A new approach to surface spectroscopy has proved 100 times more sensitive than current methods and may eventually be capable of identifying a strand of a deadly virus or a single particle of explosive residue. Spectroscopy in all its forms has long been favored as an accurate means of detecting specific atoms or molecules. But when only trace quantities of a substance are present, spectroscopy can be ineffective because the particles of interest may elude the interrogating laser light. In recent years, researchers have made strides with a technique known as cavity ringdown spectroscopy, which involves sending laser pulses into a highly reflective cavity. Because the light reflects back and forth many times, even rare molecules have ample opportunity to absorb it. But there is one major drawback: Cavity ringdown spectroscopy is effective for analyzing molecules in the gas phase but not for detecting those in condensed matter. To extend this technique to solids, a glass plate or liquid cell has to be added to contain the sample within the light path, which introduces a large intrinsic loss and makes measurement much less sensitive. Furthermore, the mirrors used in conventional cavity ringdown spectroscopy have a very limited bandwidth. Andrew Pipino, a chemist at the National Institute of Standards and Technology's Process Measurement Div., has devised a solution. He designed a miniature cavity out of ultrapure fused silica glass. Laser light enters the cube by photon-tunneling, which allows the laser light to circulate internally within the ultrasmooth polished walls of the cavity. Low optical loss Because of the low-loss design of the cavity, a single pulse of light can circulate inside the cube tens of thousands of times before its energy escapes. At each point of reflection, the cavity exhibits evanescent fields that probe the ambient medium. If even a few particles of a trace substance are present, the pulse will display an absorption signature. "From a basic research point of view, we can explore a wide variety of surface reactions and processes that may have been difficult or impossible to explore previously," Pipino said. "Furthermore, many chemical-sensing applications should benefit from the significantly enhanced sensitivity." The immense improvement in sensitivity could open the doors to new fields of research and a host of potential commercial applications, including the detection of explosive residues and biological warfare agents.