High refractive index dielectric rings in liquid crystals as tunable metasurfaces

Abstract

Dielectric structures with high refractive indices give rise to Mie-type resonances with a remarkably high localization of electric and magnetic fields inside the dielectric structures due to excitation of selective multipolar modes. It is known that the specific geometries like rings give rise to magnetic responses comparable to the electric modes in metallic structures. In the present work, we study the resonant characteristics of high refractive index circular and square dielectric rings using full-wave electromagnetic simulations. Effect of geometry, thickness of the structures and refractive index on the electromagnetic response is studied in the sub-wavelength regime. Square rings are observed to exhibit sharper resonances at lower frequencies compared to the circular rings as a consequence of enhanced field localization within the dielectric structures. It is observed that the thickness plays a major role in the excitation of various resonant modes. Electric modes are predominantly sustained in the thinner rings (thickness lesser compared to the lattice constant) while thicker rings support magnetic modes predominantly. When such rings are placed in birefringent liquid crystal medium orientation of the liquid crystal molecules are observed to enhance certain modes over the other giving rise to tunable metasurfaces. Further, novel modes such as electric dipole+electric quadrupole and toroidal modes are observed, hitherto not observed in air.