Wei-Hao Xiong

Nonadiabatic tunneling ionization of atoms in elliptically polarized laser fields

We theoretically investigate the nonadiabatic effects in strong field tunneling ionization of atoms in elliptically polarized laser fields by solving the 3D time-dependent Schrodinger equation (TDSE). Comparing our TDSE results with those of two semi-classical methods, i.e., the quantum-trajectory Monte Carlo simulation (QTMC) and the Coulomb-corrected strong field approximation (CCSFA), we confirm the existence of the nonadiabatic effects with its fingerprint in the nonzero initial lateral velocity at the tunneling exit in the laser polarization plane. Our study shows that these nonadiabatic initial lateral momentum effects become significant in high ellipticity or circularly polarized laser field. These results indicate that the calibration of the experimental laser intensity in this situation should be performed nonadiabatically, which may strongly affect the results of the real tunneling time delay measurements.

Double ionization of He by time-delayed attosecond pulses

As He is exposed to double attosecond pulses with a time delay, double ionization can proceed in several different pathways. Through ab initio simulations by the fully dimensional time-dependent Schrodinger equation, we find that interferences among different pathways in the sequential two-photon double ionization can lead to interesting grid-like patterns in the joint energy spectra of the two electrons. We show that not only these interference patterns, but also the total double ionization probability critically depends on the relative carrier–envelope phase (CEP) between the two attosecond pulses. Our findings are successfully explained by a model based on second-order time-dependent perturbation theory. The present study demonstrates the feasibility of CEP control of double ionization, and provides an alternative way to characterize the relative CEP of attosecond pulses.

Half-cycle cutoff in near-threshold harmonic generation

A half-cycle cutoff is identified in the harmonic generation spectra near the ionization potential for short driving laser pulses. Unlike the half-cycle cutoff in the high-energy region, the newly found low-energy cutoff is strongly affected by the ionic potential and multiple return trajectories. We show their contribution clearly in the observable harmonic spectrum based on the reference of this cutoff structure. Our results are calculated from a numerical solution to the 3D time-dependent Schrodinger equation and a classical trajectory Monte Carlo method. By comparing the results from both methods, we analyze the time-dependent sub-cycle electron dynamics and provide a transparent explanation to this half-cycle cutoff. We further investigate the low-energy harmonic yield as a function of CEP for different pulse durations. The modulation depth of this yield drops rapidly when the pulse duration is increased, which can explain the CEP-dependence recently observed in the experiment.