Oct. 11, 1999 DURHAM, N.C. - The OK-4, a Russian-built machine operating at Duke University, has become the first free-electron laser (FEL) to emit laser light at deep ultraviolet wavelengths shorter than 2,000 angstroms, a region some scientists call "vacuum" ultraviolet.
One of two FELS at Duke's Free-Electron Laser Laboratory, the OK-4 successfully lazed at an extremely short 1,937-angstrom wavelength (one angstrom is about .0000000039 inches long) on Aug. 10, said Vladimir Litvinenko, the lab's associate director for light sources.
"I call it our Y2K problem," Litvinenko said in an interview. "I think it was a psychological barrier for free-electron lasers to reach 2,000 angstroms before the year 2000."
Laser physicists considered 2,000 angstroms a special technical barrier because the mirrors normally used to create FEL light lose much of their reflectivity and also quickly degrade at such short wavelengths, said Litvinenko, who is also a Duke associate professor of physics.
Some conventional lasers, including excimer lasers, also can operate at such short light wavelengths using mirrors made of other materials. But those materials suffer damage from the X-ray radiation created by the magnets FEL use to generate light, so they cannot be used in free-electron lasers.
Seong Hee Park, a graduate student studying for her doctorate under Litvinenko, collaborated with the Lumonics Optics Group in Nepean, Ontario, to develop special new mirrors that can deal with both the X-ray and the ultraviolet radiation.
Ying Wu, a former research scientist at the Duke lab now at Lawrence Berkeley National Laboratory in California, also played a key role in enhancing the quality of the electron beam that powers the OK-4.
That enhancement allowed the amount of light reflecting between the laser's mirrors to be substantially increased relative to the amount that is dissipated - a concept known as gain. "Gain is a measure of the amplification of light," Litvinenko said. "Gain is very important to obtain lazing. What you are fighting against is losses in the optical cavity."
To achieve their 1,937-angstrom milestone wavelength, Litvinenko's Duke group had to increase gain in the OK-4 by "a factor of 2," he said.
He also credited the rest of the Duke lab's 15 member team. "This success would be impossible without the indispensable contribution from the technical staff of a very small but very efficient free-electron laser laboratory," Litvinenko said.
The OK-4 previously became the first FEL to emit laser light in the ultraviolet when it set a previous 2,400-angstrom wavelength record in October 1998 that endured for about eight years. It was then operating at its former home in Novosibirsk, Russia, under a team that included both Litvinenko and Igor Pinayev. Pinayev, another key research scientist now at the Duke lab, modified the OK-4 to allow its use in medical research.
All lasers work by coaxing electrons to emit light, which is then intensified and concentrated by bouncing it between mirrors within a laser "cavity." But FEL are the only kinds of lasers to use electrons that have been separated from their normal captivity within atoms.
Because the electrons empowering FELs are thus "free," the light they emit can be "tuned" to a large range of different wavelengths, a flexibility that is strongly limited when electrons are under the control of atoms.
This tunability makes FELs especially useful research tools because investigators have the freedom to try out various laser wavelengths to learn which works best for a particular need. Medical researchers, for example, might be searching for the best wavelength laser beam to cut into bone or tissue. Materials scientists might want to find the best wavelength beam to act as a probe to study molecules.
Another plus for the OK-4 is its ability to emit laser light either in a continuous beam or in pulses. Each option has its advantages, depending on the experiments that scientists want to use the ultraviolet laser to study. Excimer lasers operating at such short wavelengths can operate only in the pulsed mode and they are not tunable, Litvinenko said.
Ultraviolet light has wavelengths shorter than those humans can see. Those shorter wavelengths are also more energetic, which explains why extreme ultraviolet light can actually damage materials it strikes.
Another special problem of laser light below 2,000 angstroms is that it is absorbed by water vapor in the air. "There is a lot of deep ultraviolet light absorption, especially in North Carolina's humid air," Litvinenko said. As a result, to work successfully the beam must be contained within a vacuum. The OK-4 is empowered by a storage ring capable of operating at 1.1 billion electron volts of energy, though a number of technical problems have prevented the laser from taking full advantage of that power source.
Plans are in place to upgrade those systems and to increase the average power of the OK-4 to several watts, which will make it the most powerful tunable laser in the deep ultraviolet range, he said.
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