Hybrid Dielectric-Plasmonic Nanoantenna with Multiresonances for Subwavelength Photon Sources

The enhancement of the photoluminescence of quantum dots induced by an \noptical nanoantenna has been studied considerably, but there is still \nsignificant interest in optimizing and miniaturizing such structures, \nespecially when accompanied by an experimental demonstration. Most of the \nrealizations use plasmonic platforms, and some also use all-dielectric \nnanoantennas, but hybrid dielectric-plasmonic (subwavelength) nanostructures \nhave been very little explored. In this paper, we propose and demonstrate \nsingle subwavelength hybrid dielectric-plasmonic optical nanoantennas coupled \nto localized quantum dot emitters that constitute efficient and bright \nunidirectional photon sources under optical pumping. To achieve this, we \ndevised a silicon nanoring sitting on a gold mirror with a 10 nm gap \nin-between, where an assembly of colloidal quantum dots is embedded. Such a \nstructure supports both (radiative) antenna mode and (nonradiative) gap mode \nresonances, which we exploit for the dual purpose of out-coupling the light \nemitted by the quantum dots into the far-field with out-of-plane directivity, \nand for enhancing the excitation of the dots by the optical pump. Moreover, \nalmost independent control of the resonance spectral positions can be achieved \nby simple tuning of geometrical parameters such as the ring inner and outer \ndiameters, allowing us to conveniently adjust these resonances with respect to \nthe quantum dots emission and absorption wavelengths. Using the proposed \narchitecture, we obtain experimentally average fluorescence enhancement factors \nup to $654\\times$ folds mainly due to high radiative efficiencies, and \nassociated with a directional emission of the photoluminescence into a cone of \n$\\pm 17\\degree$ in the direction normal to the sample plane. We believe the \nsolution presented here to be viable and relevant for the next generation of \nlight-emitting devices.