| The investigation of high-order harmonic generation (HHG) has recently emerged as an important area in strong field physics. It can be applied as a coherent ultrashort radiation source in the extreme ultraviolet and soft x-ray regions. Theoretically, HHG can be well studied for atoms and molecules by solving the time-dependent Schr?dinger equation in intense laser field. Within the strong field approximation,We study the high-order harmonic behavior of atoms and molecules exposed to an intense laser field using Lewenstein model in this thesis. Firstly, the derivation of the Lewenstein model for atoms is introduced, then we extend the Lewenstein model to calculate HHG from aligned molecules. Finally, we present the explicit expressions for the calculation of harmonic generation from molecules in a linearly polarized laser field. The three step process of HHG can be semicalssically explained by the model.The interference effect from the high-order harmonic generation of N2 molecules are investigated by using Lewenstein model in intense laser field in this thesis. Compared the result with the contribution from one nucleus of N2,it exhibits a minimum because of the interference effect. It is shown that the minimum decreases with the increasing order of HHG,while for 900 the interference effect vanishes. This trend is agreement with the predictions of the two-centre interference model. In general,the presence of the interference is attributed to the generation of high-order harmonics and the change of the harmonic phase. Finally, the influence of optical maser wavelength and the distance of the nucleus on the minimum of the interference are discussed. HHG order according to the minimum increases with the increasing wavelength of laser and decreases with the increasing distance of the nucleus. These trends are agreement with the predictions of the two-centre interference model. Some summarization and a vista of the future work have been given in the end of this thesis.
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