Research On Wavelength Tunable Coherent Ultraviolet Light Source Based On High-order Harmonic Generation

High-order harmonics possess the characteristics of short duration,wide spectrum and tunable wavelength.Moreover,it covers the spectral range from ultraviolet to extreme ultraviolet and even soft X ray(0.01~100~?A).Therefore,it provides us an effective method to produce wide band tunable spectrum.High-order harmonic generation(HHG)process consists of two parts:the single atom response in the laser field and the macroscopic propagation effect of the laser field and the harmonic field in the gas medium.In the experiments,the rarefied gas is always adopted to interact with laser field.Therefore,the dispersion and absorption of the gas medium on laser and harmonic fields can be ignored.By adjusting the phase matching conditions,we can realize the tuning of the harmonic frequency and efficiency.However,except for the phase matching effect,the characteristics of gas medium also affect the harmonic efficiency.Based on this,the main contents of our work are as follows:(1)We present a method and an experimental device for generating wavelength tunable coherent ultraviolet light source.The high-order harmonics are generated by using a 800nm monochromatic laser to interact with nitrogen molecules.Then by adjusting the drive laser pulse energy and pressure to change the instantaneous phase matching conditions.As a result,it promote the frequency shift of the harmonic and realize the continuous tuning of the output wavelength.We find out the relationship between the output wavelength and the driving laser pulse energy or pressure.We can precisely control the variation of the driving laser pulse energy and the pressure to achieve the precise tuning of the output wavelength of the extreme ultraviolet light source.(2)Probing the harmonic efficiency of hydrogen molecule in the experiment,and analysis the influence of the laser intensity and gas pressure on the harmonic efficiency.We find that the laser intensity and gas pressure both have an optimal value(under this condition we can obtain the highest harmonic efficiency).Above this optimal value,the harmonic efficiency will decrease.(3)Probing the harmonic efficiency of hydrogen,argon and krypton gas in the experiment,to qualitatively analyze the influence of scattering cross section,ionization energy and multi electron effect on the high order harmonic efficiency.By comparing the harmonic efficiency of hydrogen and argon,hydrogen and krypton,krypton and argon,we find that the harmonic efficiency is not only affected by the phase matching but also the scattering cross section,the electron effect and the ionization energy.