New sensor for detecting trace gases. Practitioners in many fields, including environmental science, homeland security, and health care, are keenly interested in identifying exquisitely tiny amounts of certain gases in, for example, the atmosphere or one’s breath. Laser-based techniques are useful because they can selectively probe spectral lines with great sensitivity. In the mid-IR, where many molecules have a multitude of lines, the quantum cascade laser is fast becoming the instrument of choice. (For more on QCLs, see the article in PHYSICS TODAY, May 2002, page 34.) Typically, an optical cavity is loaded with the gas at millibar pressure and the laser light is finely tuned to resonate with both a single cavity mode and a spectral line of interest. In a new twist, Gottipaty Rao and Andreas Karpf (Adelphi University, New York) use gases at atmospheric pressure, combining so-called off-axis cavity enhanced spectroscopy with multiple-line integrated absorption spectroscopy, which...

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