Nanostructures based on metallic materials can modulate the amplitude, phase, and polarization of electromagnetic waves owing to their surface plasmon resonance (SPR) properties. The interference between bright and dark modes forms Fano resonances in metamaterials. Excitation of the dark mode can effectively suppress far-field radiation and enhance near-field radiation. However, the significant heat loss of metallic materials limits their application in optics; therefore, only a few superconfigurable materials based on surface plasma excitations can be used in practical applications. Recent studies have shown that highly refractive index all-dielectric nanostructures with low absorption properties do not undergo heat loss, thus facilitating the realization of high-performance compact devices. In this study, we designed a fully dielectric nanopillar supersurface with a high Fano resonance quality factor, Q, and modulation depth. We hope our design can provide innovative ideas for asymmetric transmission, polarization angle detection, and super-surface multifunctional multiplexing.
In this study, the Fano resonance theory was simulated around a fully dielectric supersurface material. Maxwell' s equations describe the electromagnetic-wave propagation law in space, and the equations can be solved to determine the response of the supersurface to the incident light. However, the analytical solution of Maxwell' s equations cannot be obtained in general; therefore, the simulation results are typically obtained by solving a system of equations using numerical methods. The two widely used solution methods are the finite element method (FEM) and the finite difference in the time-domain method (FDTD). We used the FDTD Solutions software to simulate the supersurface and perform high-precision simulations to replace the more expensive prototype experiments. The periodic boundary conditions were set in the x- and y-directions owing to the periodicity of the superlattice structure, and a perfect matching layer (PML) was set in the z-direction. In addition, the polarization plane wave was vertically incident in the negative direction of the axis. Simulations were performed sequentially by changing the nanopillar structure to analyze the Fano resonance generation mechanism.
The designed full-dielectric supersurface has a high-quality factor, Q, and modulation depth. Flexible modulation from single-Fano resonance to double-Fano resonance can be achieved by increasing the number of nanocolumn rows. The transmission spectrum of the first simulated single-row nanocolumn and the electromagnetic field distribution show that the Fano resonance (Fig. 3) was generated by a toroidal dipole but with a decreased quality factor. The coupling between the nanocolumns can be modulated by increasing the number of nanorows such that the toroidal dipole (TD) and magnetic dipole (MD) jointly dominate the dark mode, thus increasing the quality factor and enhancing the near-field coupling (Fig. 6). The final increase to the three rows of nanopillars achieves a double-Fano resonance. The first Fano resonance peak is formed by the TD and electric dipole (ED) resonance when the scattering power values are equal, and both interfere to cancel out each other to produce a radiation-free anapole mode. The second Fano resonance peak is formed by the resonant interference of the TD and MD to form the dark mode, whereas the remaining resonant modes act in the dark mode. The interference between the two modes forms the Fano resonance peak (Fig. 9). The sensitivity of methane volume fraction and the background refractive index can be measured simultaneously, and the simulation calculations show that the sensor has a high sensitivity (Fig. 12).
Based on the high Fano resonance quality factor, Q,
