| Terahertz waves refer to the electromagnetic waves with wavelength range between30～3000μm. Due to the fast development of the technology of ultrafast lasers, there has been great interest in terahertz wave applications nowadays. With a wide bandwidth and an excellent linear propagation direction, terahertz waves are particularly suitable for the wireless communication. So the terahertz functional devices such as intensity modulators, optical switches, filters et al. are essential for the researches of terahertz waves.In this thesis, the study including theories and applications of two kinds of terahertz subwavelength microstructure, photonic crystals and metamaterials, are discussed. Respectively, terahertz modulators and narrow bandwidth cavities made by these two artificial materials are discussed. The major contents and results are shown as follows:1. A terahertz time domain spectroscopy (THz-TDS) has been set up. Terahertz transmitter is based on photoconductive antenna, and terahertz detector is based on electro-optic crystal ZnTe. The quart-optical path (4f) of terahertz waves is constructed by four paraboloidal mirrors, and the sample are assumed to put in the point of the second’focus point. Terahertz spectroscopy of HDPE and Si plates were measured by this THz-TDS, the results showed that the indices and the spectroscopy of the two samples agree well with the available datum.2. A novel tunable photonic crystal (PC) waveguide Mach-Zehnder interferometer (MZI) based on nematic liquid crystals (LCs)5CB is proposed, which is composed of the photonic crystal line defect waveguides and splitters. The line defect modes of the PC waveguide with different liquid crystals refractive indices are analyzed by using the plane wave expansion method. Owing to the slow group velocity region of the line defect mode, when the index of5CB is changing by rotating its derector, the variation of the line defect mode’s effective indices is larger than the variation of the indices of liquid crystals. As a conclusion, the slow light in the line-defect of PC promotes the phase modulating efficiency of MZI. The efficiencies of MZI are1.8-5.7times larger than the phase modulator made of liquid crystal box.3. A pillar photonic crystal (PPC) with a tapered waveguide and a point defect to highly confine Terahertz wave is demonstrated. A PPC taper can provide low loss connections between waveguides with different cross-sectional areas. Terahertz wave is first guided into the tapered waveguide, gradually compressed to its end, and finally confined in the point defect cavity. The numerical simulations indicate that narrow band Terahertz wave is highly confined in the point defect cavity with the quality factor twice the value of the cavity without the tapered structure. Also, the coupling process is greatly extended, so the slow light effect is extremely strong. The demonstrated device may be used as an antenna for enhancing light-matter interactions in the point defect cavity at terahertz frequencies and improving the sensitivity of terahertz near field microscopy.4. A novel terahertz double split-ring resonator (DSRR) metamaterial which can optically tune the transmission of the terahertz wave is presented and demonstrated. Unlike the traditional DSRR metamaterials, the DSRR discussed in this paper is constituted by two split rings connected by two bridges. Numerical simulations with finite integration technique (FIT) method reveal that the magnetic response of this DSRR with connecting bridges is stronger than the traditional DSRR. Then by increasing the carrier density of the intrinsic gallium arsenide (GaAs) substrate, the magnetic response of the original and complementary special DSRR metamaterial can be weakened or even turned off. This metamaterial structure is promised to be a narrow-band terahertz modulator with several nanoseconds response time. |
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