Topologically Protected Plasmonic Bound States in the Continuum

Experimental realizations of bound states in the continuum (BICs) with strong robustness and advanced maneuverability in optical loss systems remain a long-standing challenge in nanophotonics. Here, we propose and fabricate a paradigm of diatomic metagratings incorporating the Su-Schrieffer-Heeger model into the design of plasmonic nanocavities to demonstrate optical resonators with a continuous "quasi-BICs (qBICs)-BICs-qBICs" transition. These resonators feature a topological band inversion, making high-quality (<i>Q</i>) resonances immune to the perturbation of incident angles and geometrical parameters. Furthermore, we strive to establish theoretical models to verify the topological nature of BICs-inspired resonances and introduce nonlinear optical probes to quantify strongly enhanced local fields at high-<i>Q</i> resonances. Our findings may provide a simple yet feasible design strategy for facilitating the dissipationless manipulation of surface/interface-enhanced light-matter interactions at the nanoscale, substantially broadening the functional scope of metaphotonics.