Study Of Difference Frequency Generation For Terahertz Generation Based On Cavity Phase Matching From The CO₂ Laser

Terahertz wave has become one of the most popular research fields due to its potential application value in biomedicine,safety inspection,communication technology,etc.Terahertz source technology is one of the most important and basic research directions.Because of its high efficiency,high power,simplified structure and tunability,the difference frequency generation?DFG?has become an important technology for the development of terahertz sources,and it is very important that how to achieve the phase matching in the process of DFG.The cavity phase matching?CPM?technology proposed by Bloemberg et al.has been experimentally verificated in recent years,and shows potential advantages in the preparation of small terahertz sources.At present,there are few theoretical or experimental studies of DFG based on CPM.According to this research situation,this paper studies several theoretical schemes of DFG for terahertz generation based on CPM from the CO₂ laser.Specifically,it includes the following aspects:1.Firstly,the phase matching principle of CPM in the process of non-linear interaction is briefly introduced,and a model of DFG based on CPM is established theoretically.Starting from Maxwell equation,the theoretical formula of DFG based on CPM is derived.Two branches which can satisfy the optimal operating conditions of the model are found in CO2 laser:9.5524?m[9P?20?],9.7937?m[9P?46?],and the corresponding crystal thickness is 756.6?m.The calculation results show that the peak power conversion efficiency of the model can reach 2.04%,with a photon conversion efficiency of 83%.2.Considering the characteristics of CPM and quasi-phase matching?QPM?,a new phase matching scheme is proposed:the periodically polarized crystal is placed in a parallel planar cavity with high-reflective coating on both sides.The theoretical results show that,compared with the DFG model using CPM and QPM alone,the conversion efficiency of the model can be improved by about two orders of magnitude under the same conditions.This phase matching method,which combines the advantages of CPM and QPM,is expected to replace CPM or QPM and become a new phase matching method.3.According to the theoretical model above,the corresponding experimental scheme is designed.According to the problem of low pump energy utilization in CPM,an experimental scheme is designed to obliquely incident the pump light into the microcavity.A CO₂ laser is used as the pumping source to achieve dual-pulse synchronous output.The preparation methods and parameters of the non-linear interaction cavity in each scheme are introduced,and the feasibility of the above series of experimental designs is briefly explained from the practical point of view.Overall,this paper makes a deep theoretical study on the DFG technology based on CPM,which provides a theoretical basis for the relevant non-linear optical experiment.