Quantum Electrodynamics of Strong Fields

The last decade has seen an intense renewed debate on tunnelling time, both from a theoretical and an experimental perspective. Here, we review recent developments and new insights in the field of ...

https://www.tandfonline.com/doi/pdf/10.1080/09500340.2019.1596325

Attoclock revisited on electron tunnelling time

The exact nondipole Volkov solutions to the Schrodinger equation and Pauli equation are found, based on which a strong field theory beyond the dipole approximation is built for describing the nondipole effects in nonrelativistic laser driven electron dynamics. This theory is applied to investigate momentum partition laws for multiphoton and tunneling ionization and explicitly shows that the complex interplay of a laser field and Coulomb action may reverse the expected photoelectron momentum along the laser propagation direction. The magnetic-spin coupling does not bring observable effects on the photoelectron momentum distribution and can be neglected. Compared to the strong field approximation within the dipole approximation, this theory works in a much wider range of laser parameters and lays a solid foundation for describing nonrelativistic electron dynamics in both short wavelength and midinfrared regimes where nondipole effects are unavoidable.

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Strong Field Theories beyond Dipole Approximations in Nonrelativistic Regimes.

We explore ionization and rescattering in strong mid-infrared laser fields in the nondipole regime over the full range of polarization ellipticity. In three-dimensional photoelectron momentum distributions (3D PMDs) measured with velocity map imaging spectroscopy, we observe the appearance of a sharp ridge structure along the major polarization axis. Within a certain range of ellipticity, the electrons in this ridge are clearly separated from the two lobes that commonly appear in the PMD with elliptically polarized laser fields. In contrast to the well-known lobes of direct electrons, the sharp ridge is created by Coulomb focusing of the softly recolliding electrons. These ridge electrons are directly related to a counterintuitive shift of the PMD peak opposite to the laser beam propagation direction when the dipole approximation breaks down. The ellipticity-dependent 3D PMDs give access to different ionization and recollision dynamics with appropriate filters in the momentum space. For example, we can extract information about the spread of the initial wave packet and the Coulomb momentum transfer of the rescattering electrons.

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Probing the ionization wave packet and recollision dynamics with an elliptically polarized strong laser field in the nondipole regime

We investigate tunneling ionization of a model helium atom in a strong circularly polarized short laser pulse using the classical backpropagation method and compare ten different tunneling criteria on the same footing, aiming for a consistent classical picture of the tunneling dynamics These tunneling criteria are categorized into velocity-based, position-based, and energy-based criteria according to different notions of a tunnel exit We find that velocity-based criteria give consistent tunneling exit characteristics with nonadiabatic effects fully included Other criteria are either inconsistent or only able to include nonadiabatic effects partially Furthermore, we construct a simple tunneling rate formula, identify a term in the rate responsible for the nonadiabatic effects, and demonstrate the importance of this term

Tunneling criteria and a nonadiabatic term for strong-field ionization

We investigate the ionization dynamics of atoms irradiated by an intense laser field using a semiclassical model that includes magnetic Lorentz force in the rescattering process. We find that the electrons tunneled with different initial transverse momenta (i.e., perpendicular to the instantaneous electric field direction) distributed on a specific circle in the momentum plane can finally converge to the same transverse momentum after experiencing Coulomb forward scattering. These electron trajectories lead to a bright spot structure in the two-dimensional transverse momentum distribution, and particularly in the long-wavelength limit, a nonzero momentum peak in the direction antiparallel to the laser propagation (or radiation pressure) direction. By analyzing the subcycle dynamics of rescattering trajectories, we unveil the underlying mechanism of the anti-intuitive peak. Beyond the strong-field approximation and the dipole approximation, we quantitatively predict the spot center and the peak position. Our results are compared with a recent experiment and some theoretical predictions are given.

Coulomb rescattering in nondipole interaction of atoms with intense laser fields

Coulomb effects in photon-momentum partitioning during atomic ionization by intense linearly polarized light

We theoretically investigate the electron-tunneling process for a helium atom irradiated by an elliptical strong laser field. The momentum distribution for an electron ionized during the cycle when the laser intensity reaches its maximum is captured, such that we can ignore the interference between the wave packets ionized in different laser cycles and precisely determine the center of the momentum distribution. The quantum mechanical prediction of the center position is further compared to the semiclassical single-trajectory simulation as well as the experimental data. We find that the electron momentums along the minor axis of the laser polarization show good agreement with the nonadiabatic semiclassical calculation for a wide range of laser intensities, indicating the existence of a nonzero lateral momentum when the electron exits the barrier. On the other hand, the offset angles obtained by our quantum mechanical approach for different laser intensities are larger than the nonadiabatic semiclassical results, indicating the importance of the quantum effects during the electron's under-the-barrier dynamics.

Nonadiabaticity of electron-tunneling-ionization processes in elliptical strong laser fields

In the present paper, we perform the classical trajectory Monte Carlo simulation of the complex dynamics of accelerating neutral atoms with linearly or circularly polarized intense laser pulses. Our simulations involve the ion motion as well as the tunneling ionization and the scattering dynamics of valence electron in the combined Coulomb and electromagnetic fields, for both helium (He) and magnesium (Mg). We show that for He atoms, only linearly polarized lasers can effectively accelerate the atoms, while for Mg atoms, we find that both linearly and circularly polarized lasers can successively accelerate the atoms. The underlying mechanism is discussed and the subcycle dynamics of accelerating trajectories is investigated. We have compared our theoretical results with a recent experiment [Eichmann Nature (London) 461, 1261 (2009)].

Classical-trajectory simulation of accelerating neutral atoms with polarized intense laser pulses

In this paper, we explore the intensity-dependent strong-field double ionization of Ne with orthogonally polarized two-color laser pulses consisting of 800- and 400-nm laser fields. The yield of ${\text{Ne}}^{2+}$ as a function of the relative phase $\mathrm{\ensuremath{\Delta}}\ensuremath{\phi}$ of the two colors experiences a qualitative transition as the laser intensity decreases from the saturation regime to the far-below-saturation regime. In the saturation regime, our simulations well reproduce the recent experimental observations [Phys. Rev. Lett. 112, 193002 (2014)]. Turning to the far-below-saturation regime, however, we find that the observed small knee structure totally disappears and the maximum yield of ${\text{Ne}}^{2+}$ is shifted by a $\ensuremath{\pi}/2$ phase. This is explained by the competition between the trajectory concentration effect and the $\mathrm{\ensuremath{\Delta}}\ensuremath{\phi}$-dependent ionization rate of the tunneling electron. The possibility of controlling over the two-electron emission direction along the 400-nm field through the laser intensity is also investigated. We show that the two-electron emission direction can be reversed by changing the laser intensity for some vales of $\mathrm{\ensuremath{\Delta}}\ensuremath{\phi}$, while this fails for some other values of $\mathrm{\ensuremath{\Delta}}\ensuremath{\phi}$.

Q. Z. Xia

Papers

Coulomb rescattering in nondipole interaction of atoms with intense laser fields

Coulomb effects in photon-momentum partitioning during atomic ionization by intense linearly polarized light

Nonadiabaticity of electron-tunneling-ionization processes in elliptical strong laser fields

Classical-trajectory simulation of accelerating neutral atoms with polarized intense laser pulses

Intensity-dependent two-electron emission dynamics with orthogonally polarized two-color laser fields