Investigation On The Properties And Applications Of Subwavelength Metallic Structure In Terahertz Regime

Terahertz wave, which is also called sub-millimeter wave, is a kind of electromagnetic wave lying between millimeter wave and infrared. Recently, with the rapid technical progress in terahertz generation and detection, the interest in terahertz research has enhanced for the potential application of terahertz in imaging, communication, biosensing and defense security inspection. However, since there are only a few natural materials which could response in terahertz regime, basic optical elements for building terahertz system are quite lacking. So, it is of much importance to investigate and fabricate various terahertz devices. One the other hand, metamaterial, which is an artificial composite material composed of subwavelength metallic structure, by changing unit cell construction, could exhibit any macroscopic electromagnetic character including those unexisting in nature. This property adds much freedom to designing terahertz devices. Hence, investigation on the properties of subwavelength metallic structure in terahertz regime, not only is useful for designing terahertz devices and practical realizations, but also is helpful for understanding metamaterial as well as providing reference designing method for metamaterial in other frequency regime.Utilizing numerical simulation by finite element method, in conjunction with analytical model by effective medium theory, efficient transmission line theory, frequency selective surface and other optical theroy, the resonance principle of subwavelength metallic cut-wire pairs in teraertz regime is investigated, and two terahertz polarization devices are designed. The primary results of this thesis are outlined as follows:1. We have investigated the principle of symmetric and unsymmetric resonances of subwavelength cut-wire pairs in terahertz regime. By analyzing the influence of wire width and wire spacing on the transmission resonance shape, it is concluded that the symmetric resonance is mainly attributed to the Fabry-Pérot interference while the unsymmetric resonance results from coherence between continius dipolar resonance background and the discrete Fabry-Pérot resonance spectrum.2. We have designed and optimized a terhaerz polarizer with very high extinction ratio. After arguing the possibility of realizing a higher extinction ratio with double metallic grating, we designed a terahertz polarizer working from 0.3THz to 3THz with more than 180dB extinction ratio and more than 97% transmission of TM wave. The grating period is only 3μm. 3. We have designed an achromatic quarter-wave retarder for terahertz radiation. The phase retarder is proposed to work in reflection mode with a thickness of about 38μm. From 1.8THz to 2.8THz, it only produces 2°variation around 90°of phase retardation, whose working frequency band is about ten times of the ordinary phase retarder's. Furthermore, we discovered that this achromaticity stems from the Fabry-Pérot interference.