Acr. Pipino et al., EVANESCENT-WAVE CAVITY RING-DOWN SPECTROSCOPY WITH A TOTAL-INTERNAL-REFLECTION MINICAVITY, Review of scientific instruments, 68(8), 1997, pp. 2978-2989
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.