Attosecond Pulse Generation And Molecular Structure Detection Based On High Harmonic Generation

High-order harmonic generation (HHG) is a highly non-linear and non-perturbative process during strong-field light matter interactions. The recent advent of attosecond sources of extreme ultraviolet (XUV) and soft x-ray radiation has opened wide perspectives for probing dynamics in matter at the natural timescale of electron motion. It was recently realized that these same sources could also provide Angstrom resolved images of electronwave packets in molecules. The combination of these extreme spatial and temporal resolutions may fulfil the dream of imaging the most fundamental processes occurring in physics, chemistry or biology in real time. Understanding these processes is of utmost importance for future developments in science and technology as it is the key for controlling them.The applications can be grouped into two categories. The first one concerns ’conventional’pump-probe experiments, referred to as’direct’schemes, where the attosecond bunch of photons is used to excite or probe ultrafast dynamics in another system-a direct extension of femtosecond spectroscopy and femtochemistry to the attosecond domain. A number of field-driven and intrinsic processes with attosecond dynamics have already been studied in atoms, molecules and solids using direct schemes. In the second category of experiments, the harmonic signal is analyzed to retrieve temporal and structural insight into the generating system itself. In such’self-probing’schemes, the recolliding electronwave packet plays the role of a probe that encodes information on the generating molecule in the emitted radiation. By theoretical reconstrcuction algorithm, one can detect direct information on the radiating system with unprecedented temporal and spatial resolutions.Based on previous works, the contents of this thesis include:(1) We perform a systematical quantum-orbit analysis of HHG in intense elliptically polarized laser field. Both the dependence of the harmonic yield on laser ellipticity and the harmonic polarization properties is investigated. For the former, it is shown that when increasing the laser ellipticity, the initial velocity of the electron increases, causing the reducing of the ionization rate, thereby diminish the generation of harmonics. A physically transparent formula has been presented to calculate the ellipticity dependence of harmonic yield. For the latter, we show that the harmonic ellipticity originates from quantum uncertainty of the electron momentum of the relevant quantum electron trajectory.(2) Two schemes of generating isolated attosecond pulse with high efficiency are proposed. First, by using pre-excited medium, i.e. populating part of the electronwave packets to the excited state, the atom is easier to ionize. Thereby, the intensity of the attosecond pulse is increased. Second, isolated attosecond pulse generation with polarization gating Bessel-Gauss beam in strongly-ionized media is investigated. The results show that Bessel-Gauss beam has the ability to suppress the spatial plasma dispersion effects caused by high density of free electrons. Significant improvement of spatiotemporal properties of harmonics is achieved and an isolated attosecond pulse with high beam quality is filtered out.(3) Ultrafast imaging of molecular orbitals with different symmetries using attosecond photoelectron diffraction is studied. The internuclear distance of the molecule can be determined from the diffraction pattern of the photoelectron momentum distribution and the molecular orbitals can be successfully reconstructed using an inversion algorithm based on two-center interference model.(4) We develop a theory of molecular orbital tomography beyond plane-wave approximation. Using more accurate molecular continuum-wave functions, the mapping been molecular orbital and the harmonic signal is established and it is proved to be invertible. As an example, we reconstruct the orbital of the N2molecule by using two-center Coulomb waves and the retrieved orbital show quantitative agreement with the exact one. This work strengthens the theoretical basis of molecular-orbital tomography.