The incredibly powerful beams of light produced by X-ray free-electron lasers (XFELs) have helped researchers perform unprecedented studies of the ultrafast motions of atoms in matter. But to interpret data taken with these extraordinary light sources, researchers need a solid understanding of how the X-ray pulses interact with matter and how those interactions affect measurements.
Now, computer simulations by scientists from the Department of Energy’s SLAC National Accelerator Laboratory suggest that a new method could turn random fluctuations in the intensity of laser pulses from a nuisance into an advantage, facilitating studies of these fundamental interactions. The secret is applying a method known as “ghost imaging,” which reconstructs what objects look like without ever directly recording their images.
According to James Cryan from the Stanford PULSE Institute, a joint institute of Stanford University and SLAC, instead of trying to make XFEL pulses less random, which is the approach most often pursued for experiments, researchers actually want to use randomness in this case. The results show, that by doing so, the researchers can get around some of the technical challenges associated with the current method for studying X-ray interactions with matter. The research team published their results in Physical Review X.
Scientists commonly look at these interactions through pump-probe experiments, in which they send pairs of X-ray pulses through a sample. The first pulse, called the pump pulse, rearranges how electrons are distributed in the sample. The second pulse, called the probe pulse, investigates the effects these rearrangements have on the motions of the sample’s electrons and atomic nuclei. By repeating the experiment with varying time delays between the pulses, researchers can make a stop-motion movie of the tiny, fast motions. One of the challenges is that X-ray lasers generate light pulses in a random process, so that each pulse is actually a train of narrow X-ray spikes whose intensities vary randomly between pulses.
According to SLAC’s Daniel Ratner, the study’s lead author, pump-probe experiments therefore typically require that the researchers first prepare well-defined, short pulses that are less random. In addition they need to control the time delay between them very well. In the new approach, the team wouldn’t have to worry about any of that. They would use X-ray pulses as they come out of the XFEL without further modifications. In fact, in this new way of thinking each pair of spikes within a single X-ray pulse can be considered a pair of pump and probe pulses, so researchers could do many pump-probe measurements with a single shot of the XFEL.
To produce snapshots of a sample’s molecular motions with this method, Ratner and his team want to apply the technique of ghost imaging. In conventional imaging, light falling on an object produces a two-dimensional image on a detector – whether the back of your eye, the megapixel sensor in your cell phone or an advanced X-ray detector. Ghost imaging, on the other hand, constructs an image by analyzing how random patterns of light shining onto the object affect the total amount of light coming off the object.
According to co-author Siqi Li, a graduate student at SLAC and Stanford and lead author of a previous study that demonstrated ghost imaging using electrons, in the new method, the random patterns are the fluctuating spike structures of individual XFEL pulses. To do the image reconstruction, they need to repeat the experiment many times – about 100,000 times in their simulations. Each time, the team measured the pulse profile with a diagnostic tool and analyzed the signal emitted by the sample. In a computational process that borrows ideas from machine learning, researchers can then turn these data into a visualization of the X-ray pulse’s effects on the sample.
So far, the new idea has been tested only in simulations and awaits experimental validation, for instance at SLAC’s Linac Coherent Light Source (LCLS) X-ray laser, a DOE Office of Science user facility. Yet, the researchers are already convinced their method could complement conventional pump-probe experiments. According to Ratner, if future tests are successful, the method could strengthen the ability to look at very fundamental processes in XFEL experiments. It would also offer a few advantages that researchers would like to explore. These include more stability, faster image reconstruction, less sample damage and the prospect of doing experiments at faster and faster timescales.
Other co-authors of the paper are SLAC’s TJ Lane and Gennady Stupakov. The project was financially supported by the DOE Office of Science. Click here to view the published paper.