It looks like an X-ray laser…
…but actually isn't one. Two teams of scientists have succeeded in generating coherent light in the X-ray range with relatively little effort. With their technology, medium-sized laboratories will soon be able to examine living cells with X-rays - with a temporal resolution that has so far been denied even to large institutes. Two teams of researchers were able to create laser-like beams of light that have much shorter wavelengths than traditional laser beams. The results were presented by a group from the University of Michigan in the Physical Review Letters and by a group from the Technical University of Vienna in Science. They show that X-rays with a wavelength short enough to image living cells can be generated with a table-top device. This typically requires huge, multi-million dollar facilities. "This is a whole new and very exciting technique," says physicist Roger Falcone of the University of California, Berkeley.
The method is called high harmonic generation. Although technically not a laser, it mimics a laser surprisingly well. "You wouldn't realize that this isn't an X-ray laser," says Margaret Murnane, a physicist at the University of Michigan, Ann Arbor. The X-rays were generated by firing ultra-short shots from infrared lasers at a small beam of helium gas. Like light in general, laser light consists of rapidly oscillating electric and magnetic fields. In this case, the electric field is so large and the laser flash so short that it can extract an electron from a helium atom and force it back again in a single oscillation. Through this violent reunion with its parent atom, the electron emits a high-energy (X-ray) photon. Since numerous neighboring atoms in the gas are hit simultaneously by the oscillating light of the infrared laser, the X-rays emitted by the gas are coherent, i.e. they oscillate in step. The result is a laser-like beam.
Both teams credit their success to improved laser technology. According to Ferenc Krausz, leader of the team at the Technical University of Vienna, his group produced X-rays with a wavelength of 2.4 nanometers (nm) by truncating the laser pulses to 5 femtoseconds. (A femtosecond is one millionth of a billionth of a second.) The Michigan group used pulses lasting 26 femtoseconds to produce X-rays with a wavelength of 2.7 nm. "In terms of physics, there is nothing fundamentally new in these two experiments," adds Krausz.
However, the consequences could still be far-reaching. X-rays with a wavelength of less than 4.4 nm can be used to image living cells, since carbon then absorbs more than water. Generating coherent X-rays in this spectral range is "the Holy Grail of soft X-ray research," says physicist Neal Burnett of the University of Alberta in Canada, co-author of the article in Science. Until now, only large institutes like the Lawrence Livermore National Lab in California have been able to image living cells with X-rays. This lab contains an X-ray laser that can only fire a pulse every 20 minutes. Burnett believes that the new benchtop devices may one day enable testing in many more labs, at lower cost and with more acquisitions per time.
