The Experimental Investigation Of Generation And Optimization Of High Order Harmonic By Intense Laser Field

With the development of the laser technology, the electric strength of the laser has already exceeded Coulomb field which is existing in the size of the internal electronic of atomic and molecular. While a series of non-linear phenomena produced by the interaction between the laser and substance, such as higher harmonic and the sequence of double ionization. And the higher harmonic is being to EUV-X ray source due to the character of short wavelength, high coherence and ultrashort pulse duration. Meanwhile, it is significance on the imaging of molecular orbital and attosecond science. Therefore, the efficiency of the higher harmonic generation and supercontinuum spectrum to be hotspots and difficulties for the researches.In this thesis, with the homemade high order harmonic spectrometer system, we analyze and calibration the high order harmonic, based on flat-field spectrograph. And taking the gases of Ar、Kr、and N2, as examples, we analyzed how to optimized high harmonic in terms of the gas pressure, the laser intensity, and the ellipticity of femtosecond laser.The result proved that, the harmonic intensity began to increase with the gas pressure. When the pressure exceeded a certain value, the free electrons increased with the pressure added which is leading to the phase mismatch, and then the intensity of the harmonic begins to recede. The harmonic intensity began to increase with the laser intensity increased within a certain laser energy range. When the laser energy exceeded a certain value, the strength of the harmonic began to decrease. The increase of the laser energy, made gas ionization more easily, thus producing more free electrons as well as causing harmonic phase mismatch. In addition, the optical defocus caused by the increase of free electrons, which is leading to reduce the peak intensity of the laser, and it is observed that the intensity of the actual participation of laser in the harmonic generation process has not been a corresponding increase. It could be seen that the existence of the optimum pressure and intensity of laser energy made the intensity of harmonic be the maximum. Experimental results indicated that a intensity of harmonic was the maximum with the linear polarized laser;harmonic was restrained with a elliptical polarized laser.