Yankee Ingenuity: Dartmouth Physicists Convert A Microcope Into A Free-Electron Laser

The work by Dartmouth physics professor John Walsh and colleagues provides a versatile new tool for probing the largely hidden far infrared spectrum, opening a window into such things as the dynamics in mesoscopic structures -- man-made molecular machines as small as a few hundred atoms -- and the behavior of long-chain molecules such as DNA.

Walsh has been given the 1998 Free Electron Laser Award by his peers, several thousand scientists around the world who work on free-electron physics. The work was reported in the January 19 issue of Physical Review Letters and in the May 8 issue of Science.

Free-electron lasers (FELs) are large-scale laser sources that were developed largely through programs such as the Strategic Defense Initiative and more recently through the Medical Free Electron Laser Initiative, a Department of Defense program intended to broaden the applications of FELs.

When Walsh began working on developing a compact, efficient FEL device about ten years ago, the equipment for generating the million electron-volt beams required by the conventional approach to FEL design could occupy a five-story building. In the version developed at Dartmouth, "the mechanism for producing a coherent beam could fit in the palm of your hand," says Walsh. "It's now a just a matter of time and engineering to adapt power supplies developed for other tasks -- such as flat panel displays -- to produce a device no bigger than a deck of cards.The compact size will allow the probe to be brought to the problem rather than the problem to the probe."

The potential uses of a portable device are enormous. A unit could be used to increase the sensitivity of far infrared telescopes by many orders of magnitude and could do the same for satellite-borne sensors that detect changes in the atmosphere. Says Dartmouth senior researcher Hayden Brownell, "This is rather like producing the PC version of the old room-sized NEAC computers. Putting this kind of technology into more hands is bound to result in all kinds of new applications."

The prototype developed by the Dartmouth team made inexpensive modifications to a scanning electron microscope, enabling it to send a beam of electrons at velocities near the speed of light across a grooved metal plate -- a laser-producing technique called the Smith-Purcell effect. The resulting electromagnetic coupling causes the speeding electrons to emit wavelets of energy in the form of infrared light -- not unlike the way sound waves are produced by running a stick across a picket fence.

Simultaneously sending a dense beam carrying multiple electrons across the grating has a synergistic effect, as the peaks of the waves reinforce each other. The result is a bright, coherent beam of radiation in the far infrared frequency range, a part of the electromagnetic spectrum lying well beyond the wavelength visible to the human eye. Changing the voltage, the angle of the grating or the spacing between the grooves all change the frequency of the radiation that is produced -- allowing the researchers to tune the device to the frequency desired.

"Reaching this kind of a research goal is always a team effort," says Walsh. "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."

Significant members of the research team include John Urata, who was the 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 now at Intel, Inc.; Dartmouth Adjunct Professor and University of Essex Emeritus Professor of Physics Maurice Kimmitt; and Senior Research Associates John Swartz and Hayden Brownell.

Vermont Photonics in Brattleboro, Vt., will produce the compact machine.