ABSORPTION SPECTROSCOPY: Optical cavity opens door to better detectors

Chemists have long used absorption spectroscopy to measure small amounts of a substance, and cavity ring-down spectroscopy (CRDS) has made the technique even more sensitive.

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Chemists have long used absorption spectroscopy to measure small amounts of a substance, and cavity ring-down spectroscopy (CRDS) has made the technique even more sensitive. Using a pair of concave, highly reflective mirrors, researchers form an optical cavity that contains a gas sample. They inject a light pulse, usually from a laser, into the cavity and let it bounce between the mirrors until its intensity decays, or rings down. When gas in the cavity absorbs the light, the decay rate speeds up, so a measurement of the rate of ring-down shows how much of a given molecule is present.

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PHOTO. Light beam enters the ring cavity through prism at left and reflects around the inner surfaces; molecules on the surface that interfere with the evanescent waves (inset) alter the ring-down time.

The problem with traditional CRDS is that it does not work in a general way for other states of matter. Andrew C. R. Pipino of the Chemical Science and Technology Laboratory at the National Institute of Standards and Technology (Gaithersburg, MD) has devised a way to extend cavity ring-down spectroscopy to surfaces and condensed matter. He uses a small ring cavity made of fused silica in the shape of a regular polygon with a convex facet, which provides for total internal reflection of laser pulses injected with photon tunneling through a pair of coupling prisms (see figure). This makes for a long lifetime for the pulses. As the light reflects back and forth against the inner surfaces, evanescent waves project outside the cavity. If a sample molecule is on the surface and absorbs the evanescent wave, the ring-down time decreases.

There have been instances of applying CRDS to samples other than gas, Pipino said, but in those cases the cavities were specifically designed for a particular part of the spectrum. In contrast, total internal reflection creates a broadband mirror. His design could, in theory, be applied from the ultraviolet to the near-infrared, although he has only gone from 440 to 680 nm experimentally. "To go to the mid-infrared (mid-IR), we need new sources, new cavity material, and so forth," he said.

Fluoride glasses, for instance, might make suitable cavities to reflect wavelengths in the mid-IR, and the lasers must be tunable to be useful. There are a lot of strategies that have to be tested to expand the technique, Pipino said.

If they can extend the technique into the mid-IR, where vibrational spectroscopy gives more information, and optimize detection, researchers may eventually be able to detect a single molecule of a chemical species. "That's a very futuristic projection at this point," Pipino said.

Based on current abilities, he projects that he could detect a concentration of approximately 600 water molecules, and even that may turn out to be optimistic. Still, he said, single-molecule detection by absorption spectroscopy has been achieved, and with sufficient research the same might be possible for the ring-down method.

Absorption is an important method for understanding fundamental chemical interactions. The total-internal-reflection technique can measure not only the concentration but also the physical orientation of a chemical species on a surface by measuring the ratio of absorption of in-plane to out-of-plane polarizations. That measurement is significant information because chemical reactivity is affected by orientation.

Pipino said the technique may one day enable the building of detectors for sniffing out explosives or for finding traces of chemical and biological weapons. "Again, a lot has to be done to get to that point," he said. "Hopefully, we'll be able to go to the mid-infrared, but that is probably several years off."

Neil Savage

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