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New journal of physics 23(3), 033041 (2021). doi:10.1088/1367-2630/abe79d, Published by IOP, [London]

We report on a comparative study of strong-field ionization of alkaline-earth-metal atoms by intense femtosecond laser pulses from near-infrared to midinfrared wavelengths. By collecting the ionization signals only produced within the central portion of the laser focus, the focus volume effect is largely reduced and the saturation intensities for different alkaline-earth-metal atoms are reliably determined, which permits us to directly test the strong-field-ionization theories. We demonstrate that the Perelomov-Popov-Terent'ev model accurately predicts the experimental ionization yields and saturation intensities in general for arbitrary values of the Keldysh parameter, while the Ammosov-Delone-Krainov simulations agree with the experiments for the tunneling-ionization regime and also for the regime when the Keldysh parameter is around 1. Our work presents benchmark data for strong-field ionization of alkaline-earth metals over a broad range of laser parameters and confirms the validity of Keldysh's picture for such atoms.

Using a three-dimensional semiclassical method, we theoretically investigate frustrated double ionization (FDI) of Ar atoms subjected to strong laser fields. The double-hump photoelectron momentum distribution generated from FDI observed in a recent experiment [S. Larimian et al., Phys. Rev. Research 2, 013021 (2020)] is reproduced by our simulation. We confirm that the observed spectrum is due to recollision. The laser intensity dependence of FDI is investigated. We reveal that the doubly excited states of Ar atoms and excited states of ${\mathrm{Ar}}^{+}$ are the dominant pathways for producing FDI at relatively low and high intensities, respectively. The information of which pathway leads to FDI is encoded in the electron momentum distributions. Our work demonstrates that FDI is a general strong-field physical process accompanied with nonsequential double ionization and it can be well understood within the context of recollision scenario.

High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.

Electron correlation is ubiquitous across diverse physical systems from atoms and molecules to condensed matter. Observing and controlling dynamical electron correlation in photoinduced processes paves the way to the coherent control of chemical reactionsand photobiological processes. Here, we experimentally investigate dynamics of electron correlation in double ionization of neon irradiated by intense elliptically polarized laser pulses. We find a characteristic, ellipticity-dependent, correlated electron emission along the minor axis of the elliptically polarized light. This observation is well reproduced by a semi-classical ensemble model simulation. By tracing back the corresponding electron trajectories, we find that the dynamical energy sharing during the electron emission process is modified by the ellipticity of the laser light. Thus, our work provides evidence for a possible ultrafast control of the energy sharing between the correlated electrons by varying the light ellipticity.

We theoretically demonstrate the feasibility of separation of various of intracycle interferences in photoelectron momentum distributions (PMDs) with a polarization-gated (PG) pulse constructed by a left-circularly polarized and a time-delayed right-circularly polarized few-cycle laser pulse. By numerically solving the time-dependent Schrodinger equation, our simulations show that the intercycle interference pattern is suppressed and a clear interference pattern with rightward curved stripes emerges in PMDs with appropriate laser parameters. This interference pattern can be well reproduced by a quantum-trajectory Monte Carlo (QTMC) simulation and, moreover, with the help of QTMC analysis, up to three types of intracycle interferences can be identified. It is further revealed that the nonvanishing transverse electric-field component in the PG pulse plays an important role in separating various of intracycle interferences in PMDs by properly distributing the interference stripes of electron wave packets, emitted within different time windows, into different momentum regions and suppressing the forward-rescattering photoelectron holography.

The long-range Coulomb effect (LRCE) is demonstrated experimentally and theoretically by investigating the pulse duration dependence of low-energy structure (LES) in above-threshold ionization of Ne. It is found experimentally that at 800 nm the LES shows itself as a double-hump structure (DHS) in momentum distribution of singly charged ion for Ne, and moreover, this structure is more prominent for multicycle laser fields than for few-cycle cases. This result can be reproduced and explained qualitatively with a semiclassical model and attributed to the paramount role of LRCE. That is to say, after the laser field vanishes, the electrons decelerate while flying away from the core by the long-range tail of Coulomb potential, which eventually makes DHS less notable.

Using a semiclassical approach, we theoretically study the above-threshold ionization of magnesium by intense, mid-infrared laser pulses. The formation of low-energy structures in the photoelectron spectrum is found to be enhanced by comparing with a calculation based on the single-active electron approximation. By performing electron trajectory and recollision-time distribution analysis, we demonstrate that this phenomenon is due to the laser-induced ionic core polarization effects on the recolliding electrons. We also show that the polarization effects should be experimentally detectable. Our finding provides new insight into ultrafast control of strong-field photoionization and imaging of polar molecules.

We perform an experimental study on comparison between the ionization of homonuclear diatomic molecules (O-2 and N-2) and their companion atoms (Xe and Ar) radiated by circularly polarized intense laser fields. We find that the ionization of O-2 shows suppression with respect to its companion atom Xe, which exhibits a clear wavelength and intensity dependence similar to that in linearly polarized laser field, while the ionization of N-2 behaves like its companion atom Ar. With the help of S-matrix theoretical analysis, our observations can be attributed to both the molecular orbital and the two-center interference effect in molecular ionization process.

We experimentally investigate strong-field ionization of noble-gas atoms in elliptically polarized laser fields with the Keldysh parameter gamma similar to 1, which is typical for most current strong-field experiments. Our data show that the single-ionization yields exhibit a rapid decrease with increasing elipticity and, moreover, the decrease becomes more dramatic for lower laser intensity. The experimental results are compared with the simulations of the Ammosov-Delone-Krainov (ADK) theory based on the quasistatic tunneling ionization picture and the strong-field approximation (SFA) theory, which involves the nonadiabatic effect in the tunneling ionization process. Our results show that although both the ADK simulations and the SFA calculations can qualitatively reproduce the experimental results, the SFA calculations are in a better agreement with the experimental data than the ADK simulations, which implies a nonadiabatic effect during strong-field atomic ionization for elliptical polarization with gamma similar to 1.

Diatomic molecules (e.g., O(2)) in an intense laser field exhibit a peculiar suppressed ionization behavior compared to their companion atoms. Several physical models have been proposed to account for this suppression, while no consensus has been achieved. In this Letter, we aim to clarify the underlying mechanisms behind this molecular ionization suppression. Experimental data recorded at midinfrared laser wavelength and its comparison with that at near-infrared wavelength revealed a peculiar wavelength and intensity dependence of the suppressed ionization of O(2) with respect to its companion atom of Xe, while N(2) behaves like a structureless atom. It is found that the S-matrix theory calculation can reproduce well the experimental observations and unambiguously identifies the significant role of two-center interference effect in the ionization suppression of O(2).