The Studies On Quasi-Phase-matching And Terahertz Waves

Recently much attention has been paid to QPM (Quasi-phase matched) which is useful in nonlinear frequency conversion such as Sum Frequency Generation (SFG) Different Frequency Generation (DFG), Optical Parametric Oscillation (OPO) and Optical Parametric Amplification (OPA).In this dissertation, the relationship between the conductivity and the polarization reversal quality of electric field poling method is explored. And a lot of experiments associated with it are done with the home-made power supply testing the conductivity of KTP; Theory simulations of the THz wave generation based on QPM are carried out, The DFG gain characteristic of the GaAs crystal is also researched; the dielectric property of KTP and its homologous studied by the THz time-domain spectroscopy system provides a basis for their application in THz band. Major contributions are as follows:1. The influence of conductivity in KTP crystal is researched while a high external voltage is applied to fabricate the periodically reversed domains, and appropriate conductivity is demanded for the poling. A conductivity test power is designed and developed for this purpose, and many samples are tested by use of it.2. Take GaAs crystal as an example, the theory of quasi-phase-matched different frequency generation of THz radiation is researched. The periodicity of the QPM GaAs is deduced and the DF gain characteristics and its influence factors are discussed. The error of periodicity to the gain is analyzed and tolerance curve is plotted.3. Utilizing ultrafast femtosecond pulses to pump photoconductive switches in the terahertz time-domain spectroscopy system, the dielectric property and the optical phonon oscillation characteristics of KTiOPO4 crystal are studied in the 0.5~2.0THz range. An obvious absorption peak is found when the z-axis of the crystal is parallel to the polarization of the terahertz electric field. With the crystal lattice vibration selective absorption principle and pseudo-harmonic dielectric model, the complex dielectric constant, refractive index and power absorption curves are fit well. The spectroscopic parameters of a weak optical phonon resonance and the dispersion equation of KTP in this spectrum range are achieved. The similar studies are also done to its homologous-RTP crystal.