Research On Toroidal Dipole Metamaterials Based On Multipole Expansions

In order to describe the electromagnetic field excited by the charge-current system in the far-field,the electric and magnetic multipoles are introduced to classical electrodynamics.However,according to the space-time inversion symmetry,there should be toroidal multipoles.In this multipole,toroidal dipole,which is widely present in materials such as blood cells,multiferroic and DNA,has been ignored for some time,because its electromagnetic response is far lower than that of electric and magnetic dipoles.Recently,with its flourishing development,electromagnetic metamaterials have become a research hotspot that provides a new method for achieving significant excitation of the toroidal dipole.In this thesis,we study the effective excitation method of the magnetic toroidal dipole,electric toroidal dipole and the interaction between multipoles by dielectric and metallic metamaterials,based on the theory of multipole expansion,and prove it experimentally.The main research contents are as follows:(1)The excitation of magnetic toroidal dipoles in different spatial positions is achieved,by designing a magnetic toroidal dipolar dielectric metamaterial consisting of rectangular high-resistivity silicon bars array.The metamaterial can excite magnetic toroidal dipoles inside silicon bars and between silicon bars,and they do not interfere with each other.Because of the weak spatial coupling of magnetic toroidal dipoles and the interaction between them at different spatial locations,an ultra-high Q resonance can be generated with the Q value of 63,000.The dielectric metamaterial has potential application value in high sensitivity sensors and nonlinear devices.(2)The excitation of transparency resonance of anapole mode is realized,through designing a terahertz metallic metamaterial.It is a metallic sheet with dumbbell-shaped holes array.Due to the anapole mode generated by interference between the magnetic toroidal dipole and the electric dipole,the metamaterial can produce transparency resonance with a Lorentz-type transmission curve.When the gap of the center is changed,the resonance frequency can achieve move,but the anapole mode still is excited.By reasonably changing several structural parameters at the same time,the anapole transparency resonance is finally achieved in a wide band range from 0.15 THz to 0.91 THz.In addition,because of its simple structure and suppression of other multipole interference,the metallic metamaterial can be applied as a sensor,and the sensitivity is 0.135THz/RIU.At the same time,a terahertz tunable anapole metamaterial based on graphene is also proposed.By changing the chemical potential and the number of layers of graphene,the modulation of the anapole resonance is achieved for the first time.(3)The excitation of electric toroidal dipole is achieved,by designing a terahertz low refractive index metamaterial.The metamaterial can excite a typical electric toroidal dipole resonance at 0.18 THz.The influence of changing the structure parameters on the resonance is analyzed.What we find is that changing the rod's length and width only produces an effect on the resonance position,but changing the height of rod not only affects the resonance position but also modulates the excitation of the electric toroidal dipole.Finally,3D printing technology and terahertz time-domain spectroscopy are used to process and test the designed metamaterial.The experimental results are in good agreement with the simulation results.