XuanYang Lai

Temporal and spatial interference in molecular above-threshold ionization with elliptically polarized fields

We investigate direct above-threshold ionization in diatomic molecules, with particular emphasis on how quantum interference is altered by a driving field of nonvanishing ellipticity. This interference may be either temporal, i.e., related to ionization events occurring at different times, or spatial, i.e., related to the electron emission at different centers in the molecule. Employing the strong-field approximation and saddle-point methods, we find that, in general, for nonvanishing ellipticity, there will be a blurring of the temporal and spatial interference patterns. The former blurring is caused by the electron velocity component perpendicular to the major polarization axis, while spatial interference is washed out as a consequence either of s-p mixing or of the temporal dependence of the ionization prefactor. Both types of interference are analyzed in detail in terms of electron trajectories, and specific conditions for which sharp fringes occur are provided.

Quantum Orbits: A Powerful Concept in Laser-Atom Physics

Additional support for the concept of ”quantum orbits” is presented that emphasizes in particular the importance of ”long orbits” where the time between ionization and rescattering may be many periods of the laser field. Two examples are discussed, above-threshold ionization by an elliptically polarized laser field and intensity-dependent enhancements of certain spectral regions within the backscattering plateau, where we compare experimental data and the theoretical quantum-orbit simulations. In both cases, good agreement is obtained.

Disentangling intracycle interferences in the photoelectron spectrum of argon using orthogonally polarized, two-colour laser pulses

In recent years, the widespread availability of multi-dimensional photoelectron spectroscopy has created the opportunity to systematically study the complex interference patterns inherent to multiphoton ionization of atoms and molecules. The dominant interference mechanism in multiphoton ionization leads to discrete peaks in the photoelectron spectrum (PES) that are spaced by the photon energy of the driving laser field. This mechanism is called above-threshold ionization (ATI) and has its origin in the coherence of ionization events repeating at every oscillation of the optical field [1]. However, many more interference mechanisms contribute to the observed photoelectron spectrum. Specifically, photoelectrons emitted within one cycle of the optical field can also interfere on the detector. These interferences from photoelectrons originating in adjacent quarter-cycles of the optical field are also known as the temporal double slit (e.g., [1]). In addition, scattering of the photoelectron on the parent ion potential will give rise to holographic interferences [2], and laser induced electron diffraction [3].

Laser-Induced Inelastic Diffraction from Strong-Field Double Ionization

In this Letter, we propose a novel laser-induced inelastic diffraction (LIID) scheme based on the intense-field-driven atomic nonsequential double ionization (NSDI) process and demonstrate that, with this LIID approach, the doubly differential cross sections (DDCSs) of the target ions, e.g., Ar^{+} and Xe^{+}, can be accurately extracted from the two-dimensional photoelectron momentum distributions in the NSDI process of the corresponding atoms. The extracted DDCSs exhibit a strong dependence on both the target and the laser intensity, in good agreement with calculated DDCSs from the scattering of free electrons. The LIID scheme may be extended to molecular systems and provides a promising approach for imaging of the gas-phase molecular dynamics induced by a strong laser field with unprecedented spatial and temporal resolution.

Polarization effects in above-threshold ionization of Mg with a mid-infrared strong laser field

Using a semiclassical approach, we find an enhancement of low-energy structure in above-threshold ionization spectra of Magnesium by intense, mid-infrared laser pulses. By performing electron trajectory and recollision-time distribution analysis, we demonstrate this enhancement results from a focusing of electrons by laser induced dynamical polarization.

Analytic quantum-interference conditions in Coulomb corrected photoelectron holography

We provide approximate analytic expressions for above-threshold ionization (ATI) transition probabilities and photoelectron angular distributions (PADs). These analytic expressions are more general than those existing in the literature and include the residual binding potential in the electron continuum propagation. They successfully reproduce the ATI side lobes and specific holographic structures such as the near-threshold fan-shaped pattern and the spider-like structure that extends up to relatively high photoelectron energies. We compare such expressions with the Coulomb quantum orbit strong-field approximation (CQSFA) and the full solution of the time-dependent Schrodinger equation for different driving-field frequencies and intensities, and provide an in-depth analysis of the physical mechanisms behind specific holographic structures. Our results shed additional light on what aspects of the CQSFA must be prioritized in order to obtain the key holographic features, and highlight the importance of forward scattered trajectories. Furthermore, we find that the holographic patterns change considerably for different field parameters, even if the Keldysh parameter is kept roughly the same.

Long-Range Coulomb Effect in Intense Laser-Driven Photoelectron Dynamics

In strong field atomic physics community, long-range Coulomb interaction has for a long time been overlooked and its significant role in intense laser-driven photoelectron dynamics eluded experimental observations. Here we report an experimental investigation of the effect of long-range Coulomb potential on the dynamics of near-zero-momentum photoelectrons produced in photo-ionization process of noble gas atoms in intense midinfrared laser pulses. By exploring the dependence of photoelectron distributions near zero momentum on laser intensity and wavelength, we unambiguously demonstrate that the long-range tail of the Coulomb potential (i.e., up to several hundreds atomic units) plays an important role in determining the photoelectron dynamics after the pulse ends.

Laser intensity determination using nonadiabatic tunneling ionization of atoms in close-to-circularly polarized laser fields

We conceive an improved procedure to determine the laser intensity with the momentum distributions from nonadiabatic tunneling ionization of atoms in the close-to-circularly polarized laser fields. The measurements for several noble gas atoms are in accordance with the semiclassical calculations, where the nonadiabatic effect and the influence of Coulomb potential are included. Furthermore, the high-order above-threshold ionization spectrum in linearly polarized laser fields for Ar is measured and compared with the numerical calculation of the time-dependent Schrödinger equation in the single-active-electron approximation to test the accuracy of the calibrated laser intensity.

Double Ionization Dynamics of Molecular Hydrogen in Ultrashort Intense Laser Fields

We experimentally investigate the double ionization of molecular hydrogen subjected to ultrashort intense laser pulses. The total kinetic energy release of the two coincident H+ ions, which provides a diagnosis of different processes to double ionization of H2, is measured for two different pulse durations, i.e., 25 and 5 fs, and various laser intensities. It is found that, for the long pulse duration (i.e., 25 fs), the double ionization occurs mainly via two processes, i.e., the charge resonance enhanced ionization and recollision-induced double ionization. Moreover, the contributions from these two processes can be significantly modulated by changing the laser intensity. In contrast, for a few-cycle pulse of 5 fs, only the recollsion-induced double ionization survives, and in particular, this process could be solely induced by the first-return recollision at appropriate laser intensities, providing an efficient way to probe the sub-laser-cycle molecular dynamics.