Cavity ring-down spectroscopy has proven to be a very sensitive gas-phase spectroscopic technique, suitable to record either very weak transitions of abundant gases or stronger transitions of trace gases. Here, an adaptation of the ring-down measurement principle to optical waveguides is presented. Fiber-loop ring-down spectroscopy ͑FLRDS͒ allows for the measurement of absorption spectra of minute quantities of liquid solutions. An optical fiber is wound into a loop using a fiber splice connector. A nanosecond laser light pulse (ϳ810 nm) is coupled into the loop and the light pulses are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. The characteristic ring-down time is obtained by exponential fitting of the envelope of the wave form. This method is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. The strengths of the technique are demonstrated by characterization of a variety of loss processes-in particular by the measurement of the absolute loss of the optical fiber and of the fiber connector, losses due to macrobending of a section of the fiber loop, as well as losses due to lateral and longitudinal displacement in the fiber-fiber connection. Furthermore, it is shown that FLRDS is useful as an absorption spectroscopic technique for very small sample volumes and may be applied as an absorption detection method in analytical chemistry devices. A crude 47 m channel in polydimethylsiloxane polymer was fabricated between the fiber end facets and the dye 1,1Ј-diethyl-4,4Ј-dicarbocyanine iodide ͑DDCI͒ was introduced into the channel. From the concentration dependence of ring-down time the sample volume was determined as 700 pL and the detection limit as about 10 Ϫ10 mol, or 7ϫ10 Ϫ8 g of DDCI.
Fiber-loop ring-down spectroscopy (FLRDS) is a recently developed absorption spectroscopic technique suitable for very small liquid samples. It is based on measurements of the optical decay constant of laser intensity in a loop made of optical waveguide material. This decay constant changes as small liquid samples containing absorbing species are introduced into the loop. In this report, it is demonstrated that one can also obtain the optical decay constant using a continuous wave laser beam that is intensity modulated and then coupled into an optical fiber loop. The inherent exponential decay in the fiber loop introduces a phase shift of the light emitted from the loop with respect to the pumping beam. By measuring this phase shift, one can readily determine the concentration of the analyte introduced between the two fiber ends and a model is established to describe the relationship. It is demonstrated that this technique, dubbed phase-shift fiber-loop ring-down spectroscopy (PS-FLRDS), is well suited as an absorption detector for any flow system in which the optical absorption path is limited by the instrument architecture. By measuring the phase angle as a function of concentration of 1,1'-diethyl-4,4'-dicarbocyanine iodide in dimethyl sulfoxide, the detection limit was determined as approximately 6 microM for a 30-40-microm absorption path. A temporal resolution of approximately 100 ms was demonstrated by a rapid displacement of the solutions between the two fiber ends. Proof-of-principle use of the PS-FLRDS detection in capillary flow systems using a commercial four-way microcross established that the alignment of the fiber and the capillary can be made simple and effective, while retaining both a low detection limit and a fast response.
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A search has been made for neutrinos from the hep reaction in the Sun and from the diffuse supernova neutrino background ( DSNB) using data collected during the first operational phase of the Sudbury Neutrino Observatory, with an exposure of 0.65 ktons yr. For the hep neutrino search, two events are observed in the effective electron energy range of 14:3 MeV < T eA < 20 MeV, where 3.1 background events are expected. After accounting for neutrino oscillations, an upper limit of 2:3 ; 10 4 cm À2 s À1 at the 90% confidence level is inferred on the integral total flux of hep neutrinos. For DSNB neutrinos, no events are observed in the effective electron energy range of 21 MeV < T eA < 35 MeV, and, consequently, an upper limit on the e component of the DSNB flux in the neutrino energy range of 22:9 MeV < E < 36:9 MeVof 70 cm À2 s À1 is inferred at the 90% confidence level. This is an improvement by a factor of 6.5 on the previous best upper limit on the hep neutrino flux and by 2 orders of magnitude on the previous upper limit on the e component of the DSNB flux.
We report a facile method for the absorption (characterized by the weight/weight swelling degree, Q) of a variety of chemical warfare agents (CWAs); including sulfur mustard (HD) (Q = 40) and V-series (VM, VX, i-Bu-VX, n-Bu-VX) of nerve agents (Q ≥ 45) and a simulant, methyl benzoate (Q = 55), through the use of a poly(styrene-co-vinyl benzyl chloride-co-divinylbenzene) lightly cross-linked poly high internal phase emulsion (polyHIPE). By varying the vinyl benzyl chloride (VBC) content and the volume of the internal phase of the precursor emulsion it is demonstrated that absorption is facilitated both by the swelling of the polymer and the uptake of liquid in the pores. In particular the sample prepared from a 95% internal emulsion water content showed rapid swelling (<5 min to total absorption) and the ability to swell both from a monolithic state and from a compressed state, making these systems ideal practical candidates for the rapid immobilization of CWAs.
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