Theoretical Exploration Of High-order Harmonic Generation From Atoms And Molecules

High-order harmonic generation (HHG) is a nonlinear phenomenon,which is resulting from the interaction of intense laser fields withthe atoms and molecules. In the past decade, many scholars focustheir attention on HHG, which is a method to generate extremeultraviolet (XUV) light and soft X radiation source. Then how tobroaden the plateau and enhance the intensity of the HHG, monitorthe electronic dynamics and investigate the detailed structure ofmolecule are the hot issues. In this article, not only the HHG in atomare optimized, but also the generations of molecular high-orderharmonic are investigated, and the molecular structure as well asthe motion of electrons in the laser field is traced.Our works include following contents:(1) Extending the plateau ofHHG spectrum. A scheme to expand the plateau is proposed byusing the combination of a two-color laser pulse and alow-frequency field, the results demonstrate that the width of the plateau can be expended; in addition, an isolated40as pulse isobtained finally. Besides, we explain the physical origin of theextending by virtue of the classical returning kinetic-energy and thetime-frequency distributions.(2) Improving the intensity of the HHG.Results show that preparing the initial state as a coherentsuperposition, which is the combination of the ground state andexcited state, is an effective method to enhance the intensity of theHHG, however, due to the appearance of second-recombinationphenomenon, the harmonic intensities are not enhanced the sameorders of magnitude, which leads to the failure on optimizing HHG.Then we study the parameter effects on the phenomenon ofsecond-recombination by altering control pulse relative strengthratio and relative phase in detail. The results show that relativephase is an important factor to reduce the phenomenon.(3)Exploring of the isotopic effect on molecular high-order harmonicgeneration. Comparing the HHG of the H2+and HD+, the results showthat more intense harmonics are generated in the asymmetricdiatomic molecule ion HD+than those of H2+, but the destructiveinterference more easily occurs in the harmonic of symmetricdiatomic molecule ion H2+at low vibrational level. With theincreasing of initial vibrational level v, the interference minimumalso appears apparently in the harmonic spectra of HD+ion with the same harmonic intensity but it gradually disappears for H2+ion andthe harmonic intensity is enhanced.(4) Exploring of multichannelrecombination in harmonic emission from the asymmetricmolecular ion and controlling of quantum channel in high-orderharmonic generation by two-color laser fields. We explain thephenomena of the multichannel recombination in harmonicemission of asymmetric molecular ion, firstly. Then, a scheme isproposed to achieve the aim of controlling the multiple channels byusing asymmetric laser fields which are synthesized by thefundamental pulse and its second harmonic pulse. The results showthat the periodical change of the electronic probability densitydistribution around the nucleus is the key to the periodical changeof quantum channel. And we come to a conclusion that thepotentials of different nucleus in asymmetric molecule ion aredissimilar, which leads to the degree of the potential well andpotential barrier enhanced (or reduced) are different when themolecule ion interacts with the same amplitude of laser field atdifferent sign (positive or negative) for each nucleus. And it isresponsible for the different ionization probabilities at those laserparameters, resulting in the intensities of the differentrecombination channels are dissimilar. Finally, the aim to controlthe channel recombination in HHG can be achieved by breaking the symmetry of laser fields.