We present a new approach to the dual-beam geometry for onchip optical trapping and Raman spectroscopy, using waveguides microfabricated in TripleX technology. Such waveguides are box shaped and consist of SiO 2 and Si 3 N 4 , so as to provide a low index contrast with respect to the SiO 2 claddings and low loss, while retaining the advantages of Si 3 N 4 . The waveguides enable both the trapping and Raman functionality with the same dual beams. Polystyrene beads of 1 µm diameter can be easily trapped with the device. In the axial direction discrete trapping positions occur, owing to the intensity pattern of the interfering beams. Trapping events are interpreted on the basis of simulated optical fields and calculated optical forces. The average transverse trap stiffness is 0.8 pN/nm/W, indicating that a strong trap is formed by the beams emitted by the waveguides. Finally, we measure Raman spectra of trapped beads for short integration times (down to 0.25 s), which is very promising for Raman spectroscopy of microbiological cells.
Abstract. Polymethylmethacrylate (PMMA) optical fibres are low-cost polymer fibres that are generally more physically robust than silica fibres, are more flexible, yet like silica fibres have the potential to be used for practical evanescent field absorption sensors in aqueous solutions. However, evanescent field absorption in aqueous solutions is influenced by more than just the specific absorptivity of the solution in question. The physical configuration of the optical fibre itself, as well as surface charge interactions between the fibre and the chromophore in the solution also significantly affects the sensitivity of the fibre to evanescent field absorption. This paper reports on an investigation of numerous physical phenomena that influence evanescent field absorption for PMMA fibres using an aqueous solution of the dye Amidoblack. Parameters investigated included fibre coiling configuration and bend radius, fibre interaction length, and effect of solution pH. Coiled fibres were found to be more sensitive to evanescent field absorption than straight (uncoiled) lengths, and sensitivity was found to increase with a further reduction in bend radius. At high solution pH, the absorption versus solution concentration proved to be linear whereas at low pH the absorption versus concentration relationship exhibited a clear deviation from linearity. The observed nonlinearity at low pH points to the importance of accounting for electrostatic interactions between chromophore and fibre surface when designing a PMMA sensor for evanescent field absorption measurements in aqueous solutions.
IntroductionThe use of optical fibres in the development of chemical sensors is receiving considerable interest due to numerous advantages of this approach over traditional sensing methodologies. Optical fibres are typically very small in diameter (~10 µm -1 mm), the materials which are often silica or polymers can be chemically and/or biologically inert and intrinsic sensors rely on modification of an optical signal hence are impervious to electromagnetic interference. Optical fibres are generally cylindrical and consist of a central core material, commonly fused silica or plastic, which is surrounded by a 4 Work completed as a Visiting Student
An exposed fiber core (EFC) method employing poly(methyl methacrylate) optical fibers was used to monitor
the formation of calcium oxalate monohydrate scale under laboratory conditions using the phenomenon whereby
development of a high-refractive-index scale coating on the fiber surface attenuated the guided radiation in
the fiber core. Formation of scale on the optical fibers was verified by scanning electron microscopy. The
results obtained by this method were comparable to results obtained from turbidity and conductivity
measurements, but whereas these latter methods rely on phenomena in the bulk that might or might not be
relevant to the kinetics of scaling per se, the optical fiber method directly interrogates phenomena at the
solid/liquid interface.
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