The Grating Resonant Mirror

The Resonant Mirror (RM) sensor is an optical waveguide sensor that allows the refractive index, optical absorption or fluorescence of a sample to be monitored. The change in refractive index, absorption or fluorescence can result from chemical, physical or biochemical reactions occurring within typically two hundred nanometres of the waveguide/sample interface. As such, this allows monitoring of affinity reactions, kinetic rate determinations and immunoassay reactions.The sensor chip is fabricated by depositing a low-index spacer layer onto a high-index glass substrate. Finally, a high-index waveguiding layer is deposited onto the spacer layer. The sample is placed in contact with the waveguiding layer. Optical modes in the waveguiding layer are excited by prism coupling, and the effective index of these modes is altered by changes in the refractive index of the sample. This changes the resonance angle which is observed as a change in the position of a peak on a linear detector. To monitor the resonance angle, the standard RM instrument requires the use of crossed polarisers. The polarisers required careful setting up, and must be used with sources containing no infrared, which passes through conventional sheet polarisers unmodified.We describe here an elegant modification of the standard RM, namely the grating RM. By etching the high-index layer into narrow parallel strips, the incident light is caused to diffract. By monitoring the position of the diffracted orders, one can determine the refractive index of the sample. This modification removes the necessity of using polarisers, and thereby simplifies the instrumentation, without affecting the sensitivity of the sensor.In this paper, we present both theoretical and experimental results obtained using the grating RM sensor.