Tunable multi-band terahertz sensor based on graphene plasmonic metasurfaces

This study introduces a highly sensitive and tunable plasmonic refractive index sensor based on a novel metal-dielectric-dielectric-metal (MDDM) metasurface architecture operating in the Terahertz (THz) region. The proposed structure consists of a graphene-based fractal pattern integrated with a dielectric layer, a silicon substrate, and a bottom aluminum layer, leveraging commercially available and easy-to-fabricate materials. This multilayer configuration supports strong plasmonic resonances and enhanced absorption, enabling triple-band refractive index sensing with high sensitivities of 10 μm/RIU, 3 μm/RIU, and 2.75 μm/RIU across three distinct modes, surpassing previously reported single- and dual-band plasmonic sensors. Unlike conventional metal-dielectric-metal (MDM) absorbers, the dual-dielectric configuration enhances field localization and supports three distinct resonance modes: a dipolar mode at approximately 7.69 THz, a quadrupolar mode near 25.4 THz, and a hybridized higher-order mode around 30.2 THz. These hybridized resonances arise from the coupled interactions between the central hexagonal graphene core and the concentric double nanorings, producing intense localized electromagnetic "hot spots" that significantly amplify the sensor's spectral response. The multiple resonances offer improved flexibility for detecting various analytes or extending the sensing range. The proposed MDDM sensor maintains stable and tunable performance under environmental variations, demonstrating significant potential for practical applications in biomedical diagnostics, gas sensing, and glucose monitoring.