Absorption enhancement and sensing performance optimization of surface lattice resonances in plasmonic gold nanodisk arrays

Abstract Surface lattice resonance (SLR) sensors based on periodic gold nanodisk arrays provide an effective route to overcome the intrinsically broad linewidth of localized surface plasmon resonance (LSPR). However, ultranarrow SLR modes, particularly out-of-plane quadrupole resonances, are usually accompanied by low resonance amplitude, which severely limits their practical detectability and sensing stability. In this work, the structural parameters of a gold nanodisk array are systematically optimized to significantly enhance the absorptivity of this type of resonance while preserving its ultranarrow linewidth. A reasonable figure of merit (RFOM) is further introduced, which jointly considers refractive index sensitivity, linewidth, and spectral depth, enabling a more comprehensive evaluation of sensing performance.

Finite-element simulations are employed to numerically analyze the optical response of the array. For the optimized single-layer configuration, the absorptivity of the out-of-plane quadrupole SLR peak is increased from 0.141 to 0.292, while the linewidth remains as narrow as 0.04~nm, yielding an RFOM of 3647.4~RIU$^{-1}$. To further enhance the resonance strength, a double-layer gold nanodisk structure is introduced, providing additional out-of-plane coupling degrees of freedom for effective modulation of the resonance characteristics. When the bottom disk radius is 190~nm and the top disk radius is 150~nm, near-perfect narrowband absorption of 0.958 is achieved with a linewidth of 2.47~nm, outperforming previously reported structures of the same category.In addition, numerical simulations of C-reactive protein (CRP) immunosensing are performed based on an effective biolayer model, indicating promising potential of the proposed structure for early-stage CRP detection.