Measuring Plasmonic Phase Using Radially Polarized Confocal Surface Plasmon Interferometer

Conventional surface plasmon (SP) interferometry allows a microscopic sensing of materials using the so-called V\!(z) curve, which actually refers to the module of complex V\!(z) effect while the phase information is lost. This article proposes a simple and compact configuration to measure the plasmonic phase of tested materials within the localized focal spot region of radially polarized light using \varphi (z) curve. A home-developed experimental configuration is applied, involving the basic concepts of virtual confocal annulus and embedded phase-shifting configuration. A modified model of confocal SP interferometry is established to explain why an attenuated beam profile is strictly required in the phase measurement. Moreover, the procedures on how to achieve beam profile attenuation and phase-shifting simultaneously using a single phase-sensitive spatial light modulator (SLM) is elaborated. The experimental verification is demonstrated as well.