Role of quantum trajectory in high-order harmonic generation in the Keldysh multiphoton regime

Evolution of the frequency-comb structure and coherence from a Keldysh multiphoton into a tunneling regime

We present a theoretical study of the characteristics of the frequency-comb structure and coherence via high-order harmonic generation (HHG) driven by the laser pulse trains when the ionization process is pushed from Keldysh multiphoton into tunneling regime. HHG is obtained by solving accurately the time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. We find that the nested comb structures are formed from each harmonic order in the Keldysh multiphoton ionization regime. But it is severely suppressed or even disappeared in the Keldysh tunneling ionization regime. It implies that the temporal coherence of the emitted frequency comb modes is very sensitive to the Keldysh ionization regime. To understand the evolution of frequency-comb structure and coherence, we perform the calculation of the time-dependent ionization probability and the spectral phase of frequency-comb HHG. We find that the frequency-comb HHG driven by the laser pulse trains in the Keldysh multiphoton regime has a good coherence because the ionization probability of the atom driven by each laser pulse is stable, leading to a phase-coherent frequency-comb structure rather than those cases in the Keldysh tunneling regime with high laser intensity. Our results shed light on current interest and significance to the experimental realization of controllable and frequency-comb vacuum-ultraviolet light sources.

Resolving the quantum dynamics of near cut-off high-order harmonic generation in atoms by Bohmian trajectories

We present an ab initio study of the quantum dynamics of high-order harmonic generation (HHG) near the cutoff in intense laser fields. To uncover the subtle dynamical origin of the HHG near the cutoff, we extend the Bohmian mechanics (BM) approach for the treatment of attosecond electronic dynamics of H and Ar atoms in strong laser fields. The time-dependent Schrödinger equation and the self-interaction-free time-dependent density functional theory are numerically solved accurately and efficiently by means of the time-dependent generalized pseudospectral method for nonuniform spatial discretization of the Hamiltonian. We find that the most devoting trajectories calculated by the BM to the plateau harmonics are shorter traveling trajectories, but the contributions of the short trajectories near the cutoff are suppressed in HHG. As a result, the yields of those harmonics in the region near the cutoff are relatively weak. However, for the last few harmonics just above the cutoff, the HHG intensity becomes a little higher. This is because the HHG just above the cutoff arises from those electrons ionized near the peak of the laser pulse, where the ionization rate is the highest. In addition, the longer Bohmian trajectories return to the core with lower energies, these trajectories contribute to the below-threshold harmonics. Our results provide a deeper understanding of the generation of supercontinuum harmonic spectra and attosecond pulses via near cutoff HHG.

Unravelling the dynamical origin of below- and near-threshold harmonic generation of H2+ in an intense NIR laser field

Recently, the study of near- and below- threshold regime harmonics as a potential source of intense coherent vacuum-ultraviolet radiation has received considerable attention. However, the dynamical origin of these lower harmonics, particularly for the molecular systems, is less understood and largely unexplored. Here we perform the first fully ab initio and high precision 3D quantum study of the below- and near-threshold harmonic generation of molecules in an intense 800-nm near-infrared (NIR) laser field. Combining with a synchrosqueezing transform of the quantum time-frequency spectrum and an extended semiclassical analysis, we explore in-depth the roles of various quantum trajectories, including short- and long trajectories, multiphoton trajectories, resonance-enhanced trajectories, and multiple rescattering trajectories of the below- and near- threshold harmonic generation processes. Our results shed new light on the dynamical origin of the below- and near-threshold harmonic generation and various quantum trajectories for diatomic molecules for the first time.