On The Multipole Resonances In Metamaterials

In the recent decade,metamaterials have attracted much research attention since various exotic electromagnetic properties beyond the scope of conventional materials can be achieved through designing the unit cell of metamaterials,such as negative refraction,abnormal transmission,electromagnetic cloaking,etc.The electromagnetic properties of metamaterials are mainly determined by the multipole resonances of the unit cell in the subwavelength scale.Multipole resonance is a classic topic in the field of electromagnetics.In the past,the classical electromagnetic multipole expansion fall into two major categories: electric and magnetic multipole families,such as electric and magnetic dipole,electric quadrupole and magnetic quadrupole moment,etc.In contrast,toroidal multipoles have been ignored for a long time since their far-field distribution are similar to that of the coreesponding electric/magnetic multipoles,and their response in conventional materials is very weak.However,the near-field distribution,the symmetry under time and parity inversion,the Q factor,and the interaction with external waves of the toroidal multipoles are quite different from those of electric/magnetic multipoles.Recently,researchers find the toroidal dipolar resonance in a metamaterial consisting of four metal split-rings with strong magnetic resonance.These unique characteristics of toroidal dipole moment have attracted a lot of attention and the toroidal moments become a research hotspot in recent years.Based on the previous studies,we discuss the realization of toroidal dipole by utilization of solely electric dipoles and the interaction between this toroidal dipole and other multipoles.The thesis is organized as follows:In the first chapter,we briefly introduce the metamaterial and the toroidal multipole moment.In the second chapter,we briefly introduce the relevant theories and simulation methods.In the third chapter,we come up with a discrete dipole model for the toroidal dipolar resonance.Based on this model,we found strong toroidal dipolar resonance in the plasmonic metamaterial.We then study the toroidal dipolar resonance in the plasmonic nanoparticle clusters,and find that the toroidal dipolar resonance in the nanoparticle clusters is less angularly dependent compared to that in the system of three plasmonic particles.Finally,by replacing the nanoparticles by half-wave antennas,strong toroidal dipolar resonance is demonstrated experimentally in the microwave regime.In the fourth chapter,we discuss the unidirectional scattering,Fano resonance and anapole which are both induced by the interference between toroidal dipole and other multipoles.We find the unidirectional scattering in the above discrete dipole model.The toroidal dipole plays an important role in the unidirectional scattering.Fano resonance is also realized by using the interference between toroidal dipole and electric dipole in the coaxial cylindrical metamaterials.At last,we demonstrate an anapole in the discrete dipole model due to the interference between toroidal dipole and electric dipole.These toroidal-based unidirectional scattering,Fano resonance and Anapole may have certain applications in optical sensing and nonlinear optics.In the fifth chapter,by the introduction of SRRs to spoof SPPs,we propose a one-dimensional strip of connected SRRs that can support surface modes.In our proposed metamaterial,the spoof SPPs can be modulated effectively by varying the gap width of the SRRs in addition to the corrugation depth.As an example,a slow-light rainbow effect in the graded SRR strip is demonstrated.Our proposal can find applications in slow-wave and frequency-splitting devices in the microwave and terahertz regimes.The last chapter,we give a summary and outlook.