Evanescent wave cavity ring-down spectroscopy for trace water detection

We explore the use of evanescent wave cavity ring-down spectroscopy (EW-CRDS) for water detection through a signal-to-noise ratio analysis. Cavity ring-down spectroscopy (CRDS) is an emerging optical absorption technique that employs the mean photon decay time of a high-finesse optical cavity as the absorption-sensitive observable. EW-CRDS is a novel implementation of CRDS that extends the technique to surfaces, films, and liquids by employing optical cavities which incorporate at least one total-internal-reflection (TIR) mirror. The concomitant evanescent wave is then used to probe the absorption of an ambient medium at the TIR surface also through a change in the photon decay time. By employing miniature monolithic cavities with ultra-smooth surfaces that are fabricated from ultra-high transmission materials, extreme sub-monolayer detection sensitivity is readily achieved. The detection of water by EW-CRDS with a fused-silica resonator provides an interesting and important application, since the nascent hydroxylated SiO2 surface is expected to show a high natural affinity for adsorption of water through hydrogen-bonding interactions. Furthermore, in the 1380 nm spectral region where water absorbs strongly, low-OH-content fused silica has extremely high bulk transmission. These factors potentially provide the basis for a novel water sensor.