Terahertz-wave decoding of femtosecond extreme-ultraviolet light pulses

In recent years, femtosecond extreme-ultraviolet (XUV) and X-ray pulses from free-electron lasers have developed into important probes to monitor processes and dynamics in matter on femtosecond-time and Angstroem-length-scales. With the rapid progress of versatile ultra-fast X-ray spectroscopy techniques and more sophisticated data-analysis tools, accurate single-pulse information on the arrival time, duration and shape of the probing X-ray and XUV pulses becomes essential. Here, we demonstrate for the first time that XUV pulses can be converted into terahertz electromagnetic pulses. We observe that the duration, arrival time and energy of each individual XUV pulse is encoded in the waveform of the associated terahertz pulses and can, thus, be readily deduced from single-shot terahertz time-domain detection.