Recently, optical nano-antennas (NAs) have been introduced as an alternative approach for photovoltaics devices in solar power harvesting application. In this work, we introduce a new modification to the conventional Archimedean spiral NA to improve its radiation/harvesting efficiency and directivity. The proposed design is a rectangular spiral NA of two tip-to-tip opposing arms which are separated by an air gap. The reported design performance is investigated in terms of the radiation efficiency, directivity, polarization, radiation pattern and total harvesting efficiency. The numerical study is carried out using the finite integration technique (FIT) within the wavelength range 300-1600 nm. The presented design offers a maximum radiation efficiency of 88% in free space and 97.9% on top of silicon dioxide (SiO2) substrate at a wavelength of 500 nm where the maximum radiation of the sun occurs. In addition, the proposed design has a maximum directivity of 10.8 in free space which is increased to 19.1 on top of a substrate at 500 nm. It is found that the suggested rectangular design shows an enhancement in the radiation efficiency and directivity over the counterpart Archimedean nano-spiral antenna by 10% and 208%, respectively. The proposed rectangular design introduces total harvesting efficiencies of 96.2%, 98.1% in free space and on the substrate, respectively. Moreover, the effect of round edges that may appear in the fabrication process is also considered.
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