High-harmonic Generation Of Atoms And Molecules And Its Applications

High-order harmonic generation(HHG)is an important nonlinear optical phenomenon in strong-field physics.It includes the ultrafast processes of the electron on the attosecond time-scale:ionization,acceleration and rescattering,which is a unique combination of classical mechanics and quantum mechanics.In attosecond physics,high-harmonic spectrum is a powerful tool to explore the ultrafast dynamics in atoms and molecules.On the one hand,this detection technology is based on the properties of high-harmonic spectra.On the other hand,it is based on the control of electron trajectories by non-trivial laser fields.In this thesis,by numerically solving the time-dependent Schr?dinger equation and combining with the quantitative analysis of the theoretical model,we study two important properties of the high-order harmonic spectrum,namely,the parity and frequency modulation in HHG.Further,the harmonic spectrum is applied to probe the dynamical symmetry in molecules and Coulomb field-induced ionization time delay in atomic HHG at attosecond resolution.Firstly,we study the two important properties of the high-order harmonic spectrum.(i)We find that only odd-order harmonics appear in the asymmetric-HD molecule harmonic spectrum.The HHG of the HD molecule is studied in non-Born-Oppenheimer treatment.It is found that there are only the odd harmonics in the harmonic spectrum of the HD molecule though the generation of even harmonics is possible in principle.Theoretical analysis reveals that the nuclear dipole moment can contribute to the generation of the even harmonics,but the acceleration of the nucleus is about three orders of magnitude less than that of the electron.Hence,the even harmonics cannot be observed in the harmonic spectrum of the HD molecule.(ii)We find that the change of the ionization energy is a new redshift mechanism in the HHG.We theoretically study the high-order harmonic generation ofH₂⁺and its isotopes beyond the Born-Oppenheimer dynamics.It is surprising that the spectral redshift can still be observed in high harmonic spectra of H₂⁺driven by a sinusoidal laser pulse in which the trailing(leading)edge of the laser pulse is nonexistent.The results confirm that this spectral redshift originates from the reduction of ionization energy between recombination time and ionization time,which is obviously different from the nonadiabatic spectral redshift induced by the falling edge of the laser pulse.Additionally,the improved instantaneous frequency of harmonics by considering the changeable ionization energy can deeply verify our results.Therefore,this new mechanism must be taken into account when one uses the nonadiabatic spectral redshift to retrieve the nuclear motion.Secondly,we explore the possibility of bicircular high-order harmonic spectroscopy to probe the laser-induced dynamics of molecules in a non-Born-Oppenheimer treatment.The numerical solutions of the time-dependent Schr?dinger equation for alignedH₂ and its isotopologs in?-2?bicircular fields show that the intensity ratio betweenD₂ andH₂ for harmonic orders 3q is lower than that for orders 3q±1(q?N).Based on the strong-field approximation,we demonstrate that the interplay of vibrational wave-packet motion and dynamical-symmetry breaking leads to the different ratio.In general,the vibrational motion causes the ratio between isotopologs to increase with q for both harmonic orders 3q and 3q±1.On the other hand,the emission of orders 3q is possible only because of the alignment-induced breaking of the dynamical symmetry.The faster nuclear motion inH₂enhances the symmetry breaking,resulting in the lowerD₂ H₂ ratio for the orders3q.Therefore,the harmonic orders 3q give access to the attosecond probing of dynamical symmetries in molecules.Finally,we focus on the probe of the ionization time in high-order harmonic generation.A weak probe field is applied to steer the lateral motion of the tunnelled electron in orthogonal two-color fields and then monitor its influence on the HHG,but without modifying the times of ionization and emission in HHG.It realizes the detection of the ionization and return times by observing the harmonic intensity and the amplitude ratio,as a function of the two-color delay.By solving the time-dependent Schr?dinger equation in two-color fields with tunable frequency streaking,we find that due to the mutual cancellation in the x and y directions of the effects of Coulomb forces on the observables,the ionization time from low streaking frequencies agrees perfectly with the quantum-orbit model,lacking Coulomb effects.In contrast,because of the negligible lateral Coulomb force on the outgoing electron,the ionization time extracted from high streaking frequencies preserves the Coulomb-induced time lag which is confirmed by both classical and quantum models.Our approach provides a feasible experimental tool to measure the Coulomb time delays by the time gap between the low-frequency and high-frequency streaking.