Resonant grating sensors using frustrated total-internal reflection

Abstract

The resonant mirror (RM) sensor is a leaky planar waveguide optical sensor that uses frustrated total internal reflection to couple light into and out of the waveguiding layer. Since the waveguiding layer acts as a resonant cavity, the light reflected from the RM device undergoes a full 2 π phase change across the resonance in either angle (for a fixed input wavelength) or wavelength (for a fixed input angle). This phase change can be visualised by using crossed input and output polarisers to produce a peak in intensity at the resonance angle or wavelength, which in turn is a sensitive function of surface refractive index. Disadvantages of this scheme are that it is very sensitive to birefringence in the substrate layer of the sensor device and requires careful choice and alignment of the polarisers. By forming the waveguiding layer as a set of thin parallel strips, it is possible to visualise the resonance angles or wavelengths directly by the appearance of diffracted spots of light at the resonance(s). To demonstrate the utility of this approach, a conventional RM sensor was coated with photoresist and exposed through a photomask consisting of 4 μm bars and 4 μm spaces, thus forming a 125 lines mm −1 grating. Once developed, the waveguiding layer was etched away in the exposed areas using 35% aqueous fluorosilicic acid. Finally, the remaining photoresist was removed, leaving the waveguide layer etched into a large number of parallel 4 μm wide strips. It proved possible to use both monochromatic and broadband non-coherent unpolarised light sources (such as light-emitting diodes and tungsten-filament lamps) to excite resonances and follow surface refractive index changes. The sensitivity of the grating sensor to refractive index was found to be 90.4% of that of the unmodified RM device. The grating-RM was used to detect low concentrations of xylene in water using a thin coating of phenyl siloxane polymer as a selective absorber of non-polar compounds. Xylene concentrations down to <5 ppm gave a reliably detectable peak shift.