Tailoring directivity of two-photon spontaneous emission using plasmonic nanoantennas

We report nanoantenna designs that emit entangled photons of distinct energy from two-photon spontaneous emission (TPSE) in different far-field directions with high quantum efficiency. To model the designs, we use a framework that computes the TPSE spectra of a quantum emitter near arbitrarily shaped nanostructures via the classical computation of the one-photon Purcell factors. A first structure exploits both the dipolar and the quadrupolar mode on a single silver nanorod. A second geometry employs dipolar modes on two perpendicular nanorods. An efficient and directional functionality is useful to develop highly compact, integrated entangled two-photon sources.