The Atomic-Scale Searchlight

Stanford's massive X-ray laser produces extremely fast, bright bursts of light that's giving a fresh look at matter itself.

|
Mar 12 2015, 11:00am

​Image: ​Lars Englert/Max Planck Institute

What would it take to store data faster?

The feeling of a laptop warming one's knees, or a smartphone heating up a pocket, is a familiar one. Just as frequent, but far less cozy, is the frustration of electronic devices operating too slowly. Although we may not always realize it, these experiences are connected, as our devices waste precious energy in the process of writing and reading data.

An international team of physicists is working toward a solution, using a powerful and rare type of laser in northern California. For now, only three like it exist in the world, and beam time at this one is so competitive that groups are only allowed to use it for short periods of time, often less than a week per year.

In the 90s, when most of us were buying our first keychain laser pointers, a handful of science facilities were developing high-power X-ray lasers, such as a tabletop laser at Lawrence Livermore National Lab that could deliver two rapid pulses of X-ray light consecutively: one about a nanosecond in length, and the second just a trillionth of a second long. The goal was to one day use them to observe, better understand, and create entirely new types of matter: anything from the explosive elements of a nuclear weapon to the plastic in a pair of jelly shoes.

Today, the X-ray free-electron laser at Stanford's Linear Accelerator Center (SLAC) in California is more than half a mile long and a billion times brighter than synchrotrons, the previous standard for generating X-rays for experiments. Called the Linac Coherent Light Source (LCLS), its extremely fast, bright bursts of light, with wavelengths about the size of an atom, allow scientists to observe the molecular structure and behavior of matter—things that were, until very recently, much too small and quick to see.

"The brightness is huge—like an atomic-scale searchlight that lets you probe in exquisite detail what's going on in matter," said Mike Dunne, the facility's director.

The LCLS undulator hall. Image: SLAC/Flickr

An X-ray laser is so-named because it is a laser (a beam of amplified light) whose radiation reaches near the X-ray range of the electromagnetic spectrum—you know, right between gamma rays and UV rays.

The system is like a ridiculously sophisticated, high-speed Rube Goldberg machine: First, a standard laser converts electricity to ultraviolet light, which hits a copper plate, releasing a flood of electrons. Those electrons are then bounced back and forth through thousands of specialized magnets placed pole-to-pole, getting repelled and attracted in rapid succession, accelerating faster and faster all the while, until reaching a point where the laws of physics demand that they let off some energy.

"Because of the speeds that these electrons are going, nearly the speed of light, the energy they release will be an X-ray—any slower and we would get UV light or optical light," Dunne explained.

Those X-rays align in a straight beam, and are then delivered to experiments, such as the one led by doctoral researcher Teresa Kubacka last spring that broke new ground for faster data storage.

One common data storage method, the most low-tech example of which is probably a floppy disk, uses electrical signals to magnetize sections of a material. Together, the sections of magnetized material form a binary pattern, which can be decoded into words and images.

"Right now, how the devices are built, this energy transfer is quite inefficient. We can see that it is inefficient because the device heats up," Kubacka said. "Energy is wasted just on heat, making it slower than it possibly could be."

Where today's devices use magnetic fields to help create the pattern, Kubacka's experiment used an electric field. Instead of switching magnetic domains on a strip of material, they aimed to affect the magnetization of individual atoms.

The team shot an ordinary laser—well, an ordinary optical laser fitted with a crystal custom-built to produce terahertz light—at a material sample. The sample, terbium manganite, is in a family of materials called multiferroics whose magnetic and electrical properties are well-connected, making it a good model to test their method.

Light is an oscillating electric and magnetic field, so with each flash of this optical laser, just trillionths of a second long, the material was exposed to both. While the laser's magnetic fields were too weak to influence the magnetic structure of the material, its electric fields were just strong enough to make the atoms' "tiny, tiny magnetic moments" begin to budge, Kubacka said.

Representation of one of Kubacka's experiments. The image shows the magnetic moments of terbium manganite being excited by a excited by a terahertz pulse (red beam) and probed by a pulse from the LCLS X-ray laser (blue beam). Image: ​Teresa Kubacka

All this would have been for naught, though, without the X-ray laser to image what went down. Its pulses, synchronized as much as possible with the optical laser beam, blasted the area holding the material sample with intense light at just the right fraction of a second, immortalizing their scientific proof within the frames of a fuzzy stop-motion movie.

