| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 104 === The researches on THz gap (0.1-30 THz) have attracted much attention in recent years because the THz gap is a transition regime between microwave and far-infrared (IR), i.e., the watershed between the electronic and optical responses so that matters in THz gap in nature possess weak electronic and optical responses, and such weak responses hinder generation and detection of THz signals. Therefore, we eager to apply metamaterials to THz devices based on the properties of metamaterials that is well developed in microwave, IR and even visible region such as their scalability and strong interaction with waves; based on these properties, we expect to expand the applications in the THz gap.
In this dissertation, we delve into three different applications at the THz gap. The first research topic is to construct a three-dimensional (3D) negative index medium (NIM) through rotating a split ring resonator. Such 3D NIM, unlike traditional NIM integrating two independent magnetic and electric resonators together, achieves electric and magnetic responses simultaneously and also negative index via a monolithic structure, that is, a metallic hemispherical shell; the shell could simplify the fabrication process of 3D NIM. Furthermore, this monolithic shell is independent of polarization and insensitive to incident angles due to its high symmetry that are favorable in practical applications. Next, in the second project, we modulate the frequencies of magnetic and electric resonances so that permeability and permittivity of the metamaterial intersect with each other to match the impedance of free space, thus leading to suppression of reflectance. On the other hand, via the resonance nature of metamaterials, the imaginary part of index is enhanced when resonance occurred, so transmittance is reduced. While reflectance and transmittance of a material are simultaneously approaching to zero, a perfect absorber could be achieved. Hence, in this topic, we employed stochastic design process to generate a double-sided metamaterial perfect absorber that is composed of a dielectric layer sandwiched by two identical metallic patterns and could absorb the electromagnetic wave from two sides, solving the drawback of traditional metamaterial absorbers, i.e., single operating direction. Noteworthily, such perfect absorber owns a sub-wavelength thickness, thus miniaturizing the devices and providing a promising future compared to conventional absorbers. Afterward, based on the previous stochastic design process, we consider increasing the efficiency of the design process of metamaterials with desired goals. Consequently, in the third project, instead of trial and error process, we utilize computer-aided genetic algorithms (GAs) to efficiently optimize the existing metamaterials and come out the best metamaterial patterns. A bandpass filter, an important unit on future THz communications, is chosen as our target to execute GA and then approach behaviors of ultra-broad fractional bandwidth 82.8% and band-edge transition of 58.3 dB/octave. Besides, the 38-μm-thick and optimized bandpass filter, which is much thinner compared to conventional THz filter. Such miniaturized ultra-broadband and sharp-transition filters profit the development of a THz optical system.
To summarize, in this dissertation, we devote ourselves into three different THz devices including 1. Polarization independent and high incident-angle tolerable 3D negative index media, 2. Stochastically designed double-sided perfect absorbers and finally 3. Ultra-broad bandwidth and sharp transition metamaterial THz bandpass filters via genetic algorithm. Such devices validate the exotic properties of metamaterials and would have a huge impact on the field of the THz gap.
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