Enhancing optical confinement in hybrid surface plasmonic nanocavities via external mode coupling near avoided resonance crossings

Plasmonic devices have been intensively studied in the past few decades. However, the substantial intrinsic ohmic losses imposed by noble metals highly limited further applications of such devices. Herein, we demonstrate the formation of long-lived states via external mode coupling in coupled hybrid surface plasmonic nano-resonators in which improved Q factors have been achieved. By systematically engineering the spatial intervals between three silicon nanorods on a silver substrate, we demonstrate the formation of long-lived states through destructive interference of optical fields. Numerical simulations reveal that, at a critical spacing, the Q factor of a resonant mode is significantly boosted to approximately 450—an eightfold enhancement compared to isolated single-rod resonators. Notably, this drastic improvement in photon lifetime is achieved while maintaining an ultrasmall effective mode volume Veff of the order of 10−4 μm3. These findings provide an alternative strategy to material optimization for mitigating metallic losses, offering a robust framework for developing high-efficiency, low-threshold plasmonic nanolasers, and advanced on-chip photonic components.