High- <i>Q</i> Resonances in Hybrid Plasmonic–Photonic Metasurfaces

Plasmonic systems supporting resonances with a high quality (<i>Q</i>) factor are highly desirable for applications such as optical sensing and nanolasing. However, this goal is hindered by radiative loss and Ohmic loss. This paper reports a hybrid plasmonic-photonic system that supports multiple high-<i>Q</i> resonances in the short-wave near-infrared regime, which are associated with either at-Γ or off-Γ bound states in the continuum (BICs). By engineering the hybridization between a dielectric slab waveguide and a plasmonic nanoparticle lattice, BICs can form via symmetry incompatibility or destructive interference. Concurrently, the enhanced electric fields are distributed away from the lossy metal, thus enabling high-<i>Q</i> optical resonances in plasmonic systems. Experimentally, we fabricate two-dimensional silver nanoparticle arrays embedded within a dielectric slab waveguide. The highest <i>Q</i> factor observed is ∼2140 at λ = 985 nm. In addition, by utilizing such nonlocal resonant modes near the BIC condition at the excitation wavelength, we demonstrate significantly enhanced upconversion luminescence from doped erbium ions. This type of metasurface thus offers prospects for future studies where a high <i>Q</i> factor and enhanced light-matter interactions are required.