Ghost Imaging with FELs

Ghost imaging: The randomness of subsequent X-ray pulses from an X-ray laser could be used to study the pulses’ interactions with matter. (Source: G. Stewart, SLAC)

X-ray free-electron lasers (XFELs) produce very powerful beams of light that enable unpre­cedented studies of the ultrafast motions of atoms in matter. 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. This ghost imaging reconstructs what objects look like without ever directly recording their images.

“Instead of trying to make XFEL pulses less random, which is the approach we most often pursue for our experi­ments, we actually want to use randomness in this case,” said James Cryan from the Stanford PULSE Institute. “Our results show that by doing so, we can get around some of the technical challenges associated with the current method for studying X-ray inter­actions with matter.” Scientists commonly look at these inter­actions through pump-probe experiments, in which they send pairs of X-ray pulses through a sample. The first pump pulse rearranges how electrons are distributed in the sample. The second 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. “Pump-probe experi­ments therefore typi­cally require that we first prepare well-defined, short pulses that are less random,” said SLAC’s Daniel Ratner. “In addition we need to control the time delay between them very well.” In the new approach, he said, “We wouldn’t have to worry about any of that. We 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 coworkers 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.

“In our method, the random patterns are the fluc­tuating spike structures of individual XFEL pulses,” said Siqi Li, a graduate student at SLAC author of a previous study that demonstrated ghost imaging using electrons. “To do the image reconstruction, we need to repeat the experiment many times – about 100,000 times in our simu­lations. Each time, we measure the pulse profile with a diagnostic tool and analyze 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. Yet, the researchers are already convinced their method could complement conven­tional pump-probe experiments. “If future tests are successful, the method could strengthen our ability to look at very funda­mental processes in XFEL experiments,” Ratner said. “It would also offer a few advantages that we 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. (Source: SLAC)

Reference: D. Ratner et al.: Pump-Probe Ghost Imaging with SASE FELs, Phys. Rev. X 9, 011045 (2019); DOI: 10.1103/PhysRevX.9.011045

Link: Beam Physics Dept., SLAC National Accelerator Laboratory, Menlo Park, USA

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