Dunne helped put the timescale of the X-ray laser's pulses into perspective, describing the length of each one, about 100 femtoseconds, in terms of Olympic runners.

"If you think about someone running the 100 meter, it's the time between the guy coming in first—usually Usain Bolt—and the guy coming in second. That's about a hundredth of a second," he said. "If you take that and split it into a million pieces, then split one of those into a million pieces, that's the sort of time we're talking about."

Like a compass dial flipping from north to south, or a zero switching to a one, these magnetic moments must rotate a full 180 degrees to signify meaning. And while that didn't quite happen here—the experiment stopped short of completing a full switch—the observation of some distinct movement was still enough to propel their research forward.

The study shows that electric fields can be used to trigger magnetization, and that, under the right circumstances, faster data storage is physically possible.

Nothing else could have made such an observation: A synchrotron's pulse of light, slightly longer, would have produced a blurred image, or missed the event entirely.

Now that they know that using electric fields for magnetization could work, and at these speeds, the group will continue to perfect the parameters for future experiments, possibly at a planned X-ray free-electron laser facility in Switzerland. They'd like to adjust the experimental conditions—tweak equipment settings, test different material samples—to get faster and more precise results, moving closer and closer to practical applications.

The main benefit of this alternative method in practice would be that, since an electric field can be generated in a small space more easily than a magnetic field can, a storage device could potentially be made smaller. A magnetic field requires a coil and electric current, while an electric field can be generated without current.

The ​Soft X-ray Research (SXR) Instrument for Materials Science, which is the experimental instrument used for a lot of materials research. Image: ​Steve Jurveston

Each magnetization flip could be achieved much more quickly. If the team's experiment is any indication, these switches could happen on the order of picoseconds (trillionths of a second) not nanoseconds (billionths of a second), which is a big deal considering how many switches are involved in storing data for one image or even the period at the end of this sentence.

The main obstacle at this point is that someone still has to develop a material that would be viable at room temperature for these findings to be useful outside the lab. As described, the experiment was done in a super-cooled vacuum chamber with strong electric fields set off by a terahertz laser. Few smartphones survive Wisconsin winters unscathed, let alone functioning in such an environment.

This is another field of study, and a challenge in itself. Manufacturers still have a great deal to learn about the most common, everyday materials. The type of polymer found in packing peanuts and plastic cups, for example, is an impenetrable, tangled knot of many thousands of atoms. Or at least it was until last fall, when another group of scientists at SLAC used the X-ray laser to study this polymer for the first time in limbo between its melting point and solid state, showing a glimmer of what future studies could explore.

Just as exciting for materials science, in a discovery last month SLAC scientists observed two atoms begin to form a weak bond before producing a molecule, a never-before-seen interaction. Faster and brighter than any strobelight, the laser was able to illuminate the atoms for a fraction of a second in this elusive state.

"Chemistry is all about chemical reactions, molecules reacting," said Stanford professor Anders Nilsson, who led the research, "and now we have the technique to look at them. This has implications for how we can understand chemical reactivity and how we can eventually control it."

The type of reaction they saw is at "the core of the chemical industry," he said. "We use it to make everything we know about."

An overhead view of the LCLS. Image: SLAC/Flickr

An X-ray laser might not be what eventually helps scientists find that unknown material needed for faster data storage: one that can behave similarly to the terbium manganite in Kubacka's experiment, but do it at room temperature in an ordinary device. No current research projects at SLAC have that specific focus, and it is more likely that many layers of different materials will fill that role. But having a tool that can examine materials on this level, and plans for more X-ray lasers underway, are positive steps in that direction.

In the future, physicists might use an even newer method called laser plasma acceleration to power X-ray lasers, making them much smaller—potentially just centimeters long. At Lawrence Berkeley National Lab, researchers have already used plasma acceleration to ramp up electrons' energy to record-breaking levels. Over the next few decades, Dunne said, facilities like these will work to get this technology to the level of precision and control that are necessary to do these types of experiments.

When that day comes, the X-ray free-electron laser as we know it will go the way of the synchrotron, and other technologies that came before it: updated and revamped until it inevitably becomes outmoded.

The next big thing, or the next, might be able to put an end to data storage inefficiency. Until then, we can continue to groan about our sluggish devices. But we should simultaneously marvel at these machines, built by humans, that could one day make them even better.

This story is part of The Building Blocks of Everything, a series of science and technology stories on the theme of materials. Check out more here.