Multipolar scattering analysis of a hybrid metal-dielectric stacked nanoantenna

Hybrid structures that combine dielectric resonators with plasmonic structures hold great promises due to the diversity of optical modes they possess. Here, we explore the physics underlying the scattering response of a hybrid nanoantenna made of a metal disk placed on top of a dielectric cylinder and study the hybridization of the different modes excited in the dielectric and metallic parts. Surprisingly, we note that the signature of an anapole state – usually only seen in high refractive index dielectrics – can be observed in the metallic part of the system. The Cartesian multipoles excited in the dielectric and metal interfere in a complex manner, leading to an unexpected high-order vector spherical multipolar response in the far-field. These effects are thoroughly studied in terms of the near-field and absorption enhancements. We also show that very fine control over the multipoles' resonant positions can be achieved by varying the geometry of the structure. This flexibility renders this system very promising for sensing applications. Based on these developments, we have designed and fabricated such hybrid nanoantennas using silicon and aluminum and measured a preliminary sensitivity of 160 nm/RIU, which is competing with conventional sensors based on localized surface plasmon resonances.