Study On The Mechanisms Of Terahertz Metamaterial Absorbers And Its Application

In recent 20 years, metamaterial has attracted significant attention owing to their exotic electromagnetic properties that are unavailable in nature, such as negative index of refraction, cloaking, and perfect lensing. Because most metamaterials are metallic resonant structures and rely on strong electromagnetic resonances, the absorption loss is inevitable. The existence of the absorption losses degrades their resonance performance. Different novel ways to reduce the resonance losses have been proposed. Indeed, everything is double-side. Instead of trying to reduce the losses, novel idea has been presented to build resonant absorbers with metamaterial.For the metamaterial absorbers, the absorption losses become useful and can be enhanced significantly by proper design of the structure. Based on the FDTD calculation method, the proposed thesis theoretically investigates the absorption properties of the several sub-wavelength terahertz(THz) metamaterial absorber structures, and some meaningful results are obtained.The main works of this thesis can be simply summarized as the following three parts:Multi-band or broadband terahertz metamaterial absorbers: This section presents five different kinds of THz metamaterial absorbers. Firstly, we theoretically investigate a broadband terahertz metamaterial absorber based on the multi-layer same-sized structure. Greater than 99% absorption can be obtained across a frequency range of 0.3 THz. The relative absorption bandwidth of the proposed absorber can be up to 42%. The proposed absorber operates quite well at a very wide range of incident angles. The broadband absorption is attributed to the longitudinal coupling between layers. Moreover, a hybridized charge distribution model is proposed to analyze the origin of the absorption bandwidth. Secondly, we present a simple design of ultra-broadband and polarization-insensitive terahertz metamaterial absorber based on only a double layered composite structure. Compared with previous reported absorber with 20-layer stacking, the designed absorber structure not only provides a compact unit cell but also has the characteristic of easy fabrication. Furthermore, the design provides a considerable freedom to broaden the resonance bandwidth by reshaping the geometrical dimensional of the proposed structure. Thirdly, we present a coplanar broadband and polarization insensitive THz metamaterial absorber based on twodouble-ring structure. The results show that the relative absorption bandwidth of the proposed device can be up to 42%, and its absorption property works well even when the incident angle is equal to 60. Besides, we propose a novel resonance hybrid theory to study the interaction of the two rings. Compared with previous presented stacked broadband absorber structure, the proposed coplanar metamaterial structure has more practical significance. Fourthly, a novel dual-band THz metamaterial absorber based on a metallic strip is theoretically demonstrated. The calculation results show that the besides a near unity absorption in the fundamental resonance, a notable high-order resonance with near perfect absorption is also appeared. More importantly, the quality factor(Q) and figure of merit(FOM) of the high-order are both larger than that of the fundamental mode resonance absorption peak. This absorption characteristic make the proposed structure has a great application potential in biological monitoring and sensing. Fifthly, we propose a four-band THz metamateri absorber based on four square metallic rings. The proposed structure has four distinctive resonance absorption peaks, and those absorption peaks are average over 97%. Importantly, the resonance frequency of each absorption peak can be easily controlled by changing the length of the corresponding metallic ring. The concept is suitable to other types of the structure and can readily be increased and decreased the structure sizes that are working other range of the frequencies.Tunable terahertz metamaterial absorbers: This section proposes three different kinds of THz metamaterial absorbers. Firstly, a composite metamaterial absorber structure based on two concentric patterned metallic square rings is numerically investigated and studied. It is found that there are two resonance absorption peaks when the two rings are concentric, while the number of the absorption peaks will change with the relative positions of those two rings. Particularly, the bandwidth of the device can be effectively tuned by shifting the position of the inner ring. We also investigate the effect of the incident angle on the resonance bandwidth of the proposed structure. Secondly, we demonstrate a frequency tunable metamaterial absorber in the THz region. The resonance frequency of the absorber can be continuously tuned by changing the relative position of the top and bottom metallic layers. Particularly, the dual-band, triple-band and even broadband tunable absorbers can be obtained by employing the similar method. The presented frequency continuous tunable absorbers have great potential application prospect in sensing,detection and imaging. Thirdly, we theoretically propose a new type of frequency tunable metamaterial absorber which works in the super-subwavelength scale. Up to80.2% frequency tuning can be obtained by changing the temperature of the metamaterial absorber, and there is very little change in the absorption strength. The shift of the resonance frequency is attributed to the temperature dependent refractive index. Moreover, at 0.111 THz, the ratio between the cell period and the resonance wavelength can be close to 1/36, which is much smaller than previous reported absorbers. Besides, the design concept is applicable to other types of absorber structure, like metallic cross and metallic circular patch. Our findings offer a new way of designing novel temperature-dependent metamaterial devices.Low-conductivity alloy metamaterial absorbers: This section demonstrates two different kinds of THz metamaterial absorbers. Firstly, we first study a low conductivity composite metamaterial structure, and its optical properties are theoretically investigated. Compared with the conventional high-conductivity(Au as an example) metamaterial absorber, the proposed alloy metamaterial absorber has three advantages: first and most importantly, the relative absorption bandwidth of the absorber can be up to 38.8%, which is about 3.6 times larger than that of the Au absorber; secondly, the majority of the electromagnetic energy is dissipated in the alloy layer of the absorber, which is helpful for increasing the choice the scope of the dielectric layer; thirdly, using the proposed structure can replace noble metal material,which can reduce the pollution of the environment. Moreover, we propose an ultra-broadband metamaterial absorber by simply stacking three different sized alloy patches, and the relative absorption bandwidth of the absorber can be up to 70.4%.Our study could be useful for designing the broadband or ultra-broadband terahertz metamaterial absorber based on the low conductivity alloy pattern. Secondly, a coplanar terahertz alloy metamaterial absorber formed by only a square metallic ring is numerically studied. The calculation results show that the relative absorption bandwidth of the absorber is greatly improved to 101.4%, which is much higher than previously reported results. The mechanism of the broadband absorption involves the overlapping of two different resonance modes. Compared with the broadband absorber with stacking multiple different sized alloy structures, the coplanar structure has the characteristics of simple structure and convenient construction.