Evanescent Wave Cavity Ring-down Spectroscopy with a Total-internal-reflection Minicavity

A miniature-cavity realization of the cavity ring-down concept, which permits extension of the technique to condensed matter spectroscopy, is elucidated using a wave-optics model. The novel spectrometer design incorporates a monolithic, total-internal-reflection-ring cavity of regular polygonal geometry with at least one convex facet to induce stability. Evanescent waves generated by total reflection probe absorption by matter in the vicinity of the cavity. Optical radiation enters or exits the resonator by photon tunneling, which permits precise control of input and output coupling. The broadband nature of total-internal-reflection circumvents the narrow bandwidth restriction imposed by dielectric mirrors in conventional gas-phase cavity ring-down spectroscopy. Through optimization of the cavity design, both high-sensitivity and substantial dynamic range can be achieved. Following a general discussion of design criteria, calculations of relevant resonator properties for specific cavity geometries are presented. Absorption spectra for the NO₃ radical from 450 to 750 nm in a nitric acid solution are calculated to demonstrate bandwidth and sensitivity.

*National Research Council postdoctoral associate/NIST