Manipulating polarization effect in nonsequential double ionization.

We report on a theoretical study of nonsequential double ionization (NSDI) of magnesium atoms by using combined linearly and circularly polarized fields. By employing a concise model including the dynamic ionic dipole potential, we show that the polarization effects can be controlled by tuning the subcycle waveform of the electric field of the two-color pulses. We demonstrate that the influence of the dipole potential on NSDI depends on the symmetry of two-color laser fields by tracing back the electron trajectories. Furthermore, we propose a method allowing for manipulating the returning trajectories with the initial direction of the tunneled electrons almost unchanged.

Timing Correlated-Electron Emission from Strong Field Atomic Double Ionization with Polarization-Gated Attoclock.

Attoclock provides a powerful tool for probing the ultrafast electron dynamics in strong laser fields. However, this technique has remained restricted to single electron or sequential double ionized electron dynamics. Here, we propose a novel attoclock scheme with a polarization-gated few-cycle laser pulse and demonstrate its application in timing the correlated-electron emission in strong field double ionization of argon. Our experimental measurements reveal that the correlated-electron emission occurs mainly through two channels with time differences of 234±22 as and 1043±73 as, respectively. Classical model calculations well reproduce the experimental results and deepen our understanding of ultrafast electron correlation dynamics.

Ellipticity dependence of anticorrelation in the nonsequential double ionization of Ar.

Within the framework of the improved quantitative rescattering (QRS) model, we simulate the correlated two-electron momentum distributions (CMDs) for nonsequential double ionization (NSDI) of Ar by elliptically polarized laser pulses with a wavelength of 788 nm at an intensity of 0.7 × 1014 W/cm2 for the ellipticities ranging from 0 to 0.3. Only the CMDs for recollision excitation with subsequent ionization (RESI) are calculated and the contribution from recollision direct ionization is neglected. According to the QRS model, the CMD for RESI can be factorized as a product of the parallel momentum distribution (PMD) for the first released electron after recollision and the PMD for the second electron ionized from an excited state of the parent ion. The PMD for the first electron is obtained from the laser-free differential cross sections for electron impact excitation of Ar+ calculated using state-of-the-art many-electron R-matrix theory while that for the second electron is evaluated by solving the time-dependent Schrödinger equation. The results show that the CMDs for all the ellipticities considered here exhibit distinct anticorrelated back-to-back emission of the electrons along the major polarization direction, and the anticorrelation is more pronounced with increasing ellipticity. It is found that anticorrelation is attributed to the pattern of the PMD for the second electron ionized from the excited state that, in turn, is caused by the delayed recollision time with respect to the instant of the external field crossing. Our work shows that both the ionization potential of the excited parent ion and the laser intensity play important roles in the process.

Carrier-envelope-phase measurement of few-cycle mid-infrared laser pulses using high harmonic generation in ZnO.

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.

Steering electron correlation time by elliptically polarized femtosecond laser pulses.

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.

Ionization suppression of diatomic molecules in an intense midinfrared laser field.

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).