Study On Generation Of THz Radiation Via Difference Frequency Mixing Of CO₂ Laser Lines And Theory Of Broadband Second Harmonic Generation Based On Frequency Mixing

Nowadays study on science and technology of terahertz wave is one of the most active research fields all over the world. Recently research on THz field is focused on the generation, propagation, and detection of THz wave. The generation of THz wave is one of the extremely critical techniques in THz field. The THz generation methods can mainly be divided into methods based on electronics and photonics. Among the methods based on optics and photonics, the method by using nonlinear optical difference frequency mixing could obtain compact, low-cost, room-temperature, and tunable THz radiation sources. The choice of difference frequency crystal and wavelength as well as the phase-matching problem of the three-wave interaction are the key problems in the generation of THz wave via nonlinear optical difference frequency mixing, and it should be seriously considered. The techniques of generation of THz wave via difference frequency mixing of far infrared CO2 laser lines has its own merits and should be attached importance to. In this dissertation, the phase-matching problem of the generation of THz wave via difference frequency mixing in isotropic nonlinear crystals and the generation of THz wave via difference frequency mixing of CO2 laser lines have been studied, and as last the theory of broadband second harmonic generation based on frequency mixing has also been analyzed. The main contents are summarized as follows:1. The principle of cross reststrahlen band dispersion compensation phase-matching in isotropic nonlinear crystals has been introduced. The collinear phase-matched condition of terahertz wave generation via difference frequency mixing in the crystals of GaP, GaAs, InP, ZnTe and CdTe has also been theoretical studied by using that principle. Moreover, the influence of the temperature varying on the collinear phase-matching in GaAs crystal has been discussed as well.2. Phase-matching problem in the generation of terahertz radiation via difference frequency mixing of CO2 laser lines in GaSe and GaAs crystals has been discussed in detail. The quasi phase-matching in QPM GaAs crystal for the generation of THz wave via difference frequency mixing of CO2 laser lines is proposed and calculated.3. The effective nonlinear coefficient deff in crystal of Cr:GaAs and the cut angle on the output face have been calculated in detail and the crystal cut scheme has been proposed for the experiment to generate THz wave via difference frequency mixing of CO2 laser lines in Cr:GaAs crystal. The experimental setup has been designed as well.4. Broadband second harmonic generation is studied based on the idea of frequency mixing. Broadband SHG in the crystals of BBO, KTP, and LBO has been numerical calculated in simulation.