Strong Coupling Induced Bound States in the Continuum in a Hybrid Metal–Dielectric Bilayer Nanograting Resonator

In the field of modern optics, the capability of localizing light at the nanoscale is crucial. Recently, the concept of the bound state in the continuum (BIC) has emerged, demonstrating highly resonant photonic modes within lossless dielectric nanostructures. On the contrary, implementing BICs with plasmonic resonators, despite its distinct advantages of near-field concentration, has been less preferred due to inherent material losses. This study proposes a novel BIC nanoresonator utilizing a hybrid metal–dielectric bilayer nanograting. In this structure, the metallic upper nanograting functions as a concentrator of the incident wave, whereas the dielectric lower nanograting serves as the main resonator for the concentrated field, exhibiting negligible material loss. This design facilitates strong near-field coupling between the three modes induced within the hybrid nanograting, leading to the emergence of BIC with exceptional quality factors. Our comprehensive analysis, including theoretical, numerical, and experimental investigations, reveals that the attainment of strong coupling is followed by the formation of BIC, while distinct mode hybridization and large Rabi splitting energy of about 436 meV are observed. As a result, an enhancement of more than 8-fold in electromagnetic energy is achieved within a silicon nanograting compared with the conventional single-layer resonator. It is worth noting that the trade-off between the intensity and storage lifetime of confined energies is addressed and the novel formation principle of the strong coupling induced BIC at the Γ-point is unveiled via temporal and spatial coupled mode theories for the first time, to the best of our knowledge. Our findings are expected to enhance the functionalities of resonant nanophotonic applications.