Nonsequential double ionization with polarization-gated pulses

We investigate laser-induced nonsequential double ionization by a polarization-gated laser pulse, constructed by employing two counter-rotating circularly polarized few-cycle pulses with a time delay Td. We address the problem within a classical framework and mimic the behaviour of the quantum-mechanical electronic wave packet by means of an ensemble of classical electron trajectories. These trajectories are initially weighted with the quasi-static tunnelling rate and with suitably chosen distributions for the momentum components parallel and perpendicular to the laser-field polarization in the temporal region for which it is nearly linearly polarized. We show that, if the time delay Td is of the order of the pulse length, the electron-momentum distributions, as functions of the parallel momentum components, are highly asymmetric and dependent on the carrier-envelope (CE) phase. As this delay is decreased, this asymmetry gradually vanishes. We explain this behaviour in terms of the available phase space, the quasi-static tunnelling rate and the recollision rate for the first electron for different sets of trajectories. Our results show that the polarization-gating technique may provide an efficient way to study the NSDI dynamics in the single-cycle limit without employing few-cycle pulses.