Ultrafast and\nHighly Collimated Radially Polarized\nPhotons at Room Temperature from a Colloidal Quantum Dot Coupled to\na Hybrid Nanoantenna

To harness the potential of radially polarized photons\nfor classical\nand quantum communication applications, we demonstrate an on-chip,\nroom-temperature device, which generates highly directional radially\npolarized photons at very high rates. The photons are emitted from\na giant CdSe/CdS colloidal quantum dot (gQD) accurately positioned\nat the tip of a metal nanocone centered inside a hybrid metal-dielectric\nbullseye antenna. We show that due to the large and selective Purcell\nenhancement specifically for the out-of-plane optical dipole of the\ngQD, the emitted photons can have a very high degree of radial polarization\n(>93%), based on a quantitative metric. Our study emphasizes the\nimportance\nof accurate gQD positioning for optimal radial polarization purity\nthrough extensive experiments and simulations, which contribute to\nthe fundamental understanding of radial polarization in nanostructured\ndevices and pave the way for implementation of such systems in practical\napplications using structured quantum light.