High Harmonic Emission Of Laser Ionized Gas,Backward Raman Amplification In Plasma

Due to the advancement of laser technology,it is likely to change or control atom,molecule and electronical dynamics.On the one hand,it extremely promotes the enormous advancement of laser-matter interaction.On the other hand,it also provides forceful techno-means and methodologies for the farther investigations of laser-matter interaction.The field of laser-matter interaction includes laser-solid,laser-liquid,lasergas and laser-plasmas.The laser-gas experiment has many inherence advantages,such as capable of repetition,the gas density being controllable,the plasma being conveniently diagnosed,there not being the pieces from target,etc.The laser-gas scheme plays a vital role in these fields like the high harmonic generation(HHG)in laser-gas ionization,the stimulated backward Raman amplification(BRA)besed on the laser-gas plasma,the laser-gas plasma guide.The paper opens out its works on the two application researches including HHG and BRA.The laser-induced-gas harmonic emission is important method to obtain an extreme ultraviolet(XUV)radiant source and X ray source,and to produce an ultrashort attosecond(as)pulse.Because of the application requirement of attosecond pulse in the ultrafast optics field,one is devoting a great deal of effort to obtaining a shorter attosecond pulse.The current shortest recorded attosecond pulse in the experiment is 67-as.In the experiment,general means of obtaining an ultrashort isolated attosecond pulse is to control harmonics emission with a polarization gating pulse.Furthermore,the infrared laser with long wavelength can provide larger ponderomotive energy,and it is highly advantageous to produc a HHG with broad bandwidth.The thesis utilizes a combination pulse of polarization gating pulse plus its double-frequency pulse to drive helium,and an ultrabroadband supercontinuum HHG is obtained through the theoretical calculation.As a result,an isolated 42-as pulse is generated directly by superposing the hundreds of harmonics.To explain the mechanism of the ultrashort attosecond generation,the semiclassical three-step model simulation and wavelet time-frequencytransform of the harmonic spectra are performed.Via two general laboratorial ways,the thesis perform an efficient scheme for the generation of a shorter isolated attosecond.The plasma produced by laser inducing gas plays a vital role in the plasma optical waveguide,backward Raman amplification besed on plasma,and plasma harmonic emission.For the requirements of practical application in these fields or others,the experiment diagnose and theoretic simulation of laser-gas plasma are very concerned about the issue.In the many characteristics on plasma,the electron density of plasma,as an important parameter in many researches,is attracted much attention.In this thesis,based on a common optical interference method,the 2 dimensional electron density profiles of the plasma by laser breakdown air are detected by the experimental arrangement.At present,the theoretical research works have not provided the simulation of the 2 dimensional density distribution of plasma.In this thesis,a theoretical model is established by separating the plasma dynamics process as two steps of plasma formation and plasma expansion,and the 2 dimensional electron density distribution of the plasma is simulated according to the experimental conditions.It is found that the simulation results are in good agreement with the experimental results.The works show theoretical scheme that can effectively simulate the plasma by laser breakdown gas(especially,the very high plasma electron density in the early stage is difficult to measure).Based on this model,the plasma density,temperature with the evolution of expansion times can be analysed.The plasma produced by laser inducing gas is considered as the most available medium for the backward Raman amplification based on the plasma.Due to the new amplification technology can overcome the damage of the high power laser irradiating the media.It is considered to be a promising way for implementing the ultra high power laser of EW scale output.In order to achieve an ultra high power laser output,a few groups have achieved the validation experiments of this new amplification technology under their own laser systems.Based on our own laser system,the thesis regards the plasma produced by laser breakdown Ar gas as a coupling medium,and implements validation experiments of the backward Raman amplification technology.In the experiment,we observed that the seed light is amplified by a pump light.Through an analysis of the experimental results,the low plasma density is the main cause of the low amplification efficiency.The 2 dimensional electron density distribution of plasma which satisfies the need of BRA is simulated using the theoretical model proposed in the previous section.Through the analysis of calculation results,this thesis presents experimental conditions that can realize the required plasma density.