Critically coupled high-Q plasmonic guided mode resonances in the visible

High-quality factor (Q factor) plasmonic resonances in the visible spectrum are typically limited by the intrinsic absorptive loss of metals, necessitating a trade-off between resonance linewidth and coupling efficiency. This work demonstrates that an ultrathin silver film supporting long-range surface-plasmon polaritons (LRSPPs) provides a low-loss interaction access for a square lattice of Ag nanoparticles, enabling critical coupling to a guided-mode resonance (GMR) at λ = 633 nm. The collective LRSPP-GMR yields a Q factor of 361 with a coupling efficiency of 99.8 %. By slightly under-coupling, the Q factor rises to ∼ 600 while the efficiency remains above 80 %. The resonance persists down to 558 nm, with a Q factor greater than 100, covering most of the visible spectrum. Coupled-mode theory and coupled-dipole simulations demonstrate that the silver film simultaneously suppresses both radiative leakage and resistive dissipation, thereby reconciling the two canonical loss channels that typically preclude critical coupling in plasmonic metasurfaces. Owing to the simultaneous boost in Q factor and moderate mode volume, the optimized Purcell factor of LRSPP-GMR resonators reaches ∼5 × 10²¹m^-3 at λ = 633 nm, twice that of uncoupled SPP GMRs, demonstrating its superior enhancement of light-matter interaction. This work provides a straightforward route to understand and realize ultranarrow, efficiently excited plasmonic resonances for SERS, nonlinear optics, Purcell-enhanced emission, and sensing.