Numerical Study Of Photoionization Process Of Solid Materials By Ultrashort Pulse Laser

Intense laser has been applied to new energy,new materials,national defense and other important areas.The study of the interaction between ultrashort pulse laser and solid material has become one of the most promising areas in the forefront.In order to study the interaction between them,we always use the Keldysh model to calculate the photoionization rate.Although the Keldysh model has achieved great success(almost all of the research between intense laser and solid material are used),but the theory itself has its inherent limitations.And this limitation comes from the approximations used in the derivation by Keldysh,the approximations including the saddle point method and the initial small momentum hypothesis.The Keldysh analytical model requires that the initial momentum of the electrons involved in photoionization is small.This assumption is applicable when the laser intensity is not particularly strong,but it can not be established when the laser intensity is strong.In addition,the Keldysh model does not take into account the effect of the Brillouin boundary on photoionization(existing studies have found that the Brillouin boundary will have a strong effect on the transition when the laser intensity is strong).In this paper,we use numerical methods to study the photoionization process in solids.The main contents of this paper as follows:(1)The photoionization process of ZnO materials was studied by numerical method,and the limitations and applicability of the existing Keldysh analytical model were analyzed.In the process of calculation,we don't employ the approximation assumptions used in the Keldysh formula and deal with the multiple integrals of the Brillouin space using the Monte Carlo method.Our study shows that the small quasi-momentum hypothesis in the Keldysh formula is a good approximation when the laser intensity is not strong(the mechanism of the interaction between the laser and the solid material is in the multiphoton region).However,when the laser intensity is strong(the mechanism of the interaction between the laser and the solid material is the tunneling area),the small quasi-momentum assumption is not appropriate and results in a significant difference in the numerical results and the Keldysh analytical formula.(2)The effects of material parameters(band gap,electron effective mass,Brillouin size)on solid photoionization rate were analyzed by numerical method.(3)The numerical method of this paper is applied to 1)the calculation of thetransient free electron density evolution of the solid material,2)the calculation of the transient optical properties of the solid material,and 3)the calculation of the damage threshold of the femtosecond laser induced solid material.The numerical results show that the Keldysh formula is smaller when the laser pulse width is wide(when the laser peak power density is small),but when the laser pulse width is short(when the laser peak power density is high)obvious.In the calculation of different pulse width thresholds of fused silica and barium aluminum borosilicate,we found that the damage threshold calculated by Keldysh method was in good agreement with the experimental data at long pulse width,but in the short pulse width region,there is an obvious difference with the experimental data.However the numerical method we use is consistent with the experiment data in the whole pulse region.