Photonic Gap Antennas Based on High-Index-Contrast Slot Waveguides

Optical antennas made of low-loss dielectrics have several advantages over plasmonic antennas, including high radiative quantum efficiency, negligible heating, and excellent photostability. However, due to weak spatial confinement, conventional dielectric antennas fail to offer light-matter interaction strengths on par with those of plasmonic antennas. We propose here an all-dielectric antenna configuration that can support strongly confined modes ($V\ensuremath{\sim}{10}^{\ensuremath{-}4}{\ensuremath{\lambda}}_{0}^{3}$) while maintaining unity antenna quantum efficiency. This configuration consists of a high-index pillar structure with a transverse gap that is filled with a low-index material, where the contrast of indices induces a strong enhancement of the electric field perpendicular to the gap. We provide a detailed explanation of the operational principle of such photonic gap antennas (PGAs) based on the dispersion relation of symmetric and asymmetric horizontal slot waveguides. To discuss the properties of PGAs, we consider silicon pillars with air or the polymer CYTOP as the gap material. We show by full-wave simulations that PGAs with an emitter embedded in the gap can enhance the spontaneous emission rate by a factor of approximately $1000$ for air gaps and approximately $400$ for CYTOP gaps over a spectral bandwidth of $\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}\ensuremath{\approx}300\phantom{\rule{0.2em}{0ex}}\mathrm{nm}$ at $\ensuremath{\lambda}=1.25\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Furthermore, the PGAs can be designed to provide unidirectional out-of-plane radiation across a substantial portion of their spectral bandwidth. This is achieved by setting the position of the gap at an optimized off-centered position of the pillar so as to properly break the vertical symmetry of the structure. We also demonstrate that, when acting as receivers, PGAs can lead to a near-field intensity enhancement by a factor of approximately $3000$ for air gaps and approximately $1200$ for CYTOP gaps.