Zhang Jing-Tao

Intensity-dependent asymmetric photoionization in few-cycle laser pulses

The asymmetric photoionization of atoms irradiated by intense, few-cycle laser pulses is studied numerically. The results show that the pulse intensity affects the asymmetric photoionization in three aspects. First, at higher intensities, the asymmetry becomes distinctive for few-cycle pulses of longer durations. Second, as the laser intensity increases, the maximal asymmetry first decreases then increases after it has reached a minimal value. Last, the value of the carrier-envelope phase corresponding to the maximal asymmetry varies with the pulse intensity. This study reveals that the increasing of pulse intensity is helpful for observing the asymmetric photoionization.

Duration-dependent asymmetry in photoionization of H atoms in few-cycle laser pulses

The photoionization of H atoms irradiated by few-cycle laser pulses is studied numerically. The variations of the total ionization, the partial ionizations in opposite directions, and the corresponding asymmetry with the carrier-envelope phase in several pulse durations are obtained. We find that besides a stronger modulation on the partial ionizations, the change of pulse duration leads to a shift along carrier-envelope (CE) phase in the calculated signals. The phase shift arises from the nonlinear property of ionization and relates closely to the Coulomb attraction of the parent ion to the ionized electron. Our calculations show good agreement with the experimental observation under similar conditions.

A Scaling Law of Photoionization in Ultrashort Pulses

By means of the numerical solution of time-dependent Schrodinger equation, we verify a scaling law of photoionization in ultrashort pulses. We find that for a given carrier-envelope phase and duration of the pulse, identical photoionizations are obtained provided that when the central frequency of the pulse is enlarged by k times, the atomic binding potential is enlarged by k times, and the laser intensity is enlarged by k3 times. The scaling law allows us to reach a significant control over direction of photoemission and offers exciting prospects of reaching similar physical processes in different interacting systems which constitutes a novel kind of coherent control.

A Scaling Law of Photoelectron Angular Distributions in One-Cycle Laser Pulses

The photoelectron angular distributions (PADs) from above-threshold ionization of atoms irradiated by one-cycle laser pulses satisfy a scaling law. The scaling law denotes that the main features of the PADs are determined by four dimensionless parameters: (1) the ponderomotive number up = Up/ω, the ponderomotive energy Up in units of laser photon energy; (2) the binding number eb = Eb/ω, the atomic binding energy Eb in units of laser photon energy; (3) the number of absorbed photons q; (4) the carrier-envelope phase 0, the phase of the carrier wave with respect to the envelope. We verify the scaling law by theoretical analysis and numerical calculation, compared to that in long-pulse case. A possible experimental test to verify the scaling law is suggested.

A scaling law of high-order harmonic generation

Using a nonperturbative quantum electrodynamics theory of high-order harmonic generation (HHG), a scaling law of HHG is established. The scaling law states that when the atomic binding energy Eb, the wavelength λ and the intensity I of the laser field change simultaneously to kEb, λ/k, and k3I, respectively. The characteristics of the HHG spectrum remain unchanged, while the harmonic yield is enhanced k3 times. That HHG obeys the same scaling law with above-threshold ionization is a solid proof of the fact that the two physical processes have similar physical mechanisms. The variation of integrated harmonic yields is also discussed.

Carrier-Envelope Phase-Dependent Effect on Photoelectron Angular Distribution in Single-Cycle Laser Pulses

Using a nonperturbative scattering theory, we study the photoelectron angular distributions (PADs) of Kr atoms irradiated by an infinite sequence of intense single-cycle pulses of circular polarization. We demonstrate the inversion asymmetry of PADs and the dependence of PADs on the carrier-envelope phase of the single-cycle pulses. The inversion asymmetry is caused by the interference between transition channels where the different channels are characterized by different combinations of absorbed-photon numbers in the ionization process. Our results provide a possible method to determine the value of carrier-envelope phase by the detected PADs.

Optical bistability in V medium by squeezed vacuum input

We discuss the optical bistable behavior of a system of N V-type three-level atoms pumped by a coherent input field and coupled to a squeezed vacuum field by treating the optical bistability of such a system as an input-output problem. It is shown that bistability can be realized due to the squeezed vacuum input.

Imprints of molecular orbitals using photoelectron angular distribution by strong laser pulses of circular polarization

We theoretically investigate the strong-field ionization of H+2 molecules in four different electronic states by calculating photoelectron angular distributions in circularly polarized fields. We find that the structure of photoelectron angular distribution depends on the molecular orbital as well as the energy of the photoelectron. The location of main lobes changes with the symmetric property of the molecular orbital. Generally, for molecules with bonding electronic states, the photoelectrons angular distribution shows a rotation of π/2 with respect to the molecular axis, while for molecules with antibonding electronic states, no rotation occurs. We use an interference scenario to interpret these phenomena. We also find that, due to the interference effect, a new pair of jets appears in the waist of the main lobes, and the main lobes or jets of the photoelectrons angular distribution are split into two parts if the photoelectron energy is sufficiently high.

Calculation of the photoelectron spectra under a scaling transform

By solving the time-dependent Schrodinger equation, the dependence of photoelectron energy spectra on the binding energy of targets, wavelength and the intensity of laser pulse is exhibited and a scaling law of kinetic energy spectra of both the direct and the rescattered photoelectrons is concluded. The scaling law provides a convenient tool to determine the equivalent photoionization process of various atoms or molecules in various laser fields. The verification of the scaling law by independent methods provides incontestable support to the validity of the scaling law.

Multiple-plateau structure and scaling relation in photoelectron spectra of high-order above-threshold ionization

By developing a full quantum scattering theory of high-order above-threshold ionization, we study the energy spectra and the angular distributions of photoelectrons from atoms with intense laser fields shining on them. We find that real rescattering can occur many times, and even infinite times. The photoelectrons from the rescattering process form a broad plateau in the kinetic-energy spectrum. We further disclose a multiple-plateau structure formed by the high-energy photoelectrons, which absorb many photons during the rescattering process. Moreover, we find that both the angular distributions and the kinetic-energy spectra of photoelectrons obey the same scaling law as that for directly emitted photoelectrons.