Geometry-Independent Excitation of Dark Modes Using Dipole Moment Transitions

Abstract

Despite numerous potential advantages of subradiative dark modes due to an extremely high quality (Q) factor, the practicality still depends on the consistency of excitations for a tight fabrication tolerance. This article proposes a novel approach to adaptive transitions between different dipole moments to allow for consistent excitation of Fano and electric quadrupole resonances that are extremely sensitive to geometric errors. The proposed geometry of each metamolecule consists of four strips printed on a thin dielectric film with four gaps between the strips, and the geometric error is described as structural deformation by adjusting the strip lengths and gap sizes in the vertical direction. In each deformed state, current and magnetic-field distributions are analyzed to interpret operating principles, and multipole moments are calculated based on the multipole expansion using the current distributions. The results demonstrate that the consistent excitation of the electric quadrupole moment is enabled based on a simultaneous transition between toroidal and electric dipole moments, and the Fano resonance is consistently excited by the magnetic-to-electric dipole moment transition. For verification, original and deformed metasurfaces are fabricated, and their transmission and reflection coefficients are measured in a semianechoic chamber to experimentally validate the proposed dipole moment transitions. The results confirm that the proposed approach has great potential to bring advances in recent sensing and energy harvesting applications where an extremely high Q-factor is essential.