Engineering Fano-type resonances in periodic gold nanorod–dye architectures for high-performance refractive-index sensing

Abstract A hybrid plasmon–exciton nanostructure consisting of periodic gold nanorods conformally coated with a molecular dye layer is theoretically investigated as a high-performance refractive-index sensor. Three-dimensional finite-difference time-domain (3D-FDTD) simulations demonstrate that cooperative interaction among localized surface plasmon resonances (LSPRs), molecular excitons, and Fabry–Pérot cavity feedback produces a narrow Fano-like reflection resonance with a linewidth of 2.87 nm and a quality factor of Q ≈ 343. The resonance behavior was systematically investigated by varying the nanorod height, dye-layer thickness, and lattice period. Building on this parametric analysis, the study establishes a resonance-engineering framework in which cooperative plasmon–exciton coupling, lattice-assisted optical confinement, and Fabry–Pérot cavity feedback are intentionally combined to generate narrow hybrid Fano resonances with improved spectral selectivity and sensing performance. The selected configuration, comprising a 25 nm nanorod height, 30 nm dye layer, and 620 nm lattice period, achieves a maximum wavelength sensitivity of 333.93 nm RIU − 1 with a figure of merit of 116.35 RIU − 1 and an estimated detection limit of 3.0 × 10 − 5 RIU for biomaterial sensing in the refractive-index range of 1.46–1.55. For low-index gaseous environments, the same platform enables fixed-wavelength intensity interrogation with an intensity sensitivity of 17.58 RIU − 1 . The enhanced performance is attributed to strong electromagnetic field localization, efficient overlap between the hybrid optical mode and the surrounding analyte, and collective lattice effects. The proposed architecture supports both wavelength-shift and intensity-based sensing within a single platform and provides general design principles for engineering hybrid Fano resonances in compact, label-free plasmonic sensors for biochemical and environmental monitoring.\.