Speaker
Description
Stroboscopic imaging is a popular use of free electron lasers (FELs), enabling studies following the dynamics of a variety of systems ranging from fast-moving objects [1] to ultrafast magnetic dynamics [2]. However, these methods face a fundamental limitation: that each flash of light from the FEL produces a single image. Therefore, to stitch together a full movie of a time-dependent process, the process either needs to happen slower than the FEL repetition rate, or it needs to be repeated many times, with one frame of data extracted per repetition.
This causes challenges when one wants to study a system which cannot be prepared in a repeatable state, such as a thin film hosting natural magnetic domains, on the femtoscale timescales comparable to the pulse length of a FEL. To overcome this challenge, we have recently demonstrated a new method, multi-frame randomized probe imaging (RPI). In this method, a single diffractive optic introduces a series of time delays, while simultaneously producing the structured illumination patterns that enable phase retrieval with RPI [3,4].
Here, we will show the results of a demonstration experiment studying the process of ultrafast demagnetization in a thin film of CoGd. However, one key limitation of this method is the need to work with stable and nearly transform-limited FEL pulses. For this reason, we performed the experiment at the FERMI FEL [5]. More broadly, continuing to develop multi-frame RPI will place a number of unusual requirements on the light source. In this presentation, we will finish by discussing the key opportunities of multi-frame RPI, with a focus on how the limitations of the method can potentially be alleviated by future developments in FEL technology.
References
[1] Vagovič, P. et al. Megahertz x-ray microscopy at x-ray free-electron laser and synchrotron sources. Optica 6, 1106 (2019).
[2] von Korff Schmising, C. et al. Imaging Ultrafast Demagnetization Dynamics after a Spatially Localized Optical Excitation. Phys. Rev. Lett. 112, 217203 (2014).
[3] Levitan, A. L., Keskinbora, K., Sanli, U. T., Weigand, M. & Comin, R. Single-frame far-field diffractive imaging with randomized illumination. Opt. Express 28, 37103 (2020).
[4] Levitan, A. L. et al. Single-shot imaging with randomized structured illumination at a free electron laser. Opt. Express 34, 8043 (2026).
[5] Allaria, E. et al. Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet. Nature Photon 6, 699–704 (2012).