| The terahertz (THz) spectrum (0.1–10 THz, 1 THz =1012Hz) represents a particularly interesting region. Terahertz frequencies cannot clearly be attributed to be either on the "electronic" side or on the "optics" side. The THz frequency radiation has been proven to be a fertile region in the electromagnetic spectrum and a powerful tool in scientific research and many applications. THz waves has significant transmission loss in free space, so waveguide-based terahertz devices have become very important foundation for the THz transmission, also the key to the wider use. Although enormous efforts have focused on the search for terahertz materials or alternative novel techniques to enable the construction of device components, much work remains. Photonic crystals (PCs) and metamaterials as a novel artificial material would play a more and more important role in the development of THz functional device and contribute to THz systems integration and miniaturization. The main contents are summarized as follows:1. Based on plane wave expansion method, the band gaps of two-dimensional (2D) THz PCs with typical square structures are optimized by changing structural parameters. The electromagnetic field distribution of THz waves in 2D photonic crystals functional device had been simulated through the finite difference time domain method. The improved T-splitter has better amplitude-frequency characteristics in pass-band and better separation degree between the two output ports than the common T-splitter. The improved Y-splitter excels to the common Y-splitter in the amplitude and -3dB bandwidth. The 2D THz splitter PCs components had been fabricated by modern micro-fabrication processes.2. Introducing the photonic band gap structure with L-type defects composed of three defects, three high-Q resonant frequencies had been simultaneously dropped (or added). The rightward T-type structure filter can extend the interval between two dropping (or adding) frequencies to 0.342 THz. The upload (or download) PCs filter had been fabricated.3. The cavity with three defect, perpendicular to the three-defect input and output waveguide, can simultaneously select two frequencies about 100% amplitude, and can be used in the dual-points or dual-band high-Q narrow-band filter. The leftward T-shaped structure formed by five single-defects has five transmission peaks. The amplitude-frequency property can be used in high-Q narrow-band filter.4. Three geometries of split ring resonators,a circular geometry and two elliptical geometries, are fabricated by a series of micro-fabrication processes for terahertz metamaterials. The samples are measured by the transmission spectroscopy in terahertz time domain. These transmission spectra had been compared for the change effect of the structure parameters and coupling coefficient on the amplitude-frequency characteristics. An improved Transmission-line RLC circuit model is used to help us understand this coupling behavior and the extent of its effects. The gap and gap-bearings are simultaneously in-plane rotated for widening the metamaterials separation between the inductive-capacitive (LC) and dipole resonances. According to improved TL-RLC model, another kind of resonant modes is studied by simulation software, and derived its expression of resonant frequency.
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