We explore the use of evanescent wave cavity ring-down spectroscopy (BW-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 ofa 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 ofan 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 Si02 surface is expected to show a high natural affinity for adsorption ofwater through hydrogen-bonding interactions. Furthermore, in the 13 80 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.
A new optical technique is described that permits extension of cavity ring-down spectroscopy (CRDS) to surfaces, films, and liquids. As in conventional CRDS, the photon intensity decay time in a low loss optical cavity is utilized to probe optical absorption. Extension to condensed matter is achieved by employing intra-cavity total internal reflection (TIR) to generate an evanescent wave that is especially well suited for thin film chemical sensing. Tow general monolithic cavity designs are discussed: (1) a broadband, TIR-ring cavity that employs photon tunneling to excite and monitor cavity modes, and (2) a narrow bandwidth cavity that utilizes a combination of TIR and highly reflective coatings. Following a qualitative description of design features, a beam transfer matrix analysis is given which yields stability criteria and mode properties as a function of cavity length and mirror radius of curvature. A signal- to-noise ratio calculation is given to demonstrate the evaluation of sensitivity.
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 result 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, fused-silica TIR- ring cavity for which the theoretically predicted photon decay time has been achieved.
In connection with recent theoretical predictions, enhancement of optical second harmonic generation (SHG) by diffractive coupling to the silver surface-plasmon polariton (SPP) mode is shown experimentally to be maximized on a biperiodic corrugated surface. Optimized first- order diffractive coupling of incident radiation to the SPP maximizes resonance enhancement of the surface-localized electromagnetic field. Through the nonlinear susceptibility of the silver surface, an SPP-enhanced, evanescent, second harmonic wave is coherently generated which is selectively scattered into the second harmonic specular order by the second spatial harmonic in the surface profile. Biperiodic surfaces with appropriately optimized spatial harmonic composition are shown to provide enhancements in second harmonic relection of up to 104 over the corresponding flat surface response. Using the hologrpahic technique of Breidne et al. (Fourier blaze holography), biperiodic surfaces were fabricated which consisted of a superposition of an 833 nm fundamental and a phase- and amplitude-controlled second spatial harmonic. The results of angle-resolved SHG experiments are presented along with atomic force microscopy line scans of the surface profiles. The effect of coupling to the SPP mode at both the incident and second harmonic frequencies is also discussed.
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