To follow the trail of a terrorist, investigators need to
identify extremely small residues of explosives or biological
warfare agents. The 11 October PRL describes a new method for
detecting molecules in tiny concentrations that is 100 times
more sensitive than current methods and could help those
investigators. Researchers predict that the method–which places
samples on the surface of a silica cube filled with laser
light–may one day be able to spot a single molecule sitting on
the surface of the cube. Spectroscopy is the art of identifying
atoms and molecules by the way they absorb light over a range of
wavelengths. Shining a light beam through a cylinder of gas and
collecting the transmitted energy is the usual way to get its
spectrum, but for samples where only a few of the interesting
molecules are floating around, the light may never interact with
the rare molecules. To find trace concentrations of gas
molecules, spectroscopists use “cavity ring-down spectroscopy,”
where a laser pulse bounces back and