Researches On Terahertz Generation By Cascaded DFG From Three-wave Coupling Characteristics

Terahertz-wave(THz-wave)sources have received considerable attention is of great importance in a variety of applications,such as high-speed wireless communication,nondestructive evaluation,and medical diagnostics.Three-wave coupling processes enable frequency down-conversion via difference-frequency generation(DFG).A cascading DFG process in which more than one THz photon is generated from the depletion of a single pump photon can enhance the photon conversion efficiency.It involves using a strong high-frequency pump pulse and a low-frequency pump pulse,is an established way for THz-wave generation.Quasi-phase matching(QPM)has been demonstrated to be an effective method of DFG.A promising configuration for Cherenkov phase-matching(ChPM)was proposed,in which the phase-matching condition is automatically satisfied at a certain angle to the pump laser path.THz-wave generation by cascaded difference-frequency generation from three-wave coupling was studied,the main research includes the following several points:1.THz-wave generation by cascaded DFG based on periodic inverse GaAs crystal(GaAs)QPM is proposed.The property of GaAs is calculated from the couple wave equations.The results show,the ratio of nonlinear coefficient to second-order nonlinear coefficient change from zero to unity,the maximal ratio of nonlinear coefficient to second-order nonlinear coefficient become unity.The two original pump frequencies are generated by a near-degenerate parametric pump at 10 ?m,hence 1-THz generation is taken as an example to illustrate the process.The range of the polarization period GaAs is small.THz power(THz intensity)and the conversion efficiency significantly increase with the nonlinear coefficient,the polarization period within small range.Absorption is one of the main factors in the terahertz source of GaAs.2.Theoretical analysis is performed on the principles and process for THz-wave generation by cascaded DFG based on QPM in periodic inverse GaP crystal(GaP),GaAs,and periodically poled lithium niobate crystal(PPLN).The tuning characteristics of three crystals are calculated from the coupled wave equations,and compare the results.The results show that THz power generated by GaP is slightly higher than that of being generated by GaAs with a pumping frequency of about 540 THz.The polarization period of GaAs is the smallest.The wave vector mismatch and polarization period range of PPLN are the smallest,however,the conversion efficiency is the highest.A widely tunable laser source system based on the general theory of QPM in periodically poled MgO:PPLN crystal(MgO:PPLN)has been developed.Through analyzing the absorption factor on its output of the THz power in theory,the conversion efficiency of cascaded DFG was obtained.The infrared radiation was produced from cascading orders 15 with a maximum power of 3.72 MW.A maximum output conversion efficiency of 3.72 % was generated.Compared to DFG,THz power in cascading processes increased to 9.5 times,the cascading processes contributed to efficient THz-wave generation.Absorption is one of the main factors in the terahertz source.3.A scheme for THz-wave generation in a PPLN via cascaded DFG based on Cherenkov-type QPM is proposed.Photon conversion efficiency is evaluated based on a promising structure that combines QPM and ChPM with a reduction in wave-vector mismatch.Cascading processes contributed to the photon conversion efficiency,and THz radiation with a maximum photon conversion efficiency of 1154.2% in a 14-order cascaded Stokes process was obtained.Comparing the processes with and without Cherenkov-type radiation,with a 50-MW pump,the power was boosted nearly 1.9 times for the former case.These results provide a good experimental approach to high-energy THz-wave generation.4.A scheme for obtaining THz-wave based on collinear phase matching DFG by Nd:YAG laser is designed.The characteristics of muti-period structure PPLN is introduced,which high-energy tunable THz-wave could be achieved.