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.
Sub-wavelength periodic metallic nanostructures give rise to very interesting optical phenomena like effective refractive index, perfect absorption, cloaking, etc. However, such metallic structures result in high dissipative losses and hence dielectric nanostructures are being considered increasingly to be an efficient alternative to plasmonic materials. High refractive index (RI) dielectric nanostructures exhibit magnetic and electric resonances simultaneously giving rise to interesting properties like perfect magnetic mirrors, etc. In the present work, we study light-matter interaction of cubic dielectric structures made of very high refractive index material Te in air. We observe a distinct band-like structure in both transmission and reflection spectra resulting from the interaction between magnetic and electric dipolar modes. FDTD simulations using CST software are performed to analyse the different modes excited at the band frequencies. The medium when replaced with liquid crystal gives rise to asymmetry in the band structure emphasizing one of the dominant magnetic modes at resonance frequencies. This will help in achieving a greater control on the excitation of the predominant magnetic dipolar modes at resonance frequencies with applications as perfect magnetic mirrors.