| The toroidal dipole is one of basic electromagnetic responses.It has many novel electromagnetic properties,but its response is weak and it is often masked by other electrical / magnetic responses.The emergence of metamaterials has provided an effective means for studying toroidal dipoles.In order to investigate the electromagnetic characteristics of the toroidal dipole phenomenon in the terahertz band,model design and simulation were carried with the help of the CST MICROWAVE STUDIO software,and carried out processing tests for experimental verification.Firstly,terahertz toroidal dipole metamaterials based on flexible substrate polyimide(PI)was designed.Two kinds of structure of square split ring resonator(SRR)and circular SRR were designed for numerical simulation.Two obvious resonance points were observed.The multipole moment scattering power formula was used to quantitatively analyze the response mechanism of the dual-frequency resonance point.The effect of structural parameters on the resonance point,Q value,surface current,and magnetic field distribution are investigated.It was found that the change of the structure parameters caused the phenomenon of red shift and blue shift of the resonance point,and the Q value also changed accordingly.Secondly,in order to simplify the production process and improve the quality of the samples,the two-layer C-type SRR metamaterials structure was simplified to a single-layer C-type SRR metasurfaces structure.The single-frequency toroidal dipole response based on flexible dielectric PI was designed and optimized using simulation,and correspondingly samples were fabricated.The multipole moment formula and field analyses were used to prove that the response mechanism of the resonance point was toroidal dipole response.The formula of the LC resonance frequency quantitatively explains the reasons to the blue and red shifts of the resonance points with the increase of the opening angle(θ)and the outer ring radius(QR),respectively.Through the comparison of single and double metal layers structures,it is found that the bandwidth decreases and the Q value increases as the number of metal layers increases.Finally,the dual-frequency toroidal dipole response based on flexible dielectric Mylar was studied using the same C-type SRR metasurfaces structure.The scattering power was calculated using the multipole moment formula,and it was proved that the main contribution of the low-frequency and high-frequency resonance points was provided by the toroidal dipole moment(Ty).The LC circuit model was used to quantitatively explain the blue shift of the resonance point as θ increased and was proved the accuracy of the calculation and simulation.By calculating the Q and FoM values of different outer ring radius(QR),it is found that these have a good consistency of Ty and FoM,and it also shows that the increase of Ty and the decrease of radiation loss leading to high frequency Q value,which are higher than the low-frequency Q value.Through comparison of the flexible substrate materials,it is found that both the Mylar and PI materials can reflect the characteristics of the C-type SRR metasurfaces.At the same time,the experimental and simulated results agree that the Q value based on PI substrate is higher.The terahertz metamaterials structure based on flexible substrate provides a new solution for studying the characteristics of toroidal dipoles,and also provides directions for designing terahertz functional devices such as absorbers and sensors. |
