Evanescent Wave Cavity Ring-Down Spectroscopy for Ultrasensitive Chemical Detection
A C. Pipino
A new technology will be described which extends the cavity ring-down optical absorption technique to condensed matter by using a miniature, high-finesse, monolithic, total-internal-reflection-ring resonator. Evanescent waves that are generated by total-internal reflection permit input and output coupling by photon tunneling and probe the presence of absorbing species at a cavity facet. The TIR-ring design permits broadband cavity ring-down measurements of adsorbates, thin films, and liquids by eliminating the use of multilayer coatings. The basic sensing concept will first be reviewed by describing recent experiments employing a non-ring prototype in which a totally reflecting element was incorporated in a conventional ring-down cavity. The basic design issues for miniature TIR-ring cavities will then be briefly reviewed along with some numerical results obtained using a wave optics model that show the magnitude of different optical losses as a function of cavity size. A competition between losses results in an optimum size for chemical detection which occurs when the round-trip loss of the empty cavity is minimized. The first experimental results will be presented for a square (7 mm x 7 mm x 5 mm), fused-silica TIR-ring cavity for which the theoretically predicted photon decay time has been achieved.
Evanescent Wave Cavity Ring-Down Spectroscopy for Ultrasensitive Chemical Detection, SPIE series, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=831522
(Accessed March 4, 2024)