Article | November 19, 1998

Tabletop Free Electron Laser Tunes Across Far IR

Modified SEM creates free-electron laser for applications in far infrared spectroscopy and materials research.

By: Laura Vandendorpe, Laboratory Networks Online

Although the mid- and far-infrared spectral regions hold promise for new spectroscopic applications in fields ranging from semiconductors to materials to biology, scientists have so far lacked a practical, tunable infrared laser source to access these potential opportunities. Patience paid off this year. After 25 years of research in the far-infrared spectrum, John Walsh, the Frances and Mildred Sears professor of physics at Dartmouth College (Hanover, NH), discovered the solution. By revamping a scanning electron microscope (SEM), Walsh has created a tabletop free-electron laser (FEL) that can efficiently produce a tunable beam of infrared light.

Walsh's instrument provides researchers with a robust far-infrared, or terahertz, source. His instrument wasn't the first on the block, but it is the most practical. Previous infrared sources filled entire rooms; Walsh's device is roughly the size of a bass drum (see Figure 1).

Figure 1: Dartmouth Professor John Walsh (right) created the prototype FEL by modifying a SEM from the lab next door. Hayden Brownell, a research associate, was a member of the research team that made this achievement possible.

"In comparison with most other free-electron lasers, our device is very, very small," Walsh says. "In our FEL, the mechanism for producing a coherent beam could eventually fit in the palm of your hand." The next step, Walsh says, is to adapt power supplies from other applications to work with the device. By creating a portable infrared source, researchers will be able to expand their analysis capabilities by many orders of magnitude, he says.

How it works
To create the FEL, Walsh and his team modified a SEM so that it shoots electrons at velocities near the speed of light across a grooved metal plate. The resulting electromagnetic coupling causes the speeding electrons to emit wavelets of energy in the form of infrared light.

Because multiple electrons cross the grating simultaneously, the peaks of the waves reinforce each other, producing a bright, coherent beam of radiation in the far-infrared frequency range. Changing either the voltage, the angle of the grating, or the spacing between the grooves alters the frequency of the radiation that is produced, allowing researchers to tune the device as desired.

Figure 2: Inside the adapted SEM, the Smith-Purcell generates a tunable infrared output beam.

Potential applications
The tabletop FEL could potentially impact many types of applications. For example, researchers have already begun conducting primitive far-infrared spectroscopy experiments of weakly-bonded systems, yielding interesting data for developing new types of materials. The FEL would greatly enhance this research, helping to develop new varieties of plastics and liquids. The data acquired could expand scientific knowledge of carbon structures like buckyballs.

"The device is a source of coherent radiation," Walsh says. "Semiconductor oscillators work well at longer wavelengths. At the opposite end are tunable lasers that work well at small wavelengths. The FEL picks up in between these two ranges, particularly in the spectral region from about 10 µm to 1000 µm. When you have a laser that works in a new region of the spectrum, it's difficult to predict all of the new applications that may be found."

Walsh's FEL could advance research into mesoscopic structures, as well as studies of the behavior of long-chain molecules such as DNA. The far-infrared spectrum can also be used to investigate the coiling of proteins and other components of biological systems.

A growing field
Although far-infrared spectroscopy represents a small niche in the research world, the technique continues to gather proponents. "It's clear there is more and more interest every year in this field," says Michael Mross, president of Vermont Photonics Inc. (Brattleboro, VT). For example, the 1999 Photonics West conference (San Jose, January 25-29) will feature a session devoted to terahertz spectroscopy.

Mross has a vested interest in making sure that interest in the infrared spectrum continues to grow in the research community. For more than 10 years, Vermont Photonics has funded portions of Walsh's work, and has now licensed the rights to the FEL technology. Intrigued by the potential of the infrared spectrum when he was Walsh's student at Dartmouth, Mross continued to follow the professor's work after completing his studies at the university. Over a dozen years ago, Mross and his colleague Tom Lowell launched Vermont Photonics to provide a forum for rapid commercialization of cutting-edge infrared developments. To make the business viable, Vermont Photonics is also the U.S. distributor for Müoller-Wedel (Hamburg, Germany), a developer and manufacturer of optical test equipment.

"If you look at how technology is involved in each part of the electromagnetic spectrum, there is a big gap when you come to the far infrared," Mross says. "It's a big opportunity. There are other people who are trying to develop things, but Walsh's method is much simpler."

Commercialization
Although it is simpler, it still took developers at Vermont Photonics' manufacturing center several months to create the second FEL prototype, even though they were working from Walsh's blueprint. Engineers are now streamlining the design so that it can be mass-produced. While Walsh has plans to test the FEL on applications within the Dartmouth community, Mross is planning for a larger, more diverse pool of users.

"Within a year, we expect that researchers will begin employing it in their applications," Mross says. "Once you have something people can use, the step to going commercial is very short—no point in waiting around. We will probably exhibit it at Pittcon 2000."

The initial market is very small, though. Because tunable lasers in this range have not been available, research will commence at the fundamental level, with most buyers coming from government and university laboratories. In addition to funding from Vermont Photonics, Walsh's original work was funded in part by the Army Research Office of the U.S. Department of Defense. The army is interested in FELs because of their implications for military communications and atmospheric research.

Walsh's FEL has already made a significant impact on the several thousand researchers who develop free-electron lasers around the world. For his achievement, Walsh was awarded the 1998 Free Electron Laser Award by his colleagues, an annual award that recognizes the top FEL researcher in the world.

"Reaching this kind of a research goal is always a team effort," Walsh says. "We have been engaged in free-electron laser research for nearly 25 years and many tens of students, research staff and scientific visitors have left their marks." Dartmouth has been awarded three U.S. patents for the device.

Acknowledgements
Significant members of the research team include John Urata, who was the graduate student who first observed that the device had succeeded in producing a coherent beam, and Michael Goldstein, a doctoral student who first observed spontaneous emission from the device; both are now with Intel. Dartmouth adjunct professor and University of Essex emeritus professor of physics Maurice Kimmitt and senior research associates John Swartz and Hayden Brownell were also involved in the research.

About the author…
Laura Vandendorpe is managing editor of Laboratory Network, a VerticalNet online community. She can be reached at 2506 N. Clark St. P.O. Box 128, Chicago, IL 60614. Phone: 773-880-0242; fax: 773-880-0252; e-mail: lvandendorpe@laboratorynetwork.com.