Elliptical Laser Field Research Articles

Alignment dependence of photoelectron momentum distributions for diatomic molecule N2 in strong elliptical laser fields

The ionization dynamics of aligned N2 molecules are studied in strong elliptical laser fields experimentally and theoretically. The alignment-dependent photoelectron momentum distribution of N2 is measured for highlighting the molecular structure contribution by comparing to that of Ar measured synchronously. Our results show that the ionization of N2 depends strongly on the alignment of molecules, relative to the main axis of the polarization ellipse of the laser. In particular, the most-probable electron-emission angle which is often used in attosecond measurement changes remarkably when changing the relative angle between the molecular axis and the major axis of laser fields. The alignment-dependent rotation angles have been well reproduced by our theoretical calculations. We show that the interplay between molecular structure and the laser fields plays an important role in the rotation angles based on the strong-field approximation analysis and this interaction also influences remarkably on the photoelectron angle distribution of aligned N2.

Third-order harmonic generation in a bi-chromatic elliptical laser field.

The low-order harmonic generation induced by a strong laser field produces a bright, ultrashort, supercontinuum radiation ranging from the terahertz to ultraviolet band. By controlling the phase-delay and ellipticity of the bi-chromatic laser fields, the third harmonic generation is experimentally and theoretically investigated for elucidating the mechanism of the low-order harmonics. The third harmonic generation is found to be strongly suppressed in the counter-rotating bi-chromatic laser field due to the selection rule for harmonic emissions. The continuum-continuum transition in the strong field approximation is extended to explain the third harmonic generation as a function of the phase delay and ellipticity of the bi-chromatic laser fields. Compared with the semi-classical photocurrent model, the continuum-continuum transition on the basis of quantum-mechanical treatment achieves better agreement with the experimental observations. Our work indicates that the overlapping in continuum states via different quantum paths of a single electron plays a role in low-order harmonics generation under elliptical bi-chromatic laser fields.

High harmonic generation in two-color elliptical laser fields of different helicity

This paper presents the results from the generation of elliptical high harmonics in a process of two-color elliptical laser field interaction with a single atom. Numerical simulations are performed for different values of ellipticities of the two-color driving laser field (the fundamental and the second harmonics of the Ti:Sapphire laser), together with their relative helicity. The dependency of the intensity and ellipticity of harmonics as functions of the ellipticity of the driving field components and their relative helicity is discussed. The presence of the sequence of two strong and one weak harmonics in the generated spectra in the case of a counter rotating bicircular laser field is demonstrated in this paper. The origin of weak harmonics generation is also discussed.

Photoabsorption of atoms in a strong elliptical laser field

SynopsisThe photoabsorption of hydrogen atoms in a strong elliptical laser field was studied by solving the three-dimensional time-dependent Schrödinger equation. We found that the photoexcitation and ionization probabilities smoothly cross the ionization threshold and we can treate them in a unified way even in the strong elliptical laser field. Such findings may help us to understand how atomic structure affects the strong field ionization near the ionization threshold.

Coulomb effect on the dynamics of atoms in a strong elliptical laser field: Unification of the excitation and ionization

We investigate the Coulomb effect on the dynamics of atoms in a strong elliptical laser field by solving the three-dimensional time-dependent Schr\odinger equation with and without the photoelectron core Coulomb interactions and find that the Coulomb effect plays an important role for the excitation and ionization processes, while it is less important for the high-order harmonic generation. Our calculation results can serve as a benchmark for other approximative methods. Looking at the photoelectron momentum distribution, we find that the Coulomb effect is important for the momentum distribution in the polarization plane and less important for the momentum distributions along the normal direction perpendicular to the polarization plane. Similar to the photoabsorption in a weak field, we demonstrate that the excitation and ionization can be treated in a unified way, which implies the low-energy photoelectron distributions can be connected with the populations of highly excited states.

Asymmetry of the photoelectron momentum distribution from molecular ionization in elliptically polarized laser pulses

We theoretically study the photoelectron momentum distributions (PMDs) of ${{\mathrm{H}}_{2}}^{+}$ in elliptically polarized laser fields by numerically solving the time-dependent Schr\odinger equation. Depending on the internuclear distance of ${{\mathrm{H}}_{2}}^{+}$, the PMDs reveal different asymmetries with respect to the major and minor axes of the laser ellipse. At an internuclear distance corresponding to the enhanced ionization region, the asymmetry of the PMDs is opposite to that of the companion atom. We attribute this phenomenon to the effect of the internal scattered electrons, which are mainly released at an earlier ionization moment with respect to the field maximum within each half cycle. We further show that the asymmetry of the PMDs in the elliptical laser fields can be used to identify the instant when the electron wave packet is released from the molecule.

