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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... more
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.
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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... more
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.
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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... more
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.
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ABSTRACT
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Novel applications of cavity ring-down spectroscopy (CRDS) to chemical detection are described. Using a linear optical resonator with an intra-cavity double-Brewster-window flow cell, CRDS is employed to probe the optical response to... more
Novel applications of cavity ring-down spectroscopy (CRDS) to chemical detection are described. Using a linear optical resonator with an intra-cavity double-Brewster-window flow cell, CRDS is employed to probe the optical response to adsorption of the surface-plasmon resonance (SPR) of an ultra-thin (0.2 nm), nanostructured Au film. Detection limits for trichloroethylene (TCE), perchloroethylene (PCE), and NO 2 are found to be 7x10 -8 mol/L, 2x10 -8 mol/L, and 4 x 10 -9 mol/L, respectively. As the ultra-thin nanostructured film is well described by a distribution of nanospheres with a mean diameter of 4.5 nm, Mie theory is employed to account for some aspects of the optical response. In a second implementation of CRDS, evanescent wave CRDS (EW-CRDS) is used to detect TCE, cis-dichloroethylene (cis-DCE), and trans-DCE by probing the first C-H stretching overtones in the near-IR with a monolithic folded resonator (MFR), providing spectroscopic selectivity and a reversible response. In a comparison of EW-CRDS to previous sensing technologies, the sensitivity obtained using an unclad MFR for TCE detection is found to be comparable to that obtained with a long-effective-path-length optical waveguide using a TCE-enriching polysiloxane coating. By applying an analyte-enriching, protective coating to an MFR, EW-CRDS may provide a sensitive, selective, and robust technology for longterm environmental monitoring.
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A Raman spectrometer has been constructed and optimized for quantitative measurement of N2 and O2 partial pressures. Detection limits of 54 and 62 mTorr are found for oxygen and nitrogen, respectively. The single-channel,... more
A Raman spectrometer has been constructed and optimized for quantitative measurement of N2 and O2 partial pressures. Detection limits of 54 and 62 mTorr are found for oxygen and nitrogen, respectively. The single-channel, single-dispersive-element system incorporates a high-power argon-ion laser and an optical configuration designed for maximum throughput. No D/A-A/D conversions are used. The design and performance of the instrument are discussed. Application of the system to nondestructive monitoring of air contamination in argon-filled insulating glass windows is briefly discussed. Interferometric effects from glass are considered.
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Evanescent wave cavity ringdown spectroscopy (EW-CRDS) is advocated as an approach for monitoring the formation of supported lipid bilayers (SLBs) on quartz substrates in situ and for the quantitative study of fast molecular adsorption... more
Evanescent wave cavity ringdown spectroscopy (EW-CRDS) is advocated as an approach for monitoring the formation of supported lipid bilayers (SLBs) on quartz substrates in situ and for the quantitative study of fast molecular adsorption kinetics at the resulting modified biomimetic surface. This approach is illustrated using SLBs of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP). Complementary atomic force microscopy (AFM) and quartz crystal microbalance with dissipation (QCM-D) measurements confirm the formation of bilayers on quartz. The subsequent interaction of the porphyrin, 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p'',p'''-tetrasulfonic acid tetrasodium hydrate (TPPS) with the cationic bilayer-modified silica surface has been studied using EW-CRDS combined with an impinging-jet to deliver analyte to the surface in a well-defined manner. The adsorption of TPPS to the bilayer was kinetically controlled and the adsorption rate constant was found to be 1.7 (±0.6) × 10(-4) cm s(-1) from finite element modeling of the jet hydrodynamics and associated convective-diffusion equation, coupled to a first-order surface process describing adsorption. These proof-of-concept studies provide a platform for the investigation of molecular processes at biomembranes using EW-CRDS for chemical species showing optical absorbance in the visible and ultraviolet range.