Optical antennas have generated much interest in
recent years due to their ability to focus optical energy beyond
the diffraction limit, benefiting a broad range of applications
such as sensitive photodetection, magnetic storage, and surfaceenhanced
Raman spectroscopy. To achieve the maximum field
enhancement for an optical antenna, parameters such as the
antenna dimensions, loading conditions, and coupling efficiency
have been previously studied. Here, we present a framework,
based on coupled-mode theory, to achieve maximum field
enhancement in optical antennas through optimization of optical antennas’ radiation characteristics. We demonstrate that the
optimum condition is achieved when the radiation quality factor (Q_(rad)) of optical antennas is matched to their absorption quality
factor (Q_(abs)). We achieve this condition experimentally by fabricating the optical antennas on a dielectric (SiO2) coated ground
plane (metal substrate) and controlling the antenna radiation through optimizing the dielectric thickness. The dielectric thickness at
which the matching condition occurs is approximately half of the quarter-wavelength thickness, typically used to achieve
constructive interference, and leads to ∼20% higher field enhancement relative to a quarter-wavelength thick dielectric layer
