SPECTROSCOPY

If you have ever been reluctant to get rid of obsolete laboratory equipment, you are not alone. Many a back room is filled with once-impressive instruments that have long since lost their worth. But the impulse to hold onto what you already have may be the right one, at least if the experience of a group of Dartmouth College (Hanover, NH) researchers is any indication. By doing inexpensive modifications to an old scanning electron microscope (SEM), they have come up with a versatile new light so

SPECTROSCOPY

Old electron microscope generates infrared light

John Wallace

If you have ever been reluctant to get rid of obsolete laboratory equipment, you are not alone. Many a back room is filled with once-impressive instruments that have long since lost their worth. But the impulse to hold onto what you already have may be the right one, at least if the experience of a group of Dartmouth College (Hanover, NH) researchers is any indication. By doing inexpensive modifications to an old scanning electron microscope (SEM), they have come up with a versatile new light source for far-infrared (FIR) spectroscopy.

The FIR band spans wavelengths of 10 to 1000 µm and is sometimes termed terahert¥radiation. Its uses for spectroscopy are manifold, for contained within it are rotational and vibrational spectra of atoms as well as inorganic and organic molecules. As just one example, large biological molecules such as DNA have resonances at terahert¥frequencies. But although bright, coherent FIR sources exist for spectroscopy, they emit only at certain frequencies (optically pumped lasers) or are not widely available (synchrotron radiation). Incoherent FIR sources lack brightness.

The new light source, devised by Dartmouth physics professor John Walsh and his colleagues, has at its heart a metal grating over which an electron beam is passed, with the electrons being produced by a set of SEM optics (see figure). The electrons, along with their positive image charges at the surface of the metal, form oscillating dipoles that emit light. By changing the angle of the grating or the voltage of the electron beam, or by choosing a different grating order, the wavelength can be tuned from 200 to beyond 1000 µm. "And it`s all coherent light," says Walsh.

Although this combination of grating and e-beam, called a Smith-Purcell radiation source, is not new in itself, all Smith-Purcell sources until now have been incoherent emitters. It is the high current density, Walsh says, that makes this source coherent. Influenced by the proximity of the grating, the dense e-beam bunches periodically--with a period equal to that of the grating-- initiating distributed feedback of the beam. This leads to beam modulation, which leads to increased field strength and then to further beam modulation. Ideally, this synergistic effect leads to the electrons being "fully bunched," that is, having a modulation of 100%, although Walsh says this state has not yet been reached in their device. Despite the 20-µm size of the e-beam spot itself, the grating, which is 1 ¥ 1 cm in size and can have a period of anywhere from 100 to 300 µm, is excited across its entire surface and emits light as a whole. The result, Walsh says, is a "micro free-electron laser."

The output is a narrow, highly polarized beam of close to TEM00 quality and about a microwatt in power. While the researchers have not yet fully determined the bandwidth of the source, they have found that it is temporally coherent enough to produce interferograms. By dithering the electron-beam position slightly, they can modulate the FIR output intensity, an effect useful for certain means of detection such as lock-in amplifiers or pyroelectric detectors.

The parts are simple, too. "The grooves in the grating were made with a saw," says Walsh.

A bright, widely tunable FIR source would be a boon to spectroscopists, according to Michael Mross, president of Vermont Photonics Inc. (Brattleboro, VT), a company aiming to commercialize the Dartmouth light source. "Although the amount of research using FIR now is not large, its growth is exponential," he says. "And there`s not a whole lot of technology yet in the terahert¥band. We in the FIR field are where microwave researchers were before good sources were developed."

Walsh echoes this sentiment. "No FIR source being used now for spectroscopy has the right combination of simplicity, brightness, and wavelength range to be a general source," he says.

With this in mind, the Dartmouth group is pursuing improvements to its device. One of Walsh`s near-term goals is to boost the power to a milliwatt, a level more in line with the power a standard spectroscopic instrument requires. Walsh also wants to extend the wavelength range down to 100 µm. "Someday," he says, "this technology could reach the 1- to 10-µm range."

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