Like a Laser, Only Better

Two research teams have created laserlike beams of light at wavelengths shorter than any laser. The results, reported in last week's Physical Review Letters (PRL) by a Michigan group and in the current issue of Science by an Austrian group, demonstrate that x-ray wavelengths short enough for imaging living cells can be generated with a table-top machine, although large, multimillion-dollar facilities are normally needed. "This is a very new technique, and it's very exciting," says physicist Roger Falcone of the University of California, Berkeley.

The method is called high harmonic generation, and even though it isn't technically a laser, it does a good job of imitating one. "You wouldn't know that there wasn't an x-ray laser there," says Margaret Murnane, a University of Michigan, Ann Arbor, physicist who co-authored the PRL paper. To make their x-ray beam, the team aimed ultrashort bursts of infrared laser light at a small jet of helium gas. Like all light, the laser light consists of rapidly oscillating electric and magnetic fields. In this case, the laser's electric field is so intense, and the pulse so brief, that it can strip an electron from a helium atom and slam it back in a single oscillation cycle. In its violent reunion with its parent atom, the electron emits a high-energy (x-ray) photon. Because many nearby atoms in the gas are hit by the oscillating laser beam at the same time, the emitted x-rays are coherent--they oscillate in step--and emerge as a laserlike beam.

Both teams credit their success to better "pump" laser technology. Ferenc Krausz, leader of the team from the Vienna Technical University in Austria, says his group achieved 2.4 nanometer (nm) x-rays by shortening their laser pulses to 5 femtoseconds. (A femtosecond is a millionth of a billionth of a second.) The Michigan group used 26-femtosecond pulses to get 2.7 nm x-rays. "Regarding the physics, there is nothing fundamentally new in these two experiments," Krausz notes.

But the payoff could be big. Generating coherent x-rays with a wavelength shorter than 4.4 nm--in the so-called water window, where carbon absorbs better than water--is "the Holy Grail in coherent soft x-ray research," says physicist Neal Burnett, of the University of Alberta in Canada, a co-author of the Science paper, because it allows imaging of living cells. Such imaging has only been done at large facilities, such as Lawrence Livermore National Lab in California, which operates an x-ray laser capable of firing only one pulse every 20 minutes or more. These new table-top systems may some day allow such investigations in many more labs, at lower cost, and with higher pulse frequency, says Burnett.