Abstract
A theoretical analysis of the operation of a chemical sensor based on cavity-enhanced optical absorption is given for a system in which the cavity is a dielectric whispering-gallery microresonator. Continuous-wave input is assumed, and the detection sensitivity is characterized in terms of an effective absorption path length. In the case of tunable single-frequency input, it is shown that monitoring analyte-induced changes in the throughput dip depth enables detection with relative sensitivity greater than that of frequency-shift and cavity-ringdown methods. In addition, for the case of broadband input and drop-port output, an analysis applicable to microcavity-enhanced absorbance spectroscopy experiments is given.
Highlights
Dielectric microresonators that support whispering-gallery modes (WGMs) are becoming increasingly useful for numerous applications in optics
The first involves using a tunable single-frequency laser and observing changes in the depth of the throughput dip caused by absorption by the analyte in the evanescent fraction of a WGM
The relative sensitivity was shown to be determined by the intrinsic loss only, in the cases of strong underor overcoupling
Summary
Dielectric microresonators that support whispering-gallery modes (WGMs) are becoming increasingly useful for numerous applications in optics These resonators can be spherical, cylindrical, disk-shaped, or toroidal; light in a WGM circulates around the circumference of the resonator, localized near the surface by total internal reflection. The method described in this paper is more closely related to other earlier work [7] in that what is measured is the analyte absorption effect, for continuous-wave input, on either the depth of the throughput dip (Section 2) or the strength of the drop signal (Section 3) As explained below, the former can be more sensitive than other tunable single-frequency methods, and the latter lends itself well to broadband spectroscopy.
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