Credit: Adapted from N. Rohringer et al., Nature, Vol. 481 (26 January 2012)
Two years ago, physicists fired up the world's first laser to shine out hard x-rays—the high-energy, short-wavelength particles of light needed to probe atomic-scale structure. Shining 10 billion times brighter than any previous x-ray source, the Linac Coherent Light Source (LCLS) can determine the structure of crystals from samples a few nanometers across and probe changes in materials that take place in a millionth of a nanosecond. But the $410 million LCLS doesn't look anything like a laser pointer, as it relies on a 3-kilometer-long particle accelerator to generate x-rays. Now, physicists have made a much smaller x-ray laser that works much more like the conventional one you might carry around in your pocket.
The new atomic x-ray laser won't replace the LCLS and other accelerator-based systems. In fact, to make the atomic laser work, researchers blasted neon atoms with x-rays from the LCLS itself. Still, the results mark a conceptual triumph, fulfilling a 45-year-old prediction that such an atomic x-ray laser is possible. "Nobody had done this before, and everybody knew that somebody had to go out and do this," says Philip Bucksbaum, director of SLAC's PULSE Institute for Ultrafast Energy Science in Menlo Park, California, who was not involved in the work. "So this is great."
In a conventional laser, atoms in, say, a gas sit between two mirrors, one only partially reflective. The electrons in an atom can occupy cloudlike quantum states of only certain energies, and an electron that has been "excited" in some way from a lower-energy state to a higher-energy one can emit radiation of a definite wavelength as it returns to its original state. That light induces other excited atoms to radiate photons in the same direction as the original and in quantum lockstep with one another—the hallmark of laser light. The result of such "stimulated emission" is a tsunami of light that shines through the partially reflective mirror.
Until now, however, that scheme hasn't worked for generating x-ray laser light. It requires simultaneously exciting many atoms to very high-energy, very short-lived states. That means applying a staggering amount of power per unit area to the sample. So the LCLS relies on a different scheme. Physicists fire high-energy electrons through a train of magnets called undulators, which make the electrons wiggle back and forth and radiate x-rays. The x-rays then travel along with the electrons and push them into bunches that radiate far more efficiently than individual electrons. Thanks to that feedback, a hugely intense burst of x-ray laser light emerges.
Ironically, that powerful pulse is just the thing for generating x-ray laser light from atoms, too, report Nina Rohringer of the Max Planck Advanced Study Group in Munich, Germany, Jorge Rocca of Colorado State University in Fort Collins, and colleagues. They shined pulses from the LCLS, which deliver up to 200 billion megawatts for a few millionths of a nanosecond, onto neon gas. The x-rays would rip the most tightly bound electron out of an atom, leaving hosts of atoms in highly energetic states. An atom could lose its energy when another of its electrons fell into the vacant spot and emitted an x-ray. Through stimulated emission, that would cause other atoms to emit x-rays and create the laser beam, the researchers report today in Nature.
Physicists dreamed up the basic scheme in 1967. But try as they might, experimenters (at least those in civilian labs) never had enough power to push it into the x-ray regime, says Roger Falcone, a physicist at the University of California, Berkeley. Rohringer says she was excited to see the scheme work. "We were jumping up and down and shouting," she says. "I was excited for days."
So what's the classical atomic x-ray laser good for? Compared with the LCLS's beam, the beam from the atoms has a more precisely defined wavelength and better synchronization among the photons. So it might be used for precision spectroscopy and other applications, Rohringer says. However, researchers are working on other ways to stabilize the LCLS's beam, Falcone notes. The atomic laser allows researchers to generate two x-ray pulses of different wavelengths, which could be used to probe materials simultaneously, Rohringer says.
An atomic x-ray laser may have been realized in the 1980s under different circumstances. As part of President Ronald Reagan's Strategic Defense Initiative, researchers in the United States tried to develop ultrahigh-power x-ray lasers to shoot down nuclear missiles, using underground nuclear explosions to excite atoms. They may have succeeded, but the details are likely classified, Bucksbaum says. "I think this was done, but I don't think much is known about it," he says. "It wouldn't have made a very good scientific instrument."
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