Linear Molecular Alignment And Soft-X Ray High Harmonic Generationin Ultrashort Laser Pulses

High harmonic generation(HHG) with short wavelength, high coherence and ultrashort pulse width has practical value in the soft X-ray source and the attosecond science such as molecular image and ulrtrafast dynamincs. It is necessary to generate stronger and shorter-wavelength HHG for soft X-ray source and attosecond science. Thus, the conversion efficiency should be enhanced and cutoff harmonic order should be extended. Moreover, coherent tunable HHG is also necessary. Molecule has more spatial degrees of freedom. Molecule alignment is produced under the laser pulse. The aligned molecule influences molecular HHG. HHG from molecule is more easily controlled and adjusted. Thus, in this paper, the linear molecular alignment and soft X-ray harmonic emission are researched in order to make some contribution.The linear molecule is regarded as a rigid rotator. There is an interaction potential energy induced by laser electric field. We get a theorical model of linear molecular alignment generated by femtosecond laser pulses. The calculation results indicate that the molecular alignment is improved by introducing second femtosecond laser pulses. The intensity of second femtosecond laser pulses can influence linear molecular alignment. We demonstrate the possibility of good linear molecular alignment using two femtosecond laser pulses theory. It provides some theorical base for controlling high harmonic generation experimentally in the future.High harmonic generation from aligned linear moleclues(N2 and O2) is researched experimentally based on the theory of linear molecular alignment. The spectrums of high harmonic generation are obtained in the case of different angles between molecular axis and laser polarization direction. When there is a parallel between N2 molecular axis and laser polarization direction, the maximum intensity of high harmonic generation is obtained. When there is a perpendicular between N2 molecular axis and laser polarization direction, the intensity of high harmonic generation is suppressed. For O2 molecule, when the angle between molecular axis and laser polarization direction is 40°, there is a maximum intensity of high harmonic generation. When there is the parallel or perpendicular between O2 molecular axis and laser polarization direction, high harmonic generation from O2 molecule is suppressed. The experimental results indicate that the process of high harmonic generation can be controlled using technology of linear molecular alignment.Two-color(fundamental laser 800.00 nm and frequency-doubing laser 400.00 nm) femtosecond laser pulses are used to enhance the intensity of HHG. The BBO crystal is placed in the front and rear ends of the lens. Thus two-color femtosecond laser pulses with single focus and two-color femtosecond laser pulses with two foci can be obtained. The lower-order harmonic efficiency is improved by using two-color femtosecond laser pulses with single focus. The even harmonics is observed. Furthermore, due to the refraction index of the lens and the window of vacuum is different for different laser wavelengths, two-color femtosecond laser pulses with two foci can be generated. The intensity of HHG is enhanced significantly. The experimental results indicate that the intensity of H23 is enhanced by a factor of more than 65. Moreover, the effect of an aperture for HHG is observed experimentally. Moderate decrease of aperture is helpful for producing the phase-matched HHG. Finally, the ellipticity of femtosecond laser can influence HHG from Xe, Ar and N2. Due to there are more larger recollision cross-section of N2,HHG in molecule exhibit weaker dependence with ellipticity of femtosecond laser.The characteristic of He is high ionization thresold and Xe is low ionization energy. To solve this problem, Xe-He gas mixture is used as the nonlinear medium for improving photon energy of HHG. The experimental results indicate that the cutoff region of HHG is enhanced from H21(32.61 e V) to H25(38.82 e V). Furthermore, the use of Xe-Ar gas mixture is helpful to generate HHG from Xe+. The cutoff region of HHG is enhanced from H33(51.25 e V) to H37(57.46 e V). Furthermore, gas tube is used to generate HHG. The effect of the femtosecond laser energy, the filling gas pressure and the position of laser position for HHG is researched. The reason of extended harmonic cutoff is analyzed. The laser focus is moved in the gas tube. For harmonics in the cutoff region, two maximum intensities near by the excit and enter of the gas tube is observed. On the other hand, for harmonics of plateau region, there is a maximum near by the center of the gas tube.The effect of the laser intensity, the gas pressure and the position of the laser focus for the blueshifting of HHG in gas cell and gas tube are investigated experimentally. In gas cell, 0.35 nm blueshifting of harmonic wavelength is obtained. In gas tube, 0.40 nm blueshifting of harmonic wavelength is observed. Self-phase modulation leads to the blueshifting of harmonic wavelength. But for Xe with low ionization energy, nonadiabatic mechanism results in the blueshifting of harmonic wavelength.