Tennis, anyone?
A laser pulse (right) bounces off the plasma wake left by a second pulse (left)--in a process that increases the energy of the first pulse by more than 50.

Sergei Bulanov/Advanced Photon Research Center, Japan Atomic Energy Agency

Serving Up X-ray Laser Pulses

Just as tennis star Roger Federer gives a ball great energy when he smashes it with his racquet, a new method produces intense pulses of x-rays by whacking ordinary laser light with a wave of plasma. The technique might compete with gargantuan x-ray lasers under construction in research labs for a fraction of the cost.

Researchers use x-rays to probe all sorts of materials including living flesh, crystallized proteins, and high-temperature superconductors. Physicists are currently working to develop a completely new type of instrument: an x-ray laser. In a laser beam, the particles of light, or photons, all travel in quantum-mechanical lockstep, so laser light interacts with matter in ways that ordinary "incoherent" light cannot. For example, ordinary x-rays can determine the structure of a protein, but only if many of the molecules are frozen into a crystal. An ultraintense x-ray laser should be able to determine the structure using just a single molecule.

That's the goal, and several big teams are racing toward it, including researchers at the Stanford Linear Accelerator Center in Menlo Park, California; the German accelerator lab DESY in Hamburg; and the Japan Synchrotron Radiation Research Institute in Harima Science Garden City. They are building so-called x-ray free electron lasers (X-FELs) that will generate x-ray laser light by firing particle beams through special arrangements of magnets. The devices cost hundreds of millions of dollars.

However, Sergei Bulanov of the Advanced Photon Research Center at the Japan Atomic Energy Agency in Kyoto and colleagues say they have a prototype that can generate pulses of x-ray laser light on the cheap. The researchers call their technique "relativistic tennis with photons," but a more violent analogy may better convey how it works. Suppose you throw a golf ball at a locomotive that is speeding toward you. The golf ball will bounce off it and come flying back at you with tremendous energy--just before you get run over.

The golf ball is a pulse of ordinary low-energy photons. With a tabletop setup, Bulanov and colleagues create the equivalent of a locomotive by firing a different laser into a cloud of plasma, where it creates a wake that travels at near-light speed. When the photons hit the wake, their energy increases 56-fold. They are also focused into an ultrashort, ultraintense blast by the wake, which is shaped like a miniature radar dish. Bulanov and colleagues dreamt up the technique in 2003 and reported that they've made it work earlier this month on the online preprint server arXiv (www.arxiv.org). "Our work is much more elegant than any other in this field," Bulanov says. The work is still in the early stages, however.

Far more than just elegance is at stake, though, says Marco Borghesi of Queen's University Belfast, Ireland. The technique could produce pulses lasting only a billionth of a billionth of a second, far shorter than the pulses at the X-FELs and short enough to allow researchers to resolve ultrafast changes within molecules.