| The generation of terahertz (THz) waves is one of the key technologies in the THz study field. The methods of THz generation are divided into electronics and photonics. The nonlinear optical difference frequency method is one of the main methods to generate all-solid-state, low cost, compactable, room-temperature, and tunable THz wave radiation. Among the key technologies of THz generation by the nonlinear optical difference frequency, developing novel difference frequency pumping sources is the most important. With the development of THz sources and THz detectors, how to construct an integrated optical circuit for THz generation, propagation, manipulation and measurement is becoming a brand-new study focus. Photonic crystals (PCs), as a kind of new artificial material, would play an important role in the design of THz functional components. The main contents are summarized as follows:1. Based on the lasing threshold equation of a diode-end-pumped Nd:YAG laser, the relationship between the transmission ratios of the output mirror corresponding to the two wavelengths is calculated. A simultaneous dual-wavelength continuous wave (cw), quasi-cw or linearly polarized wave diode-end-pumped Nd:YAG laser operating at 1319nm and 1338nm is demonstrated. To the best of our knowledge, our experimental results achieve the highest level in the domestic domain. The simultaneous dual-wavelength laser provides an experimental basis for generating highly coherent THz wave radiation of 3.23THz by nonlinear optical difference frequency method.2. A diode-end-pumped Nd:YAG crystal, type-I critical phase matching LBO crystal intra-cavity frequency doubled, simultaneous multi-wavelength red laser at 659.5nm, 664nm and 669nm is firstly realized. Using the dual-wavelength red laser at 659.5nm and 669nm and choosing the proper difference frequency crystal, it is hopeful for us to realize high frequency THz wave radiation of 6.73THz. This is the second experimental project for generating THz waves.3. Based on the theory of nonlinear optical frequency conversion, the phase-matched angle, walk-off angle, acceptance angle and effective nonlinear coefficient of difference-frequency in GaSe and ZnGeP2 crystals are calculated under different phase-matched conditions. The optimum phase-matched conditions corresponding to different THz wave bands are summarized through comparing the whole calculated results. The calculated results provide a theoretical basis for using optical difference-frequency method in the nonlinear crystals to generate tunable THz waves.4. Based on plane wave expansion method, the complete band gaps of two-dimensional (2D) THz PCs with typical structures are optimized through varying structural parameters. Two kinds of lattice structures that are very promising for the materials of THz components are found. Using finite difference time domain method, the electromagnetic field distribution of THz wave is simulated in a THz 2D PC splitter and by plane wave expansion method the dispersion relation and defect modes are achieved in a THz rotated PC waveguide.5. A woodpile three-dimensional (3D) PC with face-centered-tetragonal (fct) symmetry is one of the most popular 3D PCs because of its favorable band gap characteristics. Based on the woodpile fct lattice structure, we propose a novel woodpile THz lattice structure with comparatively decreased symmetry, better band gap properties and easy fabrication by varying some structure parameters. Compared with the woodpile THz fct PC, the novel woodpile THz PC has a wider range of filling ratios to gain higher quality complete PBGs, which makes the manufacturing process more convenient. The novelty of the new woodpile PC has gained approvals from the international peers.
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