Differential reflectance signature of a Fano-type plasmonic response in a reflective Au diffraction-grating structure

We investigate the differential-reflectance signature of a Fano-type plasmonic response in a reflective plasmonic grating structure. A double-layer Au diffraction grating—designed to operate in contact with ethanol under 780-nm illumination from the analyte side—is fabricated on a sapphire substrate using electron-beam lithography and vacuum deposition. Rigorous coupled-wave analysis reveals an asymmetric reflectance peak with steep angular variation, while numerical differentiation of the reflectance spectrum produces a sharp differential reflectance dip. Reflectance mapping combined with Rayleigh anomaly and propagating surface plasmon dispersion relations, along with quantitative near-field analysis, indicates that the observed response is consistent with a Fano-type plasmonic response involving localized and propagating plasmonic components. Angular reflectance measurements reproduce both the asymmetric reflectance peak and differential reflectance dip, in good agreement with the simulations. The potential sensing performance is evaluated from the simulated refractive-index sensitivity and experimentally obtained linewidth. These results demonstrate that differential reflectance provides a useful readout of a Fano-type plasmonic response in reflective plasmonic grating sensors, particularly for low-refractive-index liquids.