Thermally Tunable Refractive-Index Sensing Using Dual-Interface VO2 in a Metal–Dielectric–Metal Plasmonic Waveguide

Abstract We present a thermally tunable metal–dielectric–metal plasmonic waveguide sensor that uses ultrathin VO2 layers at bothmetal–dielectric interfaces to enhance refractive-index sensitivity. The proposed sensor has a simple and compact configuration,providing high sensing performance without requiring complex resonant geometries. We analyze the effect of VO2 position andphase state on the transmission response and compare structures without VO2, with VO2 at a single interface, and with VO2 atboth interfaces. The sensing response is evaluated at 30 and 80 ◦C, corresponding to the insulating and metallic states ofVO2, respectively. The results show that locating VO2 at both interfaces provides higher sensitivity than the single-interfaceand VO2-free configurations. We further engineer the VO2 thickness from 2 to 5 nm and examine its effect on the wavelengthshift produced by changes in the sample refractive index. The thickness dependence differs between the two phases, whilethe metallic state consistently provides a stronger sensing response. The optimized structure with a 5-nm VO2 layer reachesa maximum sensitivity of 1828 nm/RIU at 80 ◦C, compared with 1093 nm/RIU at 30 ◦C. These results show that controllingthe VO2 interface configuration, phase state, and thickness provides an effective approach for improving the refractive-indexresponse of MDM plasmonic waveguides while maintaining a simple and compact structure.