Temporal cross-correlation of x-ray free electron and optical lasers using soft x-ray pulse induced transient reflectivity

Abstract

The recent development of x-ray free electron lasers providing coherent, femtosecond-long pulses of high brilliance and variable energy opens new areas of scientific research in a variety of disciplines such as physics, chemistry, and biology. Pump-probe experimental techniques which observe the temporal evolution of systems after optical or x-ray pulse excitation are one of the main experimental schemes currently in use for ultrafast studies. The key challenge in these experiments is to reliably achieve temporal and spatial overlap of the x-ray and optical pulses. Here we present measurements of the x-ray pulse induced transient change of optical reflectivity from a variety of materials covering the soft x-ray photon energy range from 500eV to 2000eV and outline the use of this technique to establish and characterize temporal synchronization of the optical-laser and FEL x-ray pulses.

X-ray induced optical reflectivity

“…1 also shows data for Si 3 N 4 (silicon nitride), a large band gap semiconductor/insulator (E gap >5 eV), whose optical reflectivity at 800 nm (1.55 eV) was probed with FEL pulses at 500-2000 eV. 10 The observed drop in reflectivity is consistent with the band filling picture: enhanced absorption due to the valence holes, and negligible reflectivity from the electrons promoted to the conduction band.…”

“…This semiconductor has E gap = 1.43 eV, and was probed with 800 nm photons (E γ = 1.55 eV), originally with ∼50 fs XFEL pump pulses at 40 eV, 5 and then with ∼100 fs XFEL pump pulses at 800 eV. 10 The data show the predicted increase in reflectivity caused by band filling, after a few picoseconds. This is preceded, however, by an initial transient of sharply reduced reflectivity.…”

“…This is preceded, however, by an initial transient of sharply reduced reflectivity. Gahl et al 5 report that the characteristic time for this drop was only ∼160 fs for 40 eV incident pulses, and Krupin et al 10 cite a ∼100 fs transition time at 800 eV, although both numbers may be upper limits due to broadening by timing uncertainties in the x-ray and optical laser pulses. This sharp drop is apparently due to a prompt increase in absorption, followed by band filling and the associated increase in reflectivity.…”

“…9 Below we analyze the basic response seen in x-ray pump optical probe reflectivity measurements of insulating silicon nitride (Si 3 N 4 ), semiconducting gallium arsenide (GaAs), and metallic gold (Au). 5,10 This analysis assumes that the band structure is not drastically changed, i.e. that the deposited power is not driving the material into a new metastable phase or creating irreversible damage.…”

“…A useful metric is the density of electrons excited into the conduction band. To estimate this, we consider the conditions described by Krupin et al 10 for XFEL pulses of 50-150 fs at 500-2000 eV, with a pulse energy of 1 mJ deposited in a 0.6 mm 2 spot size. Assuming a GaAs target, 800 eV photons with a ∼0.6 micron attenuation length, and that all of the x-ray energy ultimately goes into electron-hole pair creation at ∼10 eV per pair, 3 the induced conduction electron density is a durbin@purdue.edu 2158-3226/2012/2(4)/042151/7 C Author(s) 2012 2, 042151-1 2 × 10 21 cm −3 , or 1% of the valence electron density.…”