Plasmonics could provide compact and powerful solutions for manipulating light in deep-subwavelength dimensions, which is promising for a great range of nanophotonic technologies such as plasmonic rulers and sensors. However, the effective area of enhanced localized field induced by surface plasmon polaritons is typically restricted to the structural boundaries. In this work, we propose a method to generate high quality-factor extended electromagnetic fields via hybridizing the super-radiant state and the quasi bound state in the continuum of graphene metasurfaces. The coupling interaction involved operates as a three-level system with multiple sharp resonances immune to the polarization, which holds great promise for developing nanodevices with high sensing capacity in two dimensions.

High-index dielectric resonators support different types of resonant modes. However, it is challenging to achieve a high-Q factor in a single dielectric nanocavity due to the non-Hermitian property of the open system. We present a universal approach of finding out a series of high-Q resonant modes in a single nonspherical dielectric cavity with a rectangular cross section by exploring the quasi bound-state-in-the-continuum (QBIC). Unlike conventional methods relying on heavy brutal force computations (i.e., frequency scanning by the finite difference time domain method), our approach is built upon Mie mode engineering, through which many high-Q modes can be easily achieved by constructing avoid-crossing (or crossing) of the eigenvalue for pair-leaky modes. The calculated Q-factor of mode TE(5,7) can be up to Q_theory = 2.3 × 10⁴ for a freestanding square nanowire (NW) (n = 4), which is 64 times larger than the highest Q-factor (Q_theory ≈ 360) reported so far in a single Si disk. Such high-Q modes can be attributed to suppressed radiation in the corresponding eigenchannels and simultaneously quenched electric (magnetic) field at momentum space. As a proof of concept, we experimentally demonstrate the emergence of the high-Q resonant modes [Q ≈ 211 for mode TE(3,4), Q ≈ 380 for mode TE(3,5), and Q ≈ 294 for mode TM(3,5)] in the scattering spectrum of a single silicon NW.