Cavity-Assisted High-Efficiency Excitation of High- Q Quasi-BIC Resonance in the Hexagonal Non-Close-Packed Ellipsoidal Silver-Shells Array

Abstract Simultaneously achieving high-Q and intense plasmonic quasi-bound states in the continuum (qBICs) modes under normal incidence is beneficial for applications such as nanosensors and nanolasers, which is, however remaining a great challenge due to the huge absorption loss of metals. Here, we theoretically demonstrate that a hexagonal non-close-packed array of silver nanoshells supports qBICs under oblique incidence, arising from the excitation of the electric-octupole sphere plasmon. Second, the qBIC mode can be efficiently excited by using an ellipsoidal silver-shells at normal incidence, which remains a tunable high Q-factor (614), a large Purcell factor (234.5), a large near-field enhancement (|E| ∼ 27.9) and a strong absorption intensity of qBIC (∼0.39), indicating that the ellipsoidal silver-shells and a dielectric-core (cavity) are playing critical roles. Meanwhile, the observed qBIC modes further present pronounced polarization sensitivity and high tunability at normal incidence. Finally, we show that it can be employed to realize a dye-based nanolaser with a lasing threshold as low as 85.8 cm–1.