Peng-Cheng Li

Dynamical origin of near- and below-threshold harmonic generation of Cs in an intense mid-infrared laser field

Near- and below-threshold harmonic generation provides a potential approach to generate vacuum-ultraviolet frequency comb. However, the dynamical origin of in these lower harmonics is less understood and largely unexplored. Here we perform an ab initio quantum study of the near- and below-threshold harmonic generation of caesium (Cs) atoms in an intense 3,600-nm mid-infrared laser field. Combining with a synchrosqueezing transform of the quantum time-frequency spectrum and an extended semiclassical analysis, the roles of multiphoton and multiple rescattering trajectories on the near- and below-threshold harmonic generation processes are clarified. We find that the multiphoton-dominated trajectories only involve the electrons scattered off the higher part of the combined atom-field potential followed by the absorption of many photons in near- and below-threshold regime. Furthermore, only the near-resonant below-threshold harmonic is exclusive to exhibit phase locked features. Our results shed light on the dynamic origin of the near- and below-threshold harmonic generation.

A new time-frequency method to reveal quantum dynamics of atomic hydrogen in intense laser pulses: Synchrosqueezing transform

This study introduces a new adaptive time-frequency (TF) analysis technique, the synchrosqueezing transform (SST), to explore the dynamics of a laser-driven hydrogen atom at an ab initio level, upon which we have demonstrated its versatility as a new viable venue for further exploring quantum dynamics. For a signal composed of oscillatory components which can be characterized by instantaneous frequency, the SST enables rendering the decomposed signal based on the phase information inherited in the linear TF representation with mathematical support. Compared with the classical type of TF methods, the SST clearly depicts several intrinsic quantum dynamical processes such as selection rules, AC Stark effects, and high harmonic generation.

Optimization of three-color laser field for the generation of single ultrashort attosecond pulse

We present an efficient and realizable scheme for the generation of an ultrashort single attosecond (as) pulse. The feasibility of such a scheme is demonstrated by solving accurately the time-dependent Schrödinger equation using the time-dependent generalized pseudospectral (TDGPS) method. This scheme involves the use of the optimization of the three-color laser fields. The optimized laser pulse is synthesized by three one-color laser pulses with proper relative phases. It can provide a longer acceleration time for the tunneling and oscillating electrons, and allows the electrons to gain more kinetic energy. We show that the plateau of high-order harmonic generation is extended dramatically and a broadband supercontinuum spectra is produced. As a result, an isolated 23 as pulse with a bandwidth of 163 eV can be obtained directly by superposing the supercontinuum harmonics near the cutoff region. We will show that such a metrology can be realized experimentally.

Investigation of the characteristic properties of high-order harmonic spectrum in atoms using Bohmian trajectories

We study the electron quantum dynamics of high harmonic generation (HHG) processes of atomic hydrogen under intense near infrared (NIR) laser fields by means of the De Broglie–Bohms framework of Bohmian mechanics. The proposed accurate 3D numerical scheme is utilized to discuss the mechanism of the multiple plateau generation and the cut-off extension, as the main characteristic features of HHG spectrum. One-color (1600 nm) and two-color (1600 nm + 800 nm) laser fields with different time delays are used to investigate the effect of pulse shape on electron dynamics and HHG process. The presented results on Bohmian trajectories and their energy content, along with the analysis of the emission time period of different groups of trajectories, provide a comprehensive and fresh electron dynamical picture and uncover novel mechanisms of the HHG processes and power spectra.

High-order-harmonic generation of Ar atoms in intense ultrashort laser fields: An all-electron time-dependent density-functional approach including macroscopic propagation effects

We present an ab initio approach for the study of macroscopic high-order-harmonic generation (HHG) of rare-gas atoms (Ar) in intense ultrashort laser fields. The single-atom response is calculated by means of the self-interaction-free time-dependent density-functional theoretical (TDDFT) approach with proper long-range potential, and the macroscopic propagation effects are taken into account by solving Maxwell’s equations using the induced dipole moments of the single-atom multiple electron shell. We perform all-electron nonperturbative investigation of macroscopic HHG of rare-gas atoms with the emphasis on the role of multiple valence electron subshells. In particular, we found that the valence electron subshell features a destructive interference distribution in multiphoton resonant HHG. In addition, our results show the Cooper minimum in macroscopic HHG from Ar atoms is located at 53 eV, in good agreement with the experimentally measured minimum position.

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

We present a systematic study of spectral and temporal structure of high-order harmonic generation (HHG) by solving accurately the time-dependent Schrödinger equation for a hydrogen atom in the multiphoton regime where the Keldysh parameter is greater unity. Combining with a time-frequency transform and an extended semiclassical analysis, we explore the role of quantum trajectory in HHG. We find that the time-frequency spectra of the HHG plateau near cutoff exhibit a decrease in intensity associated with the short- and long-trajectories when the ionization process is pushed from the multiphoton regime into the tunneling regime. This implies that the harmonic emission spectra in the region of the HHG plateau near and before the cutoff are suppressed. To see the generality of this prediction, we also present a time-dependent density-functional theoretical study of the effect of correlated multi-electron responses on the spectral and temporal structure of the HHG plateau of the Ar atom.

Exploration of spectral and temporal fine structures of high-order harmonic generation1

We present an ab initio precision investigation of the time-frequency characteristics of high-order harmonic generation (HHG) calculated by solving accurately and efficiently the time-dependent Schrodinger equation by means of the time-dependent generalized pseudospectral method. We extend a new synchrosqueezing transform (SST) to obtain the time-frequency spectra of the HHG and the dynamical phase associated with the dipole-emission time profile. The SST time-frequency analysis allows us to explore the in depth dynamics and contributions of the multi-rescattering trajectories and resonant trajectories in HHG. It also exhibits the subtle details of the spectral and temporal fine structures of the HHG, providing novel insights regarding the dynamical origin of the HHG in below- and above-threshold regimes.

High-Order Harmonic Generation of hydrogen molecule ions in a large internuclear distance

Figure 1 shows the HHG spectra of H2 driven by a 1600-nm laser pulse with the peak intensity 14 2 2.14 10 W/cm I   . It is clear that a double platform structure can be found in the HHG spectra. The cutoff of the first plateau is equal to 37, which corresponds with the energy of classical calculation 0 eE R ( 0 E is the intensity of laser field) [4]. That is to say the first plateau of the HHG is due to the electron motion by migrating from one nucleus to the other.

Accurate Structure Parameters for Tunneling Ionization Rates of Gas-Phase Linear Molecules*

In the molecular Ammosov–Delone–Krainov (MO-ADK) model of Tong et al. [Phys. Rev. A 66 (2002) 033402], the ionization rate depends on the structure parameters of the molecular orbital from which the electron is removed. We determine systematically and tabulate accurate structure parameters of the highest occupied molecular orbital (HOMO) for 123 gas-phase linear molecules by solving time-independent Schrodinger equation with B-spline functions and molecular potentials which are constructed numerically using the modified Leeuwen–Baerends (LBα) model.