Modeling And Numerical Simulation Of Semiconductor Switching For Ultra-short Pulse CO₂ Laser

Ultra-short pulse CO₂ laser is a powerful tool for studying solid surface plasma and photochemistry,and has obvious advantages in particle acceleration and other fields.The semiconductor switching method has the unique advantages of high output stability,easy to install and adjust,and can achieve picosecond or femtosecond pulse width controllable output.It is a better choice to obtain the pulse width adjustableultra-short pulse CO₂ seed laser.However,there is no report of picosecond seed laser output using semiconductor switching technology in 10.6?m wavelength in China.At present,the theory of Ge semiconductor switching technology to achieve ultra-short pulse width laser output is not perfect,especially the combination of two-stagesemiconductor switching has not been completely reported in theory and numerical simulation.In this paper,the physical mechanism of ultra-short pulse width CO₂ laser output based on semiconductor switching technology is studied theoretically and numerically.Firstly,the generation,diffusion and recombination mechanism of non-equilibriumcarriers on the surface and inside of semiconductor under laser irradiation areanalyzed.It is found that the physical processes such as direct absorption,free carrier absorption,Auger recombination,Plasmon-assisted recombination,bipolar diffusion and high energy carrier relaxation are the important factors determining the change of plasma density on the surface of semiconductor switching,which can not be ignored.Then,the theoretical model of semiconductor switching is improved by deeplystudying and introducing the above physical process,and the detailed mathematical expression and numerical analysis results of the theoretical model of single-stage and two-stage semiconductor switching are given.Secondly,the mechanism of thegeneration of ultra-short CO₂ laser pulses by reflection switch and two-stagesemiconductor switching is simulated and analyzed by using the improved theoretical model.The results show that the simulation results of the improved theoretical model are basically consistent with the reported experimental results,which shows therationality and correctness of the model.Finally,the influence of the parameters of the control pulse on the output of the 100 picosecond pulse width CO₂ laser pulse fromthe single-stage reflection switching is analyzed in detail by using the numerical simulation,and the main factors influencing the quality of the ultra-short CO₂ laser pulse from the two-stage semiconductor switching are studied in depth.The results show that the control of the laser pulse parameters has a great influence on theworking efficiency of the single-stage reflection switch and the two-stagesemiconductor switching and the quality of the output ultra-short CO₂ laser pulse,and the delay time is the main factor which determines the output ultra-short CO₂ laserpulse width of the two-stage semiconductor switching.