| Static toroidal dipole plays an important role in parity violation of weak interactions and has served in many fields,such as particle physics,nuclear physics,ferroelectric systems and some molecular physics.The dynamic toroidal dipole is generated by the time varying current flowing along the meridian of a torus.It is because of its unique charge-current structure that it has many fantastic characteristics.Due to the vortex magnetic field of the toroidal dipole,the field generated by the dipole can be well limited in the structure.The toroidal dipole,which is excited by the time derivative of the electric field,has the same parity and radiation field with electric dipole.So,it is covered by the traditional multipole for a long time.By exploiting the flexible unit structure and the adjustable electromagnetic parameters of metamaterial,the toroidal dipole with very slight in natural materials can increase to the measurable range by several orders of magnitude,even higher than the electric dipoles and magnetic dipoles.It makes the toroidal metamaterial possess many novel properties,such as resonant transparency,ultra-high Q values,ultrasensitive sensing,absorbing property,and nonradiating effect due to interference with the electric dipole.In this dissertation,the toroidal dipole and anapole realized by metamaterial are studied and analyzed from the aspects of theoretical calculation,electromagnetic simulation,parameter optimization,transmission characteristics,near-field analysis and experimental verification.A series of work has been carried out through the model design,optimization and experimental verification.The main research of this dissertation is as follows:(1)An analoge EIT toroidal metamaterial is presented by using a pair of asymmetric split resonant ring.The proposed subwavelength structure has a narrower transmission window in the microwave range.The transmission peak can reach 0.91 at resonant frequency 13.4GHz and the calculated Q-factor is 55.5.As excitated by normal electromagnetic wave,the designed planar toroidal metamaterial can realize high transmission with low loss.By analyzing the scattered power of each multipole and transmission curve,it can be found that the toroidal dipole is the predominate multipole at the peak of transmittance.In other words,it is thanks to the slight asymmetry of the structure that the toroidal dipole is rapidly strengthened and the electric dipole is quickly suppressed near the transparent window.By splitting the structure of first planar toroidal dipole metamaterial,a rotational symmetry toroidal metamaterial can be reconstituted.Fano resonance with high Q-factor can be carried out at 9.11 GHz.Characteristics of toroidal dipole resonance are qualitatively analyzed by surface current and magnetic field distribution.The relationship between the polarization of the incident electromagnetic field and the toroidal dipole induced in the structure is analyzed to research the excitation mechanism of the intriguing toroidal dipole.(2)An all-dielectric toroidal dipole metamaterial is designed by using TP2 with high dielectric constant.When stimulated by the perpendicular incident electromagnetic wave,the structure achieves Fano resonance with a high Q-factor at two resonant frequencies of 10.51GHz and 16.27GHz,and its transmission peaks are 0.92 and 0.9 respectively.It can be calculated that the Q-factor of the two resonance peaks are 144 and 186 respectively.By calculating the far-field scattered power of each multipole in the metamaterial,it can be seen that the toroidal dipole is strengthened and the electric dipole is suppressed at both resonant frequencies.The experimental result is basically in agreement with the simulation.The reasons for the difference between experimental test and simulation result are discussed in detail.The relationship between transmission spectrum and scattered power of toroidal dipole has been analyzed in different structural parameters.In addition,the water toroidal metamaterials is realized by 3D-printing technology.The external structure which contains distilled water in our design is printed by 3D-printing.Due to the flexibility of 3D-printing,it also provides more possibilities for the metamaterial design.The proposed metamaterial can be excited by normal incident electromagnetic wave at 1.65GHz.And the transmission peak with high Q factor can achieve 0.995.As the conductivity of water increases,the transmittance decreases rapidly at toroidal resonant response.Due to the natural friendliness,the water toroidal metamaterial provides a new path for the application of all-dielectric metamaterials in the future.(3)A compact anapole metamaterial,which is comprised of copper strips in different layers,is proposed to absorb incident electromagnetic waves.Due to the constructive interference between toroidal and electric dipole,the transmission reaches the lowest value near the resonance frequency.At the same time,the reflection curve is down to the lowest point caused by impedance matching between structure and free space.The experimental results are basically consistent with the simulation ones.To analyze the influence of peculiar anapole in the presented construction,the relation between scattered power difference and absorptivity of anapole absorber is studied.The anapole absorber with high sensitivity in refractive index,thickness and liquid concentration can be introduced in high sensitivity sensor.The proposed anapole structure in this chapter can be used in terahertz and optical metamaterial,and provides a new implementation method for perfect absorption,ultra-sensitive sensing and invisible information transmission.(4)A toroidal planar metamaterial in terahertz range is implemented by utilizing a pair of split resonant ring to come about a high Q-factor transmission deep at 2.87THz.The Q factor of designed subwavelength structure can reach 31 at resonance frequency.By analyzing the distribution of surface current and magnetic field of the structure and the far-field scattered power of each multipole,it can be proved qualitatively and quantitatively that the toroidal dipole play an important role at the transmission valley in this construction.The proposed toroidal metamaterial has high sensitivity to the refractive index,which can be used in the design of terahertz high-sensitivity sensors.In this dissertation,the intriguing toroidal dipole can be realized by metamaterial characterized by adjustable electromagnetic parameters.The transmission characteristic of toroidal dipole are studied through modeling,simulating and experimental verification.It is of great significance to design resonant transparency,absorber,ultrasensitive sensors by using toroidal dipole in the future. |
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