Theoretical Study Of Plasma-excited Terahertz Waves Based On The Photocurrent Model

Terahertz wave refers to the electromagnetic wave whose frequency is between microwave and infrared radiation(0.1-10THz).The special position of terahertz wave in electromagnetic spectrum determines that it has some unique properties which are not possessed by microwave and infrared wave.Therefore,terahertz wave has a broad application and development prospect in many fields,such as military defense,biomedicine,astronomical observation and so on.The development of high power,high efficiency and wide band terahertz radiation source can make the terahertz wave better applied in practical production and engineering.The terahertz wave technology radiated by laser plasma can meet the above conditions well,which attracts the attention and research of many scholars in the related fields at home and abroad.In the process of generating THz wave based on laser-induced plasma,the setting of laser parameters directly affects the intensity of THz wave radiated.By adjusting the laser parameters reasonably,the terahertz emission can be regulated and the intensity of terahertz radiation can be greatly enhanced.The terahertz wave generated by laser-induced plasma is mainly composed of gas and liquid.In this paper,two kinds of media are simulated.Based on the terahertz radiation source of plasma produced by intense laser,the main factors influencing the generation of terahertz radiation are studied by theoretical analysis and simulation experiments:The process of terahertz wave generated by laser acting on gas is simulated by using photocurrent model.The influence of laser parameters on terahertz wave radiated by gas is studied.This paper mainly studies the parameters such as the contrast of monochromatic and bicolor laser,the wavelength,pulse width,the applied bias electric field,the phase difference between the frequency doubled wave and the fundamental wave,and the intensity ratio.The results show that the intensity of terahertz radiation can be enhanced under the same parameters by increasing the wavelength and pulse width of the two-color laser and the bias electric field in the same direction as the two-color laser.However,with the increase of the intensity ratio and the phase difference between the two lasers,the intensity of terahertz radiationproduced by the two-color laser field increases first and then decreases.Through the above research,the best values of these factors are obtained,and the purpose of parameter optimization is achieved.Based on the photocurrent model,combined with the ionization mechanism of the ultrashort laser and the liquid,a calculation model suitable for the two-color laser in the liquid medium to excite terahertz waves is established.This model is used to simulate the influence of monochromatic and bichromatic laser and the change of laser pulse width on terahertz radiation.The results show that the bicolor laser can greatly enhance the output intensity of THz wave.The main factor is the formation of asymmetric electric field,which promotes the asymmetry of acceleration of free electrons,and thus forms the drift current.In the case of two-color laser field,the THz field excited by 50 fs pulse width is stronger than that excited by 300 fs pulse width.This is mainly due to the fact that the pulse width of 50 fs is higher than that of 300 fs.The simulation results are compared with the existing experimental results to verify the validity of the terahertz calculation model.