Strong Near-Field Coupling of Lateral Nanodendrites on Metallic Grating for Plasmonic Sensing

Plasmonic gratings are versatile platforms for manipulating light-matter interactions; however, common unit-cell geometries, such as rectangular, sinusoidal, triangular, or blazed profiles, offer limited field enhancement and constrain further performance improvements. This paper reports a centimeter-scale plasmonic grating decorated with nanodendrites on the ridge sidewalls that enables strong near-field coupling and narrow resonance for ultrasensitive sensing. The structure was fabricated by interference lithography, where an in-plane interference pattern and an out-of-plane standing wave define double-nested unit cells. The subwavelength dendritic features promote strong near-field coupling and induce plasmon hybridization under perpendicular (x) polarization, whereas Rayleigh anomalies (RAs) with enhanced reflectance dominate under parallel (y) polarization. Dispersion analysis reveals a linear angle-dependent dispersion of the RA modes in contrast to the weak dispersion of the hybridized plasmon modes. A sharp, high-contrast RA resonance with enhanced reflection was achieved in high-index media, yielding a sensitivity of 510 nm/RIU and a figure of merit of 28.3 RIU^-1 for plasmonic sensing.