A self-referenced in-situ arrival time monitor for X-ray free-electron lasers

TL;DR

This paper introduces a self-referenced, in-situ monitor for femtosecond X-ray and optical pulse timing, achieving high accuracy and sensitivity, suitable for high-repetition-rate XFEL experiments.

Contribution

A novel in-situ timing monitor that measures femtosecond delays directly on the sample used in pump-probe experiments, with high accuracy and compatibility with MHz XFEL sources.

Findings

Achieves timing accuracy better than 25 fs.

Correlates with existing diagnostics with a Pearson coefficient of 0.98.

Sensitive to X-ray pulses with energies as low as 30 μJ.

Abstract

We present a novel, highly versatile, and self-referenced arrival time monitor for measuring the femtosecond time delay between a hard X-ray pulse from a free-electron laser and an optical laser pulse, measured directly on the same sample used for pump-probe experiments. Two chirped and picosecond long optical supercontinuum pulses traverse the sample with a mutually fixed time delay of 970 fs, while a femtosecond X-ray pulse arrives at an instant in between both pulses. Behind the sample the supercontinuum pulses are temporally overlapped to yield near-perfect destructive interference in the absence of the X-ray pulse. Stimulation of the sample with an X-ray pulse delivers non-zero contributions at certain optical wavelengths, which serve as a measure of the relative arrival time of the X-ray pulse with an accuracy of better than 25 fs. We find an excellent agreement of our monitor with the existing timing diagnostics at the SACLA XFEL with a Pearson correlation value of 0.98. We demonstrate a high sensitivity to measure X-ray pulses with pulse energies as low as 30 $\mu$J. Using a free-flowing liquid jet as interaction sample ensures the full replacement of the sample volume for each X-ray/optical event, thus enabling its utility even at MHz repetition rate XFEL sources.