Modeling And Simulation Of Femtosecond Laser Excitation Of Gases And Liquids To Generate Terahertz Processes

In the electromagnetic spectrum,terahertz(terahertz)band has many unique properties,and the terahertz band spectrum carries a lot of physical and chemical information of substances,which makes terahertz a new frontier subject and technology is highly valued by governments and researchers all over the world.Terahertz wave technology has important research significance and bright production prospect in the fields of materials science,medicine,information communication,safety inspection and so on.The performance of terahertz radiation source determines whether the frontier technology of terahertz can be put into various fields of life.The generation of terahertz(THz)from gas and liquid excited by femtosecond laser is a hot topic.This method can provide high-power and ultra wideband terahertz radiation,which is widely used in many fields.In the air medium,the terahertz wave generated by this method has serious absorption and loss in the transmission process,so it is necessary to further improve the terahertz radiation efficiency and carry out application research;but in the liquid medium,the theoretical research of this method is less,and the physical process needs to be further studied.Therefore,this paper studies these two problems.The physical process of terahertz enhancement by changing the number of laser field harmonics is studied by MATLAB simulation.Particle trajectories in the terahertz excitation process in water are studied by using the drift diffusion module in COMSOL.Firstly,based on the photocurrent model,the physical process of terahertz(THz)generation by polychromatic light in air is numerically simulated.In this paper,the influence of frequency doubling light on terahertz generation in gas medium is studied.The mechanism of terahertz enhancement is discussed from the angle of electron number density and electron current,and the terahertz enhancement effect of three color laser field is further studied and compared with that of two-color laser.In this paper,the physical mechanism of the system is described from various aspects,and the terahertz spectrum and time domain diagram under the excitation of trichromatic light are calculated.Secondly,according to the particle motion in the process of terahertz excitation in liquid,the corresponding motion model is constructed,and the charged particle tracking interface is selected to study the particle trajectory.The modeling and Simulation of two-color laser in COMSOL is carried out from four aspects:peak intensity,intensity ratio,wavelength and phase difference.The results show that:1(1)The results show that the intensity of terahertz can be effectively increased by introducing 400nm second harmonic on the basis of 800nm fundamental wave.The intensity of terahertz radiated by monochromatic light is only 10^-4 times of bichromatic light.(2)After introducing 267nm third harmonic on the basis of bichromatic light,there are three peaks in the output terahertz spectrum,and the spectrum amplitude of terahertz band is about twice that of bichromatic light.It is proved that the third harmonic can effectively improve the intensity and power of terahertz radiation.(3)The results show that:with the increase of the peak intensity of the two-color laser,the range of the particle trajectory gradually increases,and the larger the deviation between the positive and negative directions,the more net current is generated;the change of intensity ratio has little effect on the particle trajectory;with the increase of fundamental wavelength,the Gaussian light increases When the envelope of the field increases,the range of particle trajectories increases,and the asymmetry between the positive and negative directions increases,and the net current increases.Finally,when the phase difference of the laser field is changed,it is found that the difference between the positive and negative directions of the particle trajectory is the largest,and the drift current is also the largest when the phase difference is?/2.This paper provides a new method to study the relationship between THz excited wave and THz excited by water.At the same time,the relationship between the parameters and the particle motion in the water excited terahertz generation is studied by modeling,which provides the simulation basis and research method for the later research of water excited terahertz.