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Research Profile

February 2009. Assistant Professor Aaron Lindenberg is an action photographer so extreme that he’d make any sports photojournalist look like a leisurely auteur of still life portraiture. Lindenberg’s subjects are the individual molecules and atoms that vibrate and zip through the hustle and flow of matter as it changes. Their size is so small and their motion so fast that he needs quite simply the fastest and sharpest camera on Earth. He’ll have exactly that when an incredibly powerful x-ray beam known as the Linac Coherent Light Source (LCLS) turns on at the SLAC National Accelerator Laboratory this summer.

The images—and movies—that he captures at SLAC could provide previously unattainable insights about exactly how solar cells convert light to electricity, how computer chips store memory, and how nanoscale materials transform. All of these processes will be observable directly, frame-by-frame, for the first time.

“Up to now, researchers just haven’t been able to see the first steps in these processes— the‘ultrafast’ phenomena that underlie their behavior,” says Lindenberg, who holds a joint appointment in materials science and engineering, and in photon sciences. He is one of a few engineering professors who also have appointments at SLAC, which Stanford runs for the U.S. Department of Energy. “If you can understand them you can learn something about how to then make better devices.”

How fast is ultrafast? The time scale of about 100 quadrillionths of a second. In other words, a standard camera with a shutter speed of 1/250th of a second is 40 billion times slower than the LCLS. Meanwhile the beam will be millions of times more powerful than a medical x-ray, with wavelength fine enough to diffract around individual atoms, hit a detector and produce a blur-free, atomic-resolution snapshot of matter in motion.

Ready for research

This capability to stop action in increments of quadrillionths of a second is not found in other matter-imaging methods, such as x-ray crystallography or electron microscopy. It is the basis of the Stanford PULSE Institute for Ultrafast Energy Science, in which Lindenberg is a leading researcher on ultrafast materials science. He’s ready to get to work on several experiments, including an attempt to find ways to make solar cells more efficient (commercial ones typically squander 75 percent or more of the sun’s energy).

“Light from the sun comes into a solar cell and initially excites electron-hole pairs, and. if you look at the first steps right after the the photon is absorbed, there’s a very, very fast relaxation process in which energy is lost by these carriers,” Lindenberg says. “We lose that as heat. One of the things we’re interested in is trying to understand and control this ultrafast relaxation process and associated structural rearrangements in real devices, like photoelectrochemical cells.

Also in the energy arena, Lindenberg is eager to book some “studio” time for a class of nanoscale “superionic,” materials that, when heated, become a bizarre blend of liquid and crystal that happens to be very good at conducting charged particles. The material may be useful in making more efficient, futuristic batteries.

Lindenberg’s research could also apply directly to information technology. For example, he hopes the LCLS will shed some light on how to engineer an improvement in computer memory made from “ferroelectric” materials. Traditionally, a key property of these materials, called polarization, is switched between two states using an electricfield. One state is a binary 1 of information; the other state is a zero. If this switching could be done with ultrafast pulses of light, it would operate much faster. Today’s computers operate at frequencies of billions of cycles a second. Ultrafast would mean trillions of cycles, or thousands of times faster.

“No one really knows what the fundamental speed limits are for how fast these materials can switch,” he says.

Lindenberg hasn’t been sitting idle for lack of the LCLS. He’s already performed some experiments using its predecessor, the Sub Picosecond Pulse Source (SPPS). In one (pictured above) he was able to capture atomic-scale resolution images of nanoscale nucleation events, in which the surface of a semiconductor begins to boil and bubble upon exposure to an intense source of light.

Behind the scenes

More recently, however, the SPPS has been shut down to build the LCLS, which will harness a good portion of the energy generated by the world’s longest linear accelerator. To generate the potent x-rays, LCLS will greet electrons that have been accelerated down the SLAC beamline at just shy of the speed of light with undulating magnetic fields that make them wiggle. That action will cause them to throw off x-rays that are all in lock step, creating the same kind of coherent light (all in a beam instead of radiating in all directions) that characterizes a laser. The tightly packed x-ray beam, in fact, will fit entirely within a diameter of a tenth of a millionth of a meter.

Th LCLS beam will be sent into one of six large x-ray hutches where it can strike whatever sample has been laid in its path. The ray won’t simply pass through the sample, but will instead be diffracted by all the molecules and atoms within. These diffraction patters will be caught by detectors and assembled into an image.

Although the LCLS shutter speed will be in the quadrillionths of a second (i.e. pulses of x-rays beams will last that long) it will only be able to shoot 120 pulses a second. To make movies of matter in motion, therefore, Lindenberg will have to repeat the same shots several times (separated by miniscule time increments). In a sense, this will be like making a stop-action movie.

Perhaps some might find that work tedious —and given the strength of the x-rays maybe even dangerous—but it’s an opportunity that Lindenberg relishes. Educated as a physicist at Columbia University and then at UC Berkeley, he is excited to straddle the worlds of basic and applied science.

“Materials science is this beautiful intersection between fundamental science, and working on research that has real application in the real world,” he says.

In fact, the images from his research may make some of the fleeting, molecular processes of matter seem more tangible and easier to engineer than ever before.