June 4, 1998 ANN ARBOR---Researchers at the University of Michigan's Center for Ultrafast Optical Sciences have built the first table-top laser capable of generating a coherent beam of X-rays.
The work could give chemists a close-up view of the dynamics of atoms during reactions with other atoms, and open a real-time window for biologists onto microscopic events at the cellular level. It appears in the May 29 issue of the journal Science, along with an accompanying news article.
By shooting a rapidly pulsing laser through a hollow glass tube filled with gas and controlling the pressure of that gas, the team---including Andy Rundquist, Charles Durfee, and electrical engineering professors Henry Kapteyn and Margaret Murnane and colleagues---were able to generate a focused beam of X-rays that could be incorporated into a device for atomic-scale imaging.
Although it has long been possible to generate X-rays with lasers, this is the first time scientists have been able to dramatically increase their efficiency to make them useful for applications, such as imaging, Murnane said. Moreover, whereas traditional lasers emit visible and near-infrared light (with wavelengths in the 500-1000 nanometer range), those from the U-M device are about 20nm, with the possibility of being as short as 2nm. The shorter the wavelength, the higher the spatial resolution of the beam.
Another benefit of this new device is that the X-ray pulse duration is extremely short, enabling high temporal resolution imaging as well. In other words, the X-rays can be used as the world's fastest strobe light, making anything moving slower appear to be frozen in time.
The U-M device consists of a hollow glass tube filled with gas, sandwiched between a laser source and a detector. When the intense light passes through the gas, electrons---the negatively charged particles swarming around atoms---are pulled away from the atoms, then slammed back when the field reverses direction. The electron then can give off its energy in the form of an X-ray photon, the particle form of X-ray light. This X-ray light can be amplified by 100-1,000 times by propagating through the hollow fiber at the same speed as the laser.
A major hurdle for earlier attempts at such a setup was that the X-rays tended to cancel each other out, much like overlapping water waves neutralize each other if their peaks and troughs coincide. The canceled waves produce very dim, diffuse X-ray patterns on the detector. But by adjusting the pressure of the gas in the fiber optic to fine-tune the laser's speed, the U-M scientists managed to keep the X-ray peaks properly matched, so that the beam amplified itself and remained "in phase" with the laser as it passed through the tube.
Kapteyn said the most exciting part of the work was that it demonstrated that any researcher with a short-pulse laser could use this technique to improve their view of chemical and biological processes.
The Center for Ultrafast Optical Sciences is one of the world's premier research facilities for high-speed lasers. U-M engineers and physicists at CUOS are working on some of the most advanced laser technologies available today, including femtosecond lasers, which generate the shortest, most intense bursts of energy yet produced.
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