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 $\sim 880\mathrm{nm}$ 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.

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• Lasers
• Light-matter interaction
• Metasurfaces
• Optical microcavities
• Photonic crystals
• Plasmonics