X-ray short-pulse generation from femtosecond laser-produced plasmas and its application in pump-probe spectroscopy

Abstract

High-density plasmas created near a solid surface by a femtosecond laser pulse emit ultrashort x-ray pulses that are synchronized to the laser pulse. In the first part of this paper, the spectral and temporal properties of the x- ray emitted from plasma created on aluminum film by a femtosecond laser pulse are shown. The minimum pulse duration was &lt; 3 ps as measured by an x-ray streak camera. The energy conversion efficiency, from laser pulse into soft x-ray at 14 +/- 0.05 nm, was 10<SUP>-6</SUP> - 10<SUP>-5</SUP>. More than a 30-fold enhancement in soft x-ray emission was achieved by fabricating an array of nanoholes on an alumina surface. In the latter half, we demonstrate time-resolved absorption measurement in the soft x-ray region by means of pump-probe spectroscopy. Using a 10-ps x-ray pulse, we measured time-resolved absorption of optically-pumped silicon near its L<SUP>II,III</SUP> edge. We found that laser-pulse irradiation caused a more than 10% increase in soft x-ray absorption near the edge, which means that the transition of electrons in inner shells was rapidly modulated by excitation of valence electrons. The absorption change recovered within 20 ps.