High- Q Nonlocal Resonances in Mirror-Enhanced Plasmonic Lattices

Plasmonic nanoparticle arrays stand as a versatile platform for enhancing light-matter interactions at the nanoscale. Ultrahigh-quality-factor (high-Q) resonances are critical for spectrally selective applications such as label-free biosensing, low-threshold lasing, and quantum information processing. However, achieving such resonances in plasmonic systems remains a challenge. Surface lattice resonances (SLRs) and bound states in the continuum (BICs) underlie high-Q behavior. Here, we develop a robust strategy to achieve high-Q resonances in plasmonic nanoparticle lattices by harnessing mirror-induced destructive interference that converts bright SLRs into dark BICs. Experimentally, we demonstrate a Q factor of 3039 at ∼880 nm in mirror-coupled silver nanoparticle arrays and observe lasing with minimized threshold at critical coupling. By spatially tailoring the orientation of anisotropic nanoparticles within this architecture, we further realize high-Q composite plasmonic microcavities that enable decoupled control over the Q factor and local near-field topology, supporting vectorial lasing modes with programmable topological charges.