Advanced anapole effects in dielectric and metallic metasurfaces

Anapole mechanisms have been broadly investigated in nanophotonics for the possibility to obtain invisible electromagnetic devices [1]. In the context of nanostructures with dipolar response, such states are typically understood as the result of destructive interference between quasi-static electric dipole and toroidal dipole. However, the richness of the physics associated with higher order multipoles open several possibilities to achieve anapole effects in other exotic ways. In this talk, two anapole mechanisms in dielectric and plasmonic meta-optics will be presented. First, an anapole mechanism in dielectric metasurfaces due to the interplay between high order multipoles will be discussed [2]. In a dielectric metasurface with inversion symmetry, multipoles of odd [e.g., electric dipole (ED), magnetic quadrupole (MQ)] and even parity [e.g., magnetic dipole (MD), electric quadrupole (EQ)] are decoupled [3]. Multipoles of each parity can exhibit an anapole state based on the destructive interference between multipoles of the same parity. This lattice anapole mechanism explains the origin of accidental bound states in the continuum (BICs) in symmetric dielectric metasurfaces. Specifically, we demonstrate that these states are nonradiating eigenmodes of the system. We exploit such modes for the realization of quasi-BIC with high-quality factor resonances. Contrary to existing techniques, our system allows us to achieve the transformation of a nonradiating BIC into a radiating quasi-BIC under normal radiation incidence and only due to a change in the period of the metasurface [2]. In general, such transformation can happen for other rather simple perturbations of the system, such as by introducing a variable superstrate, which is particularly suitable for sensing applications. Second, the talk includes a focus on anapole states in plasmonic meta-atoms, and a close-toideal plasmonic meta-atom will be discussed. This meta-atom is characterized by low absorption, high near-field enhancement, and negligible scattering due to the anapole effect [4]. We show that the anapole state of this metaatom can be qualitatively explained via destructive interference between quasi-static electric dipoles associated with complementary subvolumes of the nanostructure. We numerically demonstrate the use of this meta-atom as a building block for transparent metasurfaces and metamaterials.