Ionization dynamics of polar molecules in strong elliptical laser fields

We study ionization dynamics of oriented polar molecules in strong elliptically polarized laser fields. The calculated photoelectron momentum distributions show an irregular half-ring structure from which we track the electron motion and identify two different ionization channels, i.e., the ground-state one and the excited-state one. We show that the excited-state channel is usually temporally delayed by a few hundred attoseconds in comparison with the ground-state one. This dynamical delay is mapped in the momentum distribution, resulting in the irregular half-ring structure. The characteristics of this structure provide potential observables for studying excited-state effects in ionization of polar molecules in strong laser fields.

A three-dimensional time-dependent Schrödinger equation solver: an application to hydrogen atoms in an elliptical laser field

We extended the two-dimensional generalized pseudo-spectral time-propagator to a three-dimensional time-propagator. With this time-propagator, we theoretically investigated the above-threshold ionization of hydrogen atoms in an elliptical field by solving the time-dependent Schrödinger equation. We found that the total ionization probabilities are suppressed about one order, while the excitation probabilities drop about eight orders from a linearly polarized laser to a circularly polarized one with a given laser intensity. The width of the lateral (perpendicular to the laser polarization plane) momentum distribution increases as the ellipticity increases, which is against our intuition that the large peak laser field results in a broad distribution in the lateral direction. Further analysis shows that the narrow lateral momentum distribution for linearly polarized laser fields originates from Coulomb focusing, which is as less or not important for the circularly polarized field. Such a high precision simulation can be used to calibrate the infrared laser intensity to a high precision.

Fully differential study on dissociative ionization dynamics of deuteron molecules in strong elliptical laser fields

Deuteron momentum distributions from the dissociative ionization of ${\mathrm{D}}_{2}$ in intense elliptically polarized laser fields have been explored in a joint experimental and numerical study. The asymmetrical charge localization in the dissociative ${{\mathrm{D}}_{2}}^{+}$ offers a large torque, and thus an elliptically polarized laser field efficiently rotates the molecular ion during its dissociation, resulting in the emission of deuterons finally deviating from the bond direction at the instant of tunneling ionization of ${\mathrm{D}}_{2}$. The rotating torque of the molecular ions increases with the field ellipticity, leading to an ellipticity-dependent tilt angle for the deuteron momentum distribution. Due to the notable rotation of ${{\mathrm{D}}_{2}}^{+}$ during its dissociation, the photoelectron angular distributions in the laboratory frame and the molecular frame are distinct, which illustrates that the axial recoil approximation is broken for discussing the photoelectron angular distributions of molecules in elliptically polarized laser fields.

Behaviour of tunnelling transition rate of argon atom exposed to strong low-frequency elliptical laser field

We considered the tunnelling ionization of an electron under the influence of a monochromatic laser beam with the elliptical polarization. Arbitrary values of ellipticity were observed. The influence of ponderomotive potential and Stark shift on the ionization rate was discussed. A brief description of the dependence of the ponderomotive potential and the Keldysh parameter on the field intensity and ellipticity is given.

Toolkit for semiclassical computations for strongly driven molecules: Frustrated ionization ofH2driven by elliptical laser fields

We study the formation of highly excited neutral atoms during the breakup of strongly driven molecules. Past work on this significant phenomenon has shown that during the formation of highly excited neutral atoms (${\mathrm{H}}^{*}$) during the breakup of ${\mathrm{H}}_{2}$ in a linear laser field the electron that escapes does so either very quickly or after remaining bound for a few periods of the laser field. Here, we address the electron-nuclear dynamics in ${\mathrm{H}}^{*}$ formation in elliptical laser fields, through Coulomb explosion. We show that with increasing ellipticity two-electron effects are effectively ``switched off.'' We perform these studies using a toolkit we have developed for semiclassical computations for strongly driven multicenter molecules. This toolkit includes the formulation of the probabilities of strong-field phenomena in a transparent way. This allows us to identify the shortcomings of currently used initial phase-space distributions for the electronic degrees of freedom. In addition, it includes a three-dimensional method for time propagation that fully accounts for the Coulomb singularity. This technique has been previously developed in the context of celestial mechanics and we currently adopt it to strongly driven systems. Moreover, we allow for tunneling during the time propagation. We find that this is necessary in order to accurately describe the fragmentation of strongly driven molecules.

Subcycle Dynamics of Coulomb Asymmetry in Strong Elliptical Laser Fields

We measure photoelectron angular distributions of noble gases in intense elliptically polarized laser fields, which indicate strong structure-dependent Coulomb asymmetry. Using a dedicated semiclassical model, we have disentangled the contribution of direct ionization and multiple forward scattering on Coulomb asymmetry in elliptical laser fields. Our theory quantifies the roles of the ionic potential and initial transverse momentum on Coulomb asymmetry, proving that the small lobes of asymmetry are induced by direct ionization and the strong asymmetry is induced by multiple forward scattering in the ionic potential. Both processes are distorted by the Coulomb force acting on the electrons after tunneling. Lowering the ionization potential, the relative contribution of direct ionization on Coulomb asymmetry substantially decreases and Coulomb focusing on multiple rescattering is more important. We do not observe evident initial longitudinal momentum spread at the tunnel exit according to our simulation.