Research On THz Metamaterials With High-Q Resonance

Metamaterials are some man-made media with all sorts of unusual functionalities that can be achieved by artificially structuring media with smaller than the length scale of the external stimulus.The exotic characteristics of metamaterials are mainly exhibited by means of resonances of the artificial structures.Therefore,High-Q(Central Frequency/FWHM,FWHM means full width at half maximum)resonance is one of the most important technical targets when we design a metamaterial.The loss in metamaterials is a main factor which deteriorates the Q value of the resonance.In the thesis,we carried out researches on how to acquire high-Q resonances by eliminating or reducing the radiative loss in metamaterials.Several new metamaterials with high-Q values are designed and the physical mechanisms are also systematically analyzed.The main research contents and innovations can be summarized as follows:1.A reflection-type electromagnetically induced transparency(EIT)metamaterial with high-Q value is studied.In this part,A CST simulation about the designed metmamaterials is performed.Reasons of the radiative loss reduction and EIT generation are analyzed by utilizing the near-field coupling effect.And constructive interference of the reflected EIT spectrum is also calculated by using the multiple-beam interference theory.We fabricated a reflection-type EIT metamaterials sample and acquired a high-Q(≈43)reflectance resonance at 0.865 THz.The experimental result is in agreement with the numerical results by CST simulation and theoretical calculation.2.A THz metamaterial with toroidal dipolar resonance is studied.In this part,we designed and fabricated a metamateral with two metallic pattern layers and acquired a-high Q(≈28)Fano line-shape resonance at 0.42 THz.Analysis shows that the high-Q Fano resonance is a combinative result of the near-field coupling between the in-plane ASRRs and the hybridization effect resulting from the interlayer.Both the magnetic-field distribution and the quantitative calculation of the scattered powers for various multi-polar moments verified that the high-Q Fano resonance is a toroidal dipolar reosonance.We also analyzed the factors which affect the stability of the torodial resonance,and found that a 15-μm-thick polyimide coating on the gold structure is necessary for the metamamterials to produce a stable resonance.3.A multi-wavelength THz metamaterial with high-Q values is studied.In this part,we designed a metamaterial which is composed by two sorts of ASRRs with different sizes.Three Fano resonances at 0.268,0.418 and 0.560 THz are acquired,and the corresponding Q values are 33,42 and 25,respectively.Analysis shows that the combination of the Fano effect due to the ASRRs,the near-field coupling of the in-plane ASRR pairs,and the hybridization effect resulting from the interlayer are the causes of the three high-Q Fano resonances.Based on the analysis,we further discussed the feasibilities of acquiring ultrahigh-Q resonance or freely controlling the degree of energy transition between the two excited ASRRs pairs by adjusting their sizes.4.A study of THz sensor based metamaterials with two high-Q resonances is performed.In this part,metamaterials which can simultaneously exhibit high Q Fano and quadrupole resonances are designed,and the Q values of the two resonances are 57 and 38,respectively.By comparing the resonance characteristics of the relative structures and their surface current distributions,we find that the broken symmetry of the structure is the cause of the two resonances.Based on the two high-Q resonances,we performed thickness sensing and refractive sensing for dielectric material,respectively.And the results show that the refractive index sensitivity levels of the two resonances are 2.06×10~4 nm/ refractive index unit(RIU)and 5.07×10~3 nm/ RIU,respectively.