Brightening single-photon emitters by combining an ultrathin metallic antenna and a silicon quasi-BIC antenna

Bright single-photon emitters (SPEs) are fundamental components in many quantum applications. However, it is difficult to simultaneously get large Purcell enhancements and quantum yields in metallic nanostructures because of the huge losses in the metallic nanostructures. Herein, we propose to combine an ultrathin metallic bowtie antenna with a silicon antenna above a metallic substrate to simultaneously get large Purcell enhancements, quantum yields, and collection efficiencies. As a result, the brightness of SPEs in the hybrid nanostructure is greatly increased. Due to the deep subwavelength field confinement (mode size $ < 10$~nm) of surface plasmons in the ultrathin metallic film (thickness $<4 $~nm), the Purcell enhancement of the metallic bowtie antenna improves by more than 25 times when the metal thickness decreases from 20~nm to 2~nm. In the hybrid nanostructures by combining an ultrathin metallic bowtie antenna with a silicon antenna, the Purcell enhancement (Fp$\,\approx 2.6\times 10^{6})$ in the hybrid nanostructures is 63 times greater than those ($\le 4.1\times 10^{4}$) in the previous metallic and hybrid nanostructures. Because of the reduced ratio of electromagnetic fields in the ultrathin metallic bowtie antenna when the high-index silicon antenna is under the quasi-BIC state, a high quantum yield (QY$\,\approx 0.70$) is obtained. Moreover, the good radiation directivity of the quasi-BIC (bound state in the continuum) mode of the silicon antenna and the reflection of the metallic substrate result in a high collection efficiency (CE$\,\approx 0.71$). Consequently, the overall enhancement factor of brightness of a SPE in the hybrid nanostructure is EF$^{\ast }\approx {\rm Fp}\times {\rm QY}\times {\rm CE}\approx 1.3\times 10^{6}$, which is $5.6\times 10^{2}$ times greater than those (EF$^{\ast }\le 2.2\times 10^{3}$) in the previous metallic and hybrid nanostructures.