People: Steve Durbin's Speedy Detector
Synchrotron physicists are missing important scenes from their favorite film—the lives of atoms. "In every physicist's imagination you have this mental picture of how things move, like you're watching a movie," said Purdue University X-ray scientist Steve Durbin. Durbin is spending a six-month sabbatical at SLAC and Stanford's PULSE Institute for Ultrafast Energy Science. He's developing ideas for detectors that could make imaging methods at synchrotron sources a hundred times faster.
To make an atomic movie, you'd need to snap each frame with a shutter speed of at least a trillionth of a second, or picosecond. With that time resolution it would be possible to watch the energy of an incoming photon excite the electrons and then transfer to the atoms of a system, for example, causing movement that leads to a molecule breaking up or forming. But so far, such movies have existed only in physicists' imaginations.
"That timeframe has been totally inaccessible to the vast majority of X-ray synchrotron experiments in the past," Durbin said. "Most X-ray beams come out of a synchrotron in pulses of 100 picoseconds, which to a layman sounds incredibly fast, but this is still 100 times too slow to get to these fundamental atomic lifetimes." Even in the seemingly lightning-fast pulse from a synchrotron, the fine details of these electronic dynamics are blurred.
"The Linac Coherent Light Source and other next generation machines will open up that frontier," Durbin said. "The one drawback is that it will have a very limited number of experimental stations, typically one experiment going on at a time. Maybe in the long run it'll be two, or four experiments. So this will always be a very special, hard to get resource."
That's why Durbin has been trying to find a way to bring atomic movies to synchrotron sources, which can support dozens of users at a time. He is working on an experimental set up that could be used to access information about time-dependent sample behavior. First, an optical laser blasts the sample, setting off a series of events that researchers would like to watch. Then a 100 picosecond-long pulse of synchrotron X-rays reflects off the sample, carrying the signature of those events. If Durbin were to catch the distorted X-ray pulse with a normal detector, the detailed bumps and wiggles in this signature would bleed together into a blot.
To solve this problem, Durbin went to the world of ultrafast optical lasers, where he found a device called an Auston switch. Made of a semiconductor material, the switch passes an extremely short-lived current when a photon hits it. This brevity is a good thing; the current closely tracks the incoming light. To make the switch's detection time extremely short, it's triggered by an ultrafast optical laser, which, unlike a synchrotron pulse, can operate on picosecond time scales. That means the switch is only active for the fleeting fraction of a picosecond that the laser pulse lasts. This results in the ability to take data relating to just a tiny sliver of time.
Durbin's proposal incorporates an Auston switch in an ultrafast detector that flashes on for a picosecond, grabbing a single bump or wiggle of the incoming X-ray pulse. To get the full signature, it's just a waiting game—the synchrotron source continues to fire X-ray pulses at the sample, each time initiating the same electronic dynamics. Durbin changes the "on" time of his detector by just a little to grab a different piece of the signature each time. By stringing these observations together, like composing still frames into a movie, he can recreate the process taking place in the sample.
Such experiments currently conducted at synchrotrons can observe only changes that are visible from one 100 picosecond chunk of time to another, the duration of the X-ray pulse. Durbin's detector would capture how a sample distorts, picosecond by picosecond. "This would be the first situation where you're actually trying to find out what's going on during the pulse," Durbin said. "The reason we can't do that experiment now is that we don't have good enough detectors. That's something that's always been true in X-ray science, the synchrotron business, we've always been detector-limited."
Durbin said he sees his idea as a small complement to the LCLS's superior time-resolution and brighter beam. "What I'm doing is a very minor contribution to field of X-ray physics," Durbin said. "But if it turns out to be a viable detector concept we might be able to do experiments at synchrotrons that would support LCLS or the other way around, that would build upon results from LCLS."
As a faculty member at Purdue, Durbin uses the Advanced Photon Source at Argonne National Lab in his synchrotron X-ray research. When he started looking for a sabbatical destination, he said, SLAC was an obvious choice. "There is an amazing concentration of talent in ultrafast science here. I can toss my ideas around with people down the hall and give little seminars and learn about applications I hadn't thought about before, and find pitfalls I didn't realize, that I can then try to solve."
For example, responses to a seminar he gave recently have got him wondering whether, instead of taking a snippet of each X-ray pulse that comes along, he could design a detector that slices the entire pulse into picosecond-long data segments in one go. This would be incredibly useful to researchers working with samples that are destroyed by a single X-ray pulse from the LCLS.
"I've gotten inspiration just from having conversations here with people that have gotten me thinking deeply about helping to solve detector issues that are directly related to the LCLS, even though when I came here I thought that I was working on a detector that would be useful everywhere but here," Durbin said. "But in fact this is what happens when you have the right mix of people together."