Current Published Papers

Cross, G.H., Ren Y., Swann, M.J., Refractometric Discrimination of Void-Space Filling and Swelling During Vapour Sorption in Polymer Films. The Analyst, (2000), 125 2173-2175.

Abstract: Thin polymeric films have been deposited as upper cladding layers on a new integrated optical interferometer fabricated from layers of silicon oxynitride on a silicon wafer. The evanescent field of the probing waveguide mode transduces refractive index changes in the polymer layer into the measured phase changes in the device. Real-time measurement of index change and its sign is obtained. Upon exposure to humid air, we record water sorption by films of poly(vinyl pyrrolidone) by a rapid positive index change for void-space filling followed by a slow negative index change for swelling. Sorption of water vapor into a thin film of the viscous liquid polymer polyethylenimine shows only swelling mode behaviour and a simple constitutive model can be applied to give the fractional water occupied volume.

Cross, G.H., Ren, Y., Freeman, N.J. Young’s Fringes from Vertically Integrated Slab Waveguides: Applications to Humidity Sensing. Applied Physics,(1999), 86 6483-6499.

Abstract: Using a multiple layer optical waveguide system consisting of two vertically slab waveguides, classical Young’s fringes may be obtained in the far-field diffraction plane. In agreement with the simple theory of diffraction interference the spacing of the far-field fringes is easily observed on mm to cm dimensions without further transformation of the output light. The simple methods of fabrication and means of optical coupling should provide a readily adaptable method for examining the principles of interferometry in an integrated optical format. The structure acts to transform polarized incident plane wave input light into separate slab modes of the device which emerge as two closely spaced and coherent sources at the output. The elements required for a classical Young’s fringe demonstration are therefore all embodied in this approach. The basic concept can be applied to an optical method for sensing. In one example of this we demonstrate measurement of the phase difference induced between the upper and lower propagating modes in structures due to water vapor diffusion into the layers which are formed from hydrophilic polymers. The Young’s fringe patterns exhibit a spatial intensity distribution which is sensitive to water vapor introduced over the surface of the structure. Differences in the effective index between the modes of the two waveguides during the diffusion of the vapor causes phase shifts which result in redistribution in the fringe pattern. The anticipated limit of detection of these devices is lower than 1 ppm for water vapor.