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We show a new class of nanogap antenna structure that has chiral dissymmetry - the field intensity at the nanogap is high for a particular handedness of circularly polarized incident light, while not for the other. To find such a structure, we employed a computational inverse design technique, called topology optimization. With circularly polarized incident light, we found the mathematical algorithm is capable to find a complex spiral-like structure that is difficult to attain only by our intuition or knowledge. We calculated the field enhancement at the nanogap to evaluate dissymmetry against the handedness of the circular polarization. The resultant factor of dissymmetry was as high as 1.40, demonstrating the validity of the chiral antenna structure as well as the topology optimization techniques for the design problem of nanophotonic structures involving a complex electromagnetic field such as circular polarization or optical vortex.
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