Pushing the limit of high-Q mode of a single dielectric nanocavity

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-thecontinuum（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 Qtheory=2.3 × 10～4 for a freestanding square nanowire（NW）(n = 4), which is 64 times larger than the highest Q-factor(Qtheory≈ 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.