Narrow Surface-Lattice Resonance of Plasmonic Metasurfaces at Normal Incidence in Asymmetric Environment

Surface lattice resonance (SLR) is a key mechanism for achieving high-Q resonances in plasmonic nanostructures. However, the oblique incidence or refractive-index matching required during excitation makes it challenging in practical applications. As a solution, we demonstrate SLR metasurfaces under normal incidence and in an asymmetric environment, with a Q-factor of 47 and a resonant transmission peak of 0.43 in the 1-μm band in experiments, by skillfully exciting the high-order multipole modes and out-of-plane resonance of ordinary gold nanorods. The influence of structural parameters on SLR characteristics is explored, providing insight into the effective regulation of the transmission response and achieving the transition from a transmission dip to a transmission peak. Benefiting from the strong light-matter interaction of the SLR systems, we experimentally demonstrate linear refractive index sensing with a bulk sensitivity of 1014 nm/RIU, along with an 80-times relative enhancement (compared with an unpatterned Au film of the same thickness) in second harmonic generation and an appreciable third-order saturable absorption response. Our work provides a simple and efficient strategy to tailor narrow-band plasmonic SLR metasurfaces, enabling promising potential in sensing, nonlinear optics, pulse optics, and other fields.