Terahertz (THz) radiation is defined in the 0.1–10 THz range of the electromagnetic spectrum. It corresponds to the 30–3000μm range of the wavelength spectrum, which is an important region between the optical wave and the microwave. THz radiation finds many applications in biomedicine, cancer inspection, inspection systems in airports, and molecular science. The THz wave can be generated through optical rectification, free electron laser, and nonlinear frequency mixing in nonlinear crystals.
In this thesis, our experiment uses the nonlinear difference frequency generation (DFG) process in periodically poled LiNbO3 (PPLN) to generate the THz wave. The useful crystal length in the DFG process is believed to be limited by the absorptive length in the strongly absorptive materials. In this thesis, we use different lengths of PPLN with the same period to generate THz wave. Results show that the use of crystal length is not limited by absorptive length.

However, in long lengths, the THz wave diffracts fast in the crystal and has weak interaction with the pump and signal wave, which decreases the power of the THz wave. In our previous work, we have demonstrated that there is two times the conversion efficiency in the non-collinearly phase-matched THz generator. In the experiment, we use a thin crystal called one-dimensional (1D) waveguide and a rectangular crystal called two-dimensional (2D) waveguide to enhance the power of the THz wave. The result shows that the power of the THz wave in the two-dimensional waveguide is about 1.82 times than that in the 1D waveguide.

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