an array of nanocylinders on a gold surface. We measured the duration of the soft x-ray emitted from the laser-produced 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 soft x-ray absorption of optically excited silicon near its L_(II,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. From experimental results, 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 spatiotemporal evolution of ablated particles in Al plasma created by a 100-fs laser pulse.