Analysing surface plasmon microscopy with rigorous diffraction theory

Surface plasmon (SP) microscopy offers great potential for bioimaging applications particularly for quantitative measurement of refractive index over localized regions. Our aim is to measure variations in refractive index so that important events such as antibody-antigen binding can be measured on smaller spatial scales than is possible with conventional prism based SP imaging. To this end we have used rigorous coupled wave analysis (RCWA) to develop a theoretical framework to understand the capability of the SP microscope to measure local changes in refractive index. Our results show that the non-interferometric microscope configuration gives results which are strongly dependent on the propagation length of the SPs, whereas in the interference mode the measured path of the SPs is determined by the sample defocus and SPs following different paths are not detected. This gives better localization of the measurement of local refractive index as well as a more predictable value as it depends on the optical set rather than sample properties.