Ultrafast X-ray Diagnostics

ultrafast-x-ray-diagnostics.jpg

The evolution of the optical reflectivity of a silicon nitride
sample after exciting with an X-ray free-electron laser
pulse at a delay of 0ps. The drop in reflectivity by several
percent is used to establish the temporal overlap between
optical and X-ray lasers.

For pump-probe measurements with an ultrashort optical laser pulse and an ultrashort X-ray pulse, both pulses need to overlap in space and time at the measurement point. The spatial overlap is often easily established with a screen that is moved into the measurement point, that scatters the optical laser into a camera and converts the X-rays to visible light. For the coarse temporal alignment, one can use photodiodes with short response times which leads to overlaps in a window of tens to hundred picoseconds width. Measuring the photoemission current from high-bandwidth optimized cathodes can lead to temporal overlap estimates below ten picoseconds.

To get temporal overlap approaching the pulse-length limit on the order of 100fs, one can rely on all-optical methods. The X-ray induced change in the optical reflectivity of different materials can be observed at intense X-ray sources like free-electron lasers and proceeds on time scales of order ten femtoseconds. So far, no intrinsic time scale could be observed, as it is shorter than the used pulses (see figure).

Especially at free-electron lasers, the length of the machines and the variations in the pulse structure due to the SASE process call for a shot-to-shot characterization of the relative arrival time of the pump and probe pulses. Recently, methods have been developed that can parasitically measure the timing and thus increase the time resolution from typically 250fs to 100fs.

References:

  • Gahl, C. et al. A femtosecond X-ray/optical cross-correlator. Nature Photon. 2, 165–169 (2008)
  • Beye, M. et al. X-ray pulse preserving single-shot optical cross-correlation method for improved experimental temporal resolution. Appl. Phys. Lett. 100, 121108 (2012)