Pairing Toroidal and Magnetic Dipole Resonances in Elliptic Dielectric Rod Metasurfaces for Reconfigurable Wavefront Manipulation in Reflection

摘要

A novel approach for reconfigurable wavefront manipulation with gradientmetasurfaces based on permittivity-modulated elliptic dielectric rods is proposed.It is shown that the required 2π phase span in the local electromagneticresponse of the metasurface can be achieved by pairing the lowest magneticdipole Mie resonance with a toroidal dipole Mie resonance, instead of using thelowest two Mie resonances corresponding to fundamental electric and magneticdipole resonances as customarily exercised. This approach allows for the precisematching of both the resonance frequencies and quality factors. Moreover,the accurate matching is preserved if the rod permittivity is varied, allowingfor constructing reconfigurable gradient metasurfaces by locally modulatingthe permittivity in each rod. Highly efficient tunable beam steering and beamfocusing with ultrashort focal lengths are numerically demonstrated, highlightingthe advantage of the low-profile metasurfaces over bulky conventionallenses. Notably, despite using a matched pair of Mie resonances, the presenceof an electric polarizability background allows to perform the wavefront shapingoperations in reflection, rather than transmission. This has the advantage thatany control circuitry necessary in an experimental realization can be accommodatedbehind the metasurface without affecting the electromagnetic response.