We investigated the radiation pattern of an optical rod antenna and found that it had many features compared with its conventional radio-wave equivalents. After defining a parameter {Lambda} = {lambda}eff /{lambda}, which was the ratio of the effective wavelength of the rod antenna to the incident wavelength, we found that {Lambda} had a great influence on the radiation pattern. Even the radiation pattern with a higher resonant order is without side lobes and results in a sharper directivity, which provides new design flexibilities in improving the directivities of the optical antennas.
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