An overarching goal of ultrafast science is to enable researchers to watch structural changes in matter on their natural timescale. Experimentally, this translates into taking two snapshots of the same nanoscale object that are only femtoseconds apart to capture the initial and final states of the system. At the European XFEL in Schenefeld, Germany, it has recently become possible to perform two-colour X-ray diffraction experiments, where independently tunable pairs of X-ray pulses can be produced with a highly controllable temporal separation.
This experimental capability may suggest that two-frame X-ray movies are within reach, but a considerable hurdle needs tackling: no light detector is fast enough to record the diffraction patterns from two such snapshots separately. The frames end up on a single image, like a double-exposure photograph. Now, an international team of researchers including members of Professor Daniela Rupp's group in the Department of Physics at ETH Zurich have found ways to disentangle the two superimposed diffraction patterns captured by a single X-ray detector exposure. The two articles reporting on their findings have recently been published in Nature Communications.
Imaging with two colours
The researchers used the Small Quantum Systems (SQS) instrument at the European XFEL. "This endstation is ideally suited to an experiment conceived to show that two-colour X-ray imaging of nanoscale dynamics work," says Dr Michael Meyer, lead scientist at the SQS. Pairs of X-ray pulses with photon energies of about 1 and 1.2 kiloelectronvolts and temporal separations of up to 750 femtoseconds were focussed on isolated free-flying helium nanodroplets. The scattered X-rays were then recorded on a pn-junction charge coupled device (pnCCD) that measures the charge created on each pixel by the incoming photons.
Based on this experimental setup, the team considered two approaches to the task of disentangling two diffraction patterns hitting the pnCCD detector almost simultaneously.
Every pixel counts
First, the researchers separated the overlapping diffraction signals based on an individual pixel count analysis combined with an image post-processing algorithm. Indeed, the high photon-energy resolution and low readout noise of the pnCCD allowed the team to assign pixels to single- and multiple-photon hits coming from either X-ray pulse as well as to identify mixed-photon events. The outcome of this pixel-based colour separation approach showed good agreement with simulated results.
"This method is very flexible. It can be applied to any kind of target system, and it's especially good at extracting valuable high-resolution information from the diffraction patterns," says Linos Hecht, PhD candidate in Rupp's group and first author of both publications. "These findings open the door to observing rapid changes in nanoparticles, comparable to what is already possible with small molecules," adds Dr Yevheniy Ovcharenko, scientist at the SQS and principal investigator of the study.
One pattern, two images
Encouraged by the performance of the pixel-based colour separation strategy, the team went a step further with an algorithmic approach they termed Dichography. Dichography extends well-established imaging algorithms for conventional diffraction patterns to the two-colour case, making it possible to recover two time-delayed snapshots of a given sample. In this case, the researchers imaged helium droplets doped with xenon to study a target system with higher structural complexity.
The results presented in the second paper show how the Dichography algorithm can reconstruct two views of xenon filaments within a nanodroplet from its two-colour diffraction pattern. "To my knowledge, these are the fastest nanoscale movies ever recorded, if by movie we mean multiple frames of the same object," says Dr Alessandro Colombo, the member of Rupp's group who led this investigation.
All eyes on target
Two-colour X-ray imaging remains experimentally challenging, as Dichography requires high brightness and similar scattering contributions from the two X-ray pulses. Colombo believes that the continued machine development at facilities like the European XFEL will help make Dichography more readily applicable. "We now have free-electron lasers that can produce two-colour pairs of X-ray pulses, we have detectors that allow us to distinguish images based on the colour of the scattered light, and we have analysis tools to reconstruct the shape of individual particles from the recorded X-ray snapshots," summarises experimental project leader Professor Marcel Mudrich from the University of Kassel.
Ultimately, what the team wishes to build is a complete, powerful toolbox for studying X-ray driven dynamics. "We're excited about the new science that will be unlocked by the ability of filming ultrafast structural changes in nanomatter," says Colombo.
