Metal Stripe Waveguide Based Interferometer-Type Sensor Working in an Aqueous Solution With a Low Refractive Index

This paper reports a metal stripe waveguide based sensor that functions like a Mach-Zehnder interferometer. It consists of three sections. The first and third sections are input and output metal stripe waveguides that support a long-range surface plasmon polariton (LRSPP). The second section is a sensing region; it comprises a substrate, which is common to the first and third sections, an insulator layer with a refractive index larger than 2 (e.g., TiO₂), and an Au layer much thicker than the skin depth of gold. For sensing, it is covered by an aqueous solution with a refractive index of about 1.3. Because of the thick Au layer, separate single-interface surface plasmon polaritons (SPPs) propagate along the top and bottom surfaces of the Au layer. Since the top and bottom SPPs rather than an LRSPP are used in the second section, it is not constrained by the condition of supporting an LRSPP, which is that the substrate should have almost the same refractive index as the solution. The top and bottom SPPs play the roles of sensing and reference arms of an interferometer, respectively. In this paper, the sensor is designed, and its bulk-sensing and surface-sensing characteristics are theoretically analyzed. The design results in the compact sensor whose sensing region is ~ 35μm long; the analysis demonstrates that the sensor has sensitivity higher than or comparable to that of previous plasmonic sensors.