Analysis Of Terahertz Single-pulse Picking Based On Plasma Mirror

Terahertz technology has developed from the 1980s to the present,and has become a hot spot due to its various outstanding characteristics.At present,the application and research of the research and technical personnel on the terahertz has penetrated into many subject areas,including imaging,spectral analysis,material detection,astronomical observation,security and communication and other technologies.In addition,with the continuous development of free electron laser technology,terahertz free electron laser light source came into being.Compared with other terahertz light sources,this terahertz light source has high power,repetition frequency and efficiency,which brings great help to terahertz imaging,spectral analysis and material detection and other technical fields.However,for the two major types of experiments in biology,materials,and spectroscopy,it is often required that the light source has a high pulse energy and a long enough time interval between pulses to better study some ultrafast processes.The first category includes biological and superconducting materials induced phase change experiments,such experiments require very high pulse energy,and also try to avoid the adverse effects caused by the high repetition frequency and high average power caused by the thermal effect on the experiment influences.The second category involves experiments with long-life photo-excited samples.If the time interval between pulses is very short,then the corresponding photo-excited state cannot be restored to equilibrium on the time scale of the free electron laser pulse period.Conducive to the study of ultra-fast process.Therefore,there is an urgent need to find a single pulse selection method for terahertz free electron laser to reduce its higher repetition frequency when necessary.In this paper,self-induced plasma switching technology is used,considering that the terahertz can be reflected by the plasma mirror generated by short-pulse laser shooting,and then the plasma mirror can be used as a gated switch from a free-electron laser terahertz source with a higher repetition frequency A single micro-pulse is selected from the macro-pulses of this paper.This article has conducted a detailed theoretical analysis of this method for this physical process.In order to theoretically explain the physical process of the plasma mirror being excited by the short pulse laser from the selected specific target,this paper starts from the principle that the laser irradiates the target material surface and interacts with it.Physical properties,and the physical process of absorbing laser energy when the germanium target is irradiated.In the process of the interaction between the short-pulse laser and the germanium target,various ionization mechanisms will participate and compete with each other,while the stage of generating a plasma mirror on the surface of the germanium target will be dominated by multiphoton ionization and avalanche ionization.Through theoretical analysis,the plasma critical density required for the plasma mirror to achieve the corresponding pulse cutting function can be calculated.Combined with the working frequency range of the terahertz free electron laser,it is determined that the plasma mirror needs to reach 2.156×1017cm-3 electron density.In addition,the theoretical calculation of the power density threshold of the short-pulse laser used to generate the plasma mirror is 8.7×109W/cm2.From this,the physical process of the laser exciting the plasma mirror on the target surface can be further calculated by software simulation.It shows that the density of the laser-excited plasma is much greater than the critical density,which proves that the selected laser can meet the demand and has experimental feasibility.On the other hand,because the switching effect of the plasma mirror mainly depends on the reflectivity of the target surface plasma to the terahertz,the physical process of the terahertz irradiated plasma is discussed in combination with the physical properties of the plasma.The theoretical calculation It is found that the reflectivity of horizontally polarized terahertz at different angles when reflected by the plasma mirror decreases with increasing operating frequency,and the reflectivity at Brewster angles is greater than 90%,which verifies the feasibility of the theory.The research results of this paper provide new reference ideas for single pulse selection and other terahertz switch modulation fields in terahertz free electron laser devices.