High-order Harmonic Generation From Aligned Molecules With Various Structures

Measuring and understanding the electronic structure and dynamics of matter on its natural timescale and spatial scale is one of the goals for scientists in the fields that encompass physics, chemistry and biology. It has been shown that high-order harmonic generation (HHG), which occurs in the interaction of strong laser field with atomic and molecular gases, can be used as an effective tool to probe the electronic structure and dynamics in atoms and molecules with attosecond temporal and Angstrom spatial resolutions. This important and amazing application of HHG has attracted a lot of interest in the past decades. The first observation of high-order harmonic emissions was in the rare atomic gases. The mechanism and spectral properties of HHG from atoms has been well studied and understood. Due to its good spectral properties, HHG from atoms has been widely used in producing coherent extreme ultraviolet (XUV) even soft x-ray light sources, as well as attosecond pulses, which can provide a powerful tool for probing the ultrafast dynamics in atoms, molecules and matters. Compared with the atomic targets, the molecular targets possess more complex structure, such as the multicenter structure, diverse structure and symmetry of the molecular orbitals. In addition, the degree of freedom in the molecular target increases, due to the multicenter structure. The complex structure of molecules and the extra degrees of freedom, such as rotation and vibration, can complicate the process of high-order harmonic generation and result in new spectral properties of the high-order harmonics. Correspondingly, new applications of the high-order harmonic generation would be found, such as high-order harmonic spectroscopy of molecular structure, molecular orbital tomography with high spatial and temporal resolutions, probing the ultrafast dynamics in molecules such as rotational dynamics. In view of the above objectives, my Ph.D study focuses on the effects of molecular structure and extra degrees of freedom on HHG and exploiting new potential applications of HHG. The contents of this thesis include:(1) We investigate the polarization properties of the high-order harmonics generated from polar molecules, of which the molecular orbitals are asymmetric, driven by a linearly polarized laser pulse. It is found that elliptically polarized harmonics are observed in a wide spectral range from the plateau to the cutoff. These results provide a new efficient method for producing large-ellipticity XUV pulses, which will benefit the application of HHG as a tool of detection in materials and biology science. Following the above investigation, we systematically analyze the feature of the harmonic ellipticity map for the molecular orbitals with different symmetry. It is found that the molecular-orbital geometry is imprinted on the ellipticity of the high-order harmonics, which invites the use of ellipticity measurements as a probe of the orbital structure for polar molecules.(2) We investigate the spectral properties of the high-order harmonic spectra generated from the molecular ensembles that are nonadiabatically aligned. First, we study the interference effects of high-order harmonics generated from molecules aligned at different angles. We well reproduce the results obtained in recent experiments and successfully explain the new phenomena observed in the harmonic spectra from the molecular ensembles, such as the anomalous harmonic cutoffs and the shift of the spectral minimum with the degree of alignment. Based on the above investigation, we further investigate the high-order harmonic spectra generated from molecular ensembles at various delays. We find that the spectral minimum position depends linearly on the inverse of the alignment parameter. Using this simple linear relationship, one can easily obtain the value of the alignment parameter from the spectral minimum position. As the variation of the alignment parameter with the time delay can well characterize the evolution of molecular rotational dynamics, our result provides an all-optical method of tracing the rotational dynamics.(3) We investigate the reconstruction of polar molecular orbitals based on the high-order harmonic generation. It has been shown that to reconstruct the asymmetric molecular orbitals, the returning electrons should be controlled to recollide with the parent ion from only one direction. To achieve this, a scheme with one-color laser field was proposed. In this scheme, however, the requirement of the laser pulse is rather stringent, i.e., the driving pulse must be a single-cycle pulse with a stabilized and controllable CEP and a sufficient intensity. Based on the efficient control of the electron dynamics by the two-color laser field, we propose a two-color scheme for tomographic reconstruction of asymmetric molecular orbitals. Our scheme can achieve the unidirectional recollisions of the returning electrons in the multicycle regime. This releases the very stringent requirement of the driving pulse in the one-color scheme. With our two-color multicycle laser scheme, the highest occupied molecular orbital of CO molecule is satisfactorily reconstructed.