Modeling and analysis of a common-path surface plasmon interferometer with confocal configuration

This paper is to present a model and analysis of a common-path surface plasmon interferometer from the aspect of the electrical field distribution, which excellently supplements the currently applied simplified ray model. We apply radial polarization to show the principle. Firstly, we study the electric field distribution in the vicinity of the focal spot and show how the defocusing properties behave on the far-field imaging plane. The diffraction orders on the far-field image plane with an axially-scanned sample contains the common-path interferometric V(z) effect and quantitatively interprets the plasmonic properties of the materials. The implementation refers to extracting the interferometric V(z) SP signals from intensity-based SPM by applying a confocal annulus on the image plane. Secondly, we specifically analyze the impact of annulus parameters, e.g. inner radius and width, on the acquisition of the interferometric V(z) curves. We also discuss the acquisition conditions of the plasmonic V(z) effect. Finally, we discuss the advantages of using radially polarized illumination and make a comparison with a conventional linearly polarized system. The established model is experimentally verified.