| Exploration and design on novel metamaterial devices and research on performance improvement of conventional microwave devices by using metamaterials have become the most cutting-edge hotspots in the world. The main contents in this dissertation are set in the background of metamaterials and include: the metamaterial absorber(MA) based on the fractal structure, the MA based on the frequency independent structure(FIS), the split-ring resonator(SRR) based on the fractal structure and applied in terahertz sensing area, the coupled-line coupler based on the composite right-/left-handed transmission line(CRLH TL).1. Two novel MAs based on the fractal Koch and Sierpinski structure respectively were presented. Both of the fractal MAs have dual-band, polarization insensitive absorption peaks and can perform well at a wide range of incident angles. Compared with the conventional dual-band MA, the fractal MAs presented in this dissertation have three notable advantages: firstly, the dual-band operation of fractal MAs is not originated from the hybrid or stacked methods as reported before, but from the self-similarity of the fractal structure. All the unit cells of the fractal MAs are arranged on the layer and have the same geometry, size and orientation, so the disadvantages of larger unit cell suffered from the hybrid method and manufacturing difficulties suffered from the stacked method can be avoided. Secondly, due to the space-filling characteristic of the fractal structure, at the same operating frequency, the unit cell of the fractal MAs has more compact size as compared with the conventional MA. Thirdly, another important characteristic of the fractal MA is that its second absorption frequency is higher than the conventional MA, which can be used to breakthrough the limitation of the machining accuracy and create an optic-band MA with higher operating frequency. Both the effective medium theory and the multi-reflection interference theory have been employed to investigate the underlying physical mechanism of the fractal MAs. Two fractal MAs based on the same Sierpinski curves but operating in THz region and microwave region respectively have been fabricated and tested. Both of them have good consistency in terms of absorption performance and dimension compression capability, which verify that the MA is a kind of structure with good scalability.2. A novel MA based on the frequency independent structure—the Archimedes spiral with four arms was proposed. The FIS is one type of structure which has the properties of self-similarity similar to the fractal structure but does not have the space-filling characteristic, so the MA based on the FIS can exhibit the performance of multiple-frequency absorption but have no advantage in size miniaturization. The fill factor of the resonator array of the FIS MA was analyzed for the first time. It is found that the sparse array with fill factor of 25% can still obtain similar absorption performance with the full-filled array at the cost of thicker substrate. By means of filling the unfilled area with tuning units, the combination of the multi-resonance unit and the sparse array can increase the absorption band or expand the existing absorption bandwidth.3. The sensitivity of SRR for terahertz sensing application has been analyzed. It is found that the resonant frequency originated from LC resonance under TE polarization has no advantages in terms of detection sensitivity as compared with the resonant frequency originated from the dipole resonance under TM polarization. Under the influence of the thickness or the constant dielectric of the analyte, both of these two kinds of resonant frequencies have same trends of frequency shifts. In fact, the detection sensitivity is only influenced by the detection frequency: the higher the detection frequency, the greater the frequency shifts and the higher the sensitivity. The fractal SRR structure based on the Sierpinski curve was proposed in the terahertz sensing application. Under TM polarization, the high order resonant frequency of the fractal structure is higher than the conventional SRR with the same unit size. In this dissertaion, the maximum frequency shifts are achieved at the second resonant frequency of the fractal SRR based on the 1st order Sierpinski curve.4. A novel coupled-line coupler constituted by two resonant-type composite right-/left-handed transmission lines(CRLH TLs) was proposed in this dissertation. CRLH TL is a kind of transmission line which has a left-handed transmission band at low frequency and a right-handed transmission band at high frequency, between them there is a frequency gap where β =0 and α >0. Different from the conventional coupled-line coupler, the coupling level of the CRLH coupler is determined by αl rather than βl. When αl is large enough, it can be verified by the formula that the CRLH coupler has the potential to realize the 0-dB coupling level. The design process consists of three steps: First, an equivalent circuit model and procedure for circuit parameters extraction are presented to reveal the inherent nature of the unit cell of the CRLH coupler. Then a CRLH TL composed of four cascaded unit cells is demonstrated to point out the way to achieve the balanced condition. At last, even/odd modes analysis based on full-wave simulation is employed to explain the operating principle of the coupler. The maximum coupling coefficient 0.52 dB is obtained at 1.96 GHz with fractional bandwidth of 25.8%. By adjusting the structure parameters, a 3-dB CRLH coupler with the bandwidth from 1.99 GHz to 2.23 GHz has also been presented to verify that this design approach not only has the ability to achieve full coupling but also has a good flexibility. Owing to its resonant structure, the CRLH coupler has a narrower coupling bandwidth, but it can take advantage of fully planar fabrication techniques and is ease of integration, because no vias are needed in this structure as compared with the transmission-line-type CRLH TL. |
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