The Modulation Of Air Plasma And Its Terahertz Radiation By Multi-color Femtosecond Laser Field

Air plasma filament is a particular nonlinear optical phenomenon generated during the propagation of high power and ultra-fast laser in gas media.There are abundant physical mechanisms and effects in plasma filaments,such as self-phase modulation,cross-phase modulation,self-focusing,photoionization,etc.Further study of plasma filament has potential guiding significance for the application of laser lightning,pulse compression,remote sensing and other fields.At the same time,the plasma filament has great application value to the basic research of attosecond pulse,high order harmonic,air laser,terahertz radiation and so on.In the above studies,researchers are concerned about two important issues: the control of ionization degree in plasma filaments,and the influence of the ionization control on secondary radiation.In this paper,a coherent femtosecond multicolor laser is used to pump air to produce plasma filament.By changing the temporal waveform of the pump pulse via the modulation of the relative phases of the multicolor field,the ionization in the air plasma is controlled.In addition,the intensity modulation of terahertz radiation from plasma is studied in detail.The main work of this paper can be divided into the following two aspects:1.Coherent control of ionization degree of gas molecules by collinear tricolor femtosecond laser pulseIn the experiment,we designed and built the collinear three-color field experimental device by using two BBO with different cutting angles.By changing the insertion quantity of optical wedge pairs and the distance between the sum frequency BBO and the focal point,the time delay between the three-color field can be precisely controlled in attosecond order,by taking advantage of the refractive index difference of different wavelength light fields in fused quartz and air.The plasma fluorescence is collected laterally with a spectrometer,by monitoring the signal of a specific wavelength,the ionization of gas inside the plasma is reflected.A commercial 800 nm femtosecond laser was used as the pump source.Specifically,this pump laser was firstly focused through a lens,and in the following focused optical path,in sequence,we placing the optical elements as: a frequency-doubling crystal BBO 1(cutting Angle 29.2° for frequency doubling,Produces 400 nm lasers),a calcite crystal(to compensate for the time delay between 800 nm and 400 nm light fields),a two-color waveplate(to correct polarization at 800 nm,so that it has the same polarization direction as 400 nm),a pair of fused quartz optical wedges(for precise adjustment of the delay between 800 nm and 400 nm),a forward and backward movable BBO 2(cutting Angle of 44.3°,to generate the 266 nm laser via sum frequency),in this way,an experimental device for collinear three-color field pumping air is constructed.In the above settings,due to the small angle of the optical wedge,the phase between 800 nm and 400 nm can be controlled precisely.By moving the BBO 2 forward and backward,the phase between 400 nm and 266 nm can be precisely adjusted by taking advantage of the difference in refractive index between the air and the laser at different wavelengths.In the sideway of the plasma filament,two short focal lenses are used to collect the fluorescence of plasma radiation,which is coupled to the fiber optical spectrometer.The relationship between ionization degree and phase can be reflected by monitoring the dependence between intensity and phase of a specific wavelength in the fluorescence spectrum.The experimental results show that the degree of ionization varies with phase changing in both two-color field and three-color field.However,when the relative phase between 800 nm and 400 nm is changed,the modulation effect of ionization is almost invisible in the two-color field.In the case of three-color field,if the relative phase between 800 nm and 266 nm is kept unchanged,only the relative phase between 800 nm and 400 nm is changed,the ionization modulation degree is about 20%.When the two relative phases of the three-color field change periodically,the ionization modulation degree reaches 45%.In addition,the dependence of ionization and phase will change when the intensity ratio between the three color fields changes.From Fourier transform analysis,we believe that different ionization dependence originated from different ionization channels.Theoretically,we simulate the dependence of the ionization modulation degree on the relative phase and the ionization degree on the intensity ratio by combining the ADK model.The calculation perfectly reproduces the experimental results with a reasonable explanation.This optical method of coherent control of ionization degree provides a precise adjustment and optimization scheme for the study of high harmonics,air laser and other ionization processes,which reference significantly in the research field of laser-matter interaction.2.Coherent control of terahertz radiation from air plasma filamentsWe have studied the terahertz radiation from air plasma.Based on the above experimental device in part 1,we flexibly remove BBO 2 to realize the switch between two-color field scheme and three-color field scheme.Firstly,in the two-color field pumping scheme,we use Air Bias Coherent Detection(ABCD)to measure the terahertz wave spectrum,and found out that the effective cutoff frequency of the terahertz wave in our system is about 20 THz.Then,the terahertz temporal waveform is measured by electro-optical sampling method.By transforming the phase between the two color fields,it is determined that the intensity of terahertz wave modulated by relative phase is more than 85%.Finally,by combining the Golay Cell with a phase-locked amplifier and changing the relative phase of the two color fields,the dependence of the energy and phase of the terahertz wave in different frequency bands is measured.The results show that the low frequency modulation is strongly dependent on the phase,while the high frequency modulation is little affected.Theoretically,the photocurrent model is used for numerical simulation.It is proved from the simulation that the low frequency terahertz wave can be easily controlled by phase but the high frequency terahertz wave,because of the increase of detuning,is hard to control by phase.This work makes up for the blank of dependence between terahertz wave intensity and phase at high frequency.In the case of three-color field pumping scheme,by using different terahertz filters,we measure the intensity of terahertz wave in different bands and compare the results with the case of two-color field pumping scheme.The results show that the amplitude of terahertz wave in the three-color field is about two times higher than that in the two-color field.Finally,we study the intensity dependence of horizontal and vertical polarized terahertz wave on phases by changing the relative phases between the three color fields.To sum up,this thesis realizes the modulation of air ionization based on the phase control of multi-color femtosecond laser field,and studies the dependence of light intensity ratio and ionization modulation degree.The ADK model is used to simulate the experimental results,and a reasonable explanation is given.At the same time,we study the modulation regime of terahertz wave in different frequency bands from plasma filament,and find that the modulation of low frequency band is much higher than that of the high frequency band.In theory,photocurrent model is used to carry out preliminary numerical simulation for this part.