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 spectroscopy of surfaces, thin f ilms, liquids, and, potentially, solids, is explored using a wave-opti cs model. The novel spectrometer design incorporates a monolithic, tot al-internal-reflection-ring cavity of regular polygonal geometry with at least one convex facet to induce stability. Evanescent waves genera ted by total-internal reflection probe absorption by matter in the vic inity of the cavity. Optical radiation enters or exits the resonator b y photon tunneling, which permits precise control of input and output coupling. The broadband nature of total-internal reflection circumvent s the narrow bandwidth restriction imposed by dielectric mirrors in co nventional gas-phase cavity ring-down spectroscopy. Following a genera l discussion of design criteria, calculations are presented for square and octagonal cavity geometries that quantify intrinsic losses and re veal an optimal cavity size for each geometry. Calculated absorption s pectra for the NO3 radical from 450 to 750 nm in a nitric acid solutio n an presented to demonstrate bandwidth and sensitivity.