The classic electromagnetic expansion of of multipoles can be decomposed into electric and magnetic multipoles, primarily including dipoles, quadrupoles, octupoles, and so on. But, it was proved that this electromagnetic theory is not complete. So, in 1957, Zel’dovich firstly proposed the toroidal dipole to explain the parity violation in the weak interaction, not only solving the puzzles in physics but also completing the electromagnetic multipolar theory. Toroidal dipole arises from a current flowing on the surface of a torus along its meridian and presents a head-to-tail distribution of magnetic field, greatly strengthening the near-field localization. However, compared with other conventional multipolar responses, toroidal dipole response couples weakly to free space and is hardly to be detected. Therefore, toroidal metamaterials connect toroidal dipole response and novel optical behaviors, in terms of suppressing other classic multipole responses while strengthening the toroidal dipole resonance. In recent years, the interesting topic of toroidal metamaterials have attracted a growing attention. In this dissertation, based on the previous research, we further explore the optical properties of toroidal dipole response and optical effects. The overall frame of the dissertation is following:
In the first chapter, the research background is introduced. First, the proposal, excitation mechanism and optical properties of toroidal dipole resonance; Then, the proposal and evolution of toroidal metamaterial.
In the second chapter, the relevant theoretical basis and computing method is presented. First, the dispersion models: Drude model and Lorentz model; Then, electromagnetic numerical analysis methods: Finite element method and finite difference time domain method; Finally, the simulated softwares: CST Microwave Studio and HFSS.
In the third chapter, we explore the influence of geometric parameters of double-disk metamaterial as well as the incident angle of electromagnetic wave, which is explained by the LC-circuit model.
In the fourth chapter, the farfield-radiating of dipolar emitter can be manipulated by the toroidal dipole resonance, and the coupling physical mechanism is also explored. The study provides potential applications in light-matter interaction.
In the fifth chapter, the gain material is embedded into the variation structure of double-disk metamaterial and the effect of gain material on the super-radiating of dipolar emitter is also studied, promoting the development of light–matter interaction.
In the sixth chapter, an analog of electromagnetically induced transparency (EIT) occurs by the coupling between toroidal and electric dipoles. Besides, by tuning the geometric parameter, the dual-band EIT appears due to an asymmetric coupling between toroidal and electric dipoles. The study provides a new way for the sensor.
In the seventh chapter, to overcome the metal loss in toroidal metamaterials, a LiTaO3 microtube with high dielectric constant is proposed and, then, the effect of geometric parameters on the toroidal dipole resonance, intensity of field hot spot and Q-factor is studied. Besides, the physical mechanism is also explored.
In the eighth chapter, a depth-asymmetry circular groove metamaterial is designed to obtain the toroidal dipole resonance under normal incidence and the physical mechanism is explored. Above all, the optical effects including the field hot-spot and perfect absorption induced by toroidal dipole resonance is deeply explored.
In the ninth chapter, the conclusion and perspective.