Nonlinear optics with ionizing radiation and ultrafast lasers: progress toward measuring the complete electric field of XFEL pulses

Newly developed X-ray free electron lasers (XFELs) promise to transform nearly all areas of modern science. However, there is a significant need for more sophisticated tools to measure the X-ray pulses themselves. Complete measurements of the pulse’s electric field are needed to improve the light source technology and to enable new spectroscopy and scattering experiments. Here we describe measurement approaches in which an optical laser interacts with an interference pattern from the unknown XFEL pulse, referred to as a transient grating or as four wave mixing. This geometry largely isolates the instantaneous wave-mixing response necessary for the FROG algorithm and removes the long-lived response seen in other X-ray-pump optical-probe measurements. The measurement also transfers the phase information of the X-ray electric field onto a readily-detectable optical laser field, allowing us to fully characterize the unknown pulses with visible spectrometers and cameras. Crucially, our strategy promises to be broadly applicable across the entire energy range accessible by existing and planned XFEL facilities. Lab-scale UV demonstration experiments will be presented as well as results from an upcoming beam time at the FERMI soft X-ray FEL.