The Numerical Study On Multiband Terahertz Left-handed Metamaterial

Left-handed materials (LHMs) are artificial materials that have negative electrical permittivity and negative magnetic permeability at the same frequency band. LHMs have attracted tremendous attention because of their exotic electric and magnetic properties and because of their potential applications to build so-called perfect lenses and invisibility cloak devices. In the past ten years there has been ample proof for the existence of LHMs in the microwave frequency range. Recently, researchers are trying hard to push the operating frequency of LHMs into terahertz and optical regimes.In the terahertz and optical ranges, constructing LHMs with low loss, broad passband, three-dimensional structure and easy fabrication to improve their performance has been a research focus in this area. On the basis of this research focus, we try to seek good terahertz LHMs. In this thesis, we propose a series of design schemes. We use numerical simulation methods to study the electromagnetic response of the left-handed materials. Moreover, we study the internal mechanism of LHMs to explore the feasibility of LHMs designs. The main content of this thesis is listed as follows.(1) We construct a dual-band THz left-handed material based on fishnet structure. The fishnet structure has the virtues of simple fabricated and easy operated in the experiment, so the structure is now considered as the best LHM suitable for the higher frequencies. We combine two different sized fishnet structure units into a dual fishnet structure. We calculate the effective material parameters using the S-parameters retrieval method, and find two left-handed bands from the results. Next, based on the effective LC circuit model we deduce the magnetic resonant frequency function, and theoretically analyze the mechanism of the dual-band negative refraction. Last, we make a detail study on losses of the metamaterial, and optimize the material components and the structural parameters to reduce the losses.(2) We propose a multiband terahertz left-handed metamaterial based onå·¥-model fractal structure. According to the self-similarity of the fractal structure, several localized resonances occur in the metallic fractal structure cell under the terahertz wave incidence, so the fractal metamaterial realizes multiple left-handed bands. We study the multiband left-handed properties under the electromagnetic wave normal incidence with two polarizations, and analyze the mechanism of the multiband negative refraction. Considering the practical application we disscuss the losses and calculate the figure of merit of the metamaterial in the multiple negative refraction bands.(3) We design a terahertz planar metamaterial with asymmetric cross plates. The metamaterial can provide multiple left-handed bands for normal and parallel incidence. There have been some reports on the realization of multiband LHMs with different geometrical structures in a unit cell. However, we propose the THz metamaterial with a single geometrical structure in a unit cell, from which multiple left-handed bands have been found. Owing to the characteristics of simple fabrication and compact structure, there is the possibility of fabricating THz compact devices suitable for multifrequencies. Under terahertz wave incidence in two different directions, the electromagnetic responses and the multiband left-handed performance of the metamaterial are analyzed and verified. According to the effective LC circuit theory, we derive the magnetic resonance frequency and electrical resonance frequency dependence on the structural parameters. Furthermore, we theoretically analyze the mechanism of multiband left-handed properties.(4) We design and investigate a three dimensional multiband left-handed metamaterial. The metamaterial is a "metal-dielectric-metal" sandwich layered structure with symmetrical cross, and then the symmetry reduces the effect of electromagnetic wave polarization on the metamaterial's properties. Meanwhile, in the manufacture, the three-dimensional structure can be obtained through mechanically stacking layer by layer, which reduce the processing difficulty of the three-dimensional left-handed materials. Then, the electromagnetic responses and the mechanism of multiband negative refraction are detail analyzed and discussed under electromagnetic wave normal incidence.