Fabrication and Operation Analysis of a Surface-Plasmon Sensor Using a Nonpropagating Mode

Our research focuses on the development of a surface-plasmon sensor that uses a stationary surface plasmon, referred to as a “nonpropagating mode.” This mode is observed when light is incident perpendicularly on a surface-plasmon sensor based on a metal diffraction grating. We performed a comprehensive analysis of the behavior of the surface-plasmon resonances within this nonpropagating mode, employing the rigorous coupled-wave analysis method. Using electron-beam lithography, sputtering, and a lift-off process, we fabricated such a surface-plasmon sensor and evaluated its optical properties rigorously. By combining simulations and experiments, we successfully utilized the nonpropagating mode to detect a liquid medium with a refractive index of 1.70. Simulations show that the nonpropagating mode arises due to a Fano resonance; i.e., to a resonant interaction between a localized surface plasmon generated at the edge of a metal grating strip during normal incidence and a propagating surface plasmon that occurs at the boundary between the metal diffraction grating and the measurement medium. The present results provide useful information for the advancement of surface-plasmon sensing technologies.