Transmuting Light Into X-rays

Despite advances in laser technology, one dream remains elusive: a table-top instrument that can pump out a beam of high-intensity, coherent x-rays. Such a device would give researchers Superman-like eyes to peer into living cells or catch glimpses of molecules during a chemical reaction. Now researchers have the beginnings of such a tool. A report in today's Science describes a trick for boosting visible laser light efficiently into the soft x-ray range.

When a laser plows into gas, it can occasionally rip an electron away from an atom before slamming it back into orbit. This violent reassembly produces a photon that can pack several hundred times the energy of an ordinary photon in the beam. This harmonic conversion, as it's called, can generate photons in the x-ray range. But the process is horribly inefficient because the gas slows down the laser light much more than it does the x-rays. The waves of x-rays emerge out of synch with each other and "tend to cancel each other out," says University of Michigan physicist Margaret Murnane.

Murnane and colleagues found a way to equalize the speed of the laser light and the x-rays in the gas. First they put argon in a precisely machined glass tube that speeds up the laser light but doesn't disturb the x-rays, which pass right down the center. Then the team adjusted the pressure of the gas, which fine-tunes the velocity of the two beams, until they marched along at the same pace.

To test their device, the team blasted the gas with light from a titanium-doped sapphire laser. At a "magic pressure of 30 torr," says physicist Andrew Rundquist, x-rays shot out of the 3-centimeter-long tube with a surprisingly high intensity. "At two o'clock in the morning I was yelling and screaming in the lab," he recalls. Roughly one in 100,000 photons made the jump to x-rays, at least 100 times the proportion seen without the glass tube, he says.

"This will be a wonderful new tool," says Ken Kulander, a physicist at Lawrence Livermore National Laboratory in California, who hopes to use it to zap molecules and watch how they fall apart. But Christian Spielmann, a physicist at the University of California, San Diego, says the x-rays aren't yet at a high enough energy to cut through water and see into cells. "This is a first step, but there are many more steps to go," he says.

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