Optical responses of metallic plasmonic arrays under the localized excitation

The metallic plasmonic array that can support both propagating surface plasmon polaritons (PSPPs) and localized surface plasmon resonance (LSPR) possesses rich optical properties and remarkable optical performance, making it a powerful platform for applications in photonics, chemistry, and materials. For practical applications, the excitation spot is usually smaller than the area of metal arrays. It is thus imperative to address "how many array units are enough?" towards a rational design of plasmonic nanostructures. Herein, we employed focused ion beam (FIB) to precisely fabricate a series of plasmonic array structures with increased unit number. By utilizing photoluminescence (PL) and surface-enhanced Raman spectroscopy (SERS), we found that the array units outside the excitation spot still have a significant impact on the optical response within the spot. Combined with the numerical simulation, we found that the boundary of the finite array leads to the loss of PSPP outside the excitation point, which subsequently affects the coupling of PSPP and LSPR in the excitation spot, leading to variations in PL and SERS intensity. Based on the findings, we further tuned the LSPR mode of the metal arrays by electrodeposition to obtain strong near-field enhancement without any influence on the PSPP mode. This work advances the understanding of near-field and far-field optical behavior in finite-size array structures and provides guidance for designing highly-efficient photonic devices.