Fig. 1. Single-photon emitter in the bound-state-in-the-continuum cavity.

a Illustration of coupling between a single-photon emitter (SPE) and a bound-state-in-the-continuum (BIC) cavity with quality factor $Q$. The BIC mode is formed by an array of vertical magnetic dipoles (pink arrows), which associates with an electric-field (blue doughnuts) circulating in the x-y plane (red arrows). An SPE with a horizontal transition dipole moment (yellow sphere with green arrow) spatially overlaps with the electric-field hotspots. For strong coupling, the theoretical condition $g\ge \mid {\kappa }_{\mathrm{cav}}-{\kappa }_{\mathrm{SPE}}\mid /2$ has to be satisfied, where $g$ is the coupling strength, ${\kappa }_{\mathrm{cav}}$ and ${\kappa }_{\mathrm{SPE}}$ are dissipative decay rates of the cavity and the emitter, respectively. b 30° tilted scanning electron microscope (SEM) image of the fabricated TiO₂ nanopillar array (diameter $~$ 260 nm, gap $~$4 nm). c SEM image of the array after transferring a 3-nm thick hexagonal boron nitride (hBN) film on top. Scale bars in (b) and (c) are 200 nm.