Title: Anapole plasmonic meta-atom enabled by inverse design for metamaterials transparency

Abstract: Anapole states are broadly investigated in nanophotonics for their ability to provide field enhancement and transparency. While low extinction has been achieved in dielectric nanoparticles due to the absence of intrinsic losses, in the case of plasmonic nanostructures this is still lacking. Here, we report an easy-to-fabricate planar plasmonic nanostructure found via topology optimization, which exhibits an anapole state with close-to-ideal characteristics in the visible regime including weak absorption, high near-field enhancment, and strong suppression of scattering. The nanonantenna can act as an individual meta-atom because, due to low inter-coupling, it preserves its optical response even when used in highly packed metasurfaces and metamaterials. The low losses are due to the optimized topology which provides concentration of the field outside the structure with minimum penetration inside the metal. Compared to anapole states in dielectric structures, the accessibility of the volume of enhanced field is suitable for sensing applications. Anapole states are typically interpreted as the result of the interference between electric and toroidal dipole moments. Here and based on a novel approach, in the context of secondary multipole analysis, we introduce the concept of anapole state without using the contribution of toroidal dipole moments. The article provides new insight into anapoles in plasmonic nanostructures and ways to achieve them, while remarking the power of topology optimization to unlock designs with novel functionalities.