# Corner Reflector Plasmonic Nanoantennas for Enhanced Single-Photon Emission
## 1 Introduction  
Efficient and reliable single-photon sources, capable of producing on-demand high-purity single-photon states, are key elements within the emerging field of photon-based quantum technologies [1; 2]. Atom-like emitters, based either on solid-state or molecular systems, are a promising alternative for this purpose [3; 4; 5; 6; 7]. In the past years, nanoscale elements have been introduced to enhance the emission rates of photon sources and to tailor their spatial radiation properties. Such nanoantennas (NAs) reproduce at the nanoscale the action of traditional radio frequency (RF) aerials. Various traditional antenna geometries have been considered for applications in the optical realm: dipole [8; 9], patch [10], bowtie [11] or the Yagui-Uda array [12; 13; 14]. Antennas for light [12] serve the dual purpose of enhancing emitter fluorescence and facilitating the outcoupling of the generated photons. The performance requisites for the two functions are antithetical: whereas a small mode volume is required for an enhanced emission rate, high antenna directivity is well known to be the result of a large electrical size, i.e, the extent of the structure measured using the wavelength of the electromagnetic field as unit reference [15; 16]. In [4], such antipodal requirements have been reconciled by photomodifying a nanopatch antenna assembled around a nanodiamond with a nitrogen-vacancy center to reduce the cavity mode size while keeping a large size emitter to spatially shape the radiation produced. In this work, we tackle this issue by using a hybrid plasmonic structure built from the combination of a conventional NA acting as the feeder element of a corner reflector (CR) [17]. This strategy simultaneously permits optimization of the emission rate at the feeder design and the capacity to use a CR to enhance the directivity of the photons produced. The use of CRs in antenna design has been prevalent since [PERSON] introduced them in 1940 [17], owing to their outstanding performance, simplicity, and ease of construction. Thiswork demonstrates that these advantageous properties can also be applied to nanoscale photonics. The potential of CRs to enhance photon emission was highlighted in [18], where the radiation patterns generated by an electric dipole emitter with a perfect conductor CR were analyzed. Here, we present a thorough numerical study of the proposed NA, utilizing two alternative noble metals: silver and gold. The feeder is assumed to be constituted by an atom-like dipole emitter coupled to a spherical nanoparticle (NP), though the proposed CRs can also accommodate more complex feeder geometries, such as dimers or bowtie structures [8; 11], which may offer enhanced emission. The analysis is conducted using the finite difference time domain method (FDTD) [19; 20], enabling the calculation of the radiative emission rate enhancement factor, losses within the metallic structures, and the far-field patterns of the emitted photons. This approach provides a comprehensive characterization of the NA. The geometry employed in the analyses is depicted in Figure 1. A spherical NP with diameter \(d\) is positioned at the origin. The atom-like emitter is represented by a point dipole source \(\mathbf{p}\) located at a distance \(s\) from the surface of the metal nanosphere that is placed at a distance \(S\) from the corner apex along the bisector. The corner angle is \(\psi\), while the metal films have a length \(L\) and height \(H\), as shown in the figure. The thickness of the metal films forming the reflector is \(W\). The planes of the CR are assumed to be made of the same metal as used for the NP in the feeder system. A metallic plane typically reflects a substantial portion of an incident wave originating from a large distance, due to the significant mismatch between its optical properties and those of the surrounding dielectric.