Double Ionization of He by an Intense Elliptically-Polarized, Few-Cycle Attosecond Pulse

By solving the six-dimensional two-electron, time-dependent Schr\"{o}dinger equation for He interacting with an arbitrarily-polarized intense attosecond XUV pulse, we demonstrate numerically the control of He double ionization by means of the pulse polarization and its carrier-envelope phase (CEP).\footnote{J.M. Ngoko Djiokap \emph{et al.}, Phys.~Rev.~Lett. \textbf{113}, 223002 (2014).} Using perturbation theory (PT), we predict a new type of CEP-sensitive polarization asymmetry that is normally absent in single photon double ionization of He, but does occur for an elliptically-polarized, few-cycle attosecond XUV pulse. We call this new effect \emph{nonlinear dichroism},\footnote{Djiokap \textit{et al.}} which is sensitive not only to the ellipticity, peak intensity $I$, and temporal duration of the pulse, but also to the energy-sharing. This dichroic effect (i.e., the difference of the two-electron angular distributions for opposite helicities of the ionizing XUV pulse) originates from interference of first- and second-order PT amplitudes, allowing one to investigate and control S- and D-wave channels of the two-electron continuum. Nonlinear dichroism probes electron correlation on its natural timescale since it vanishes for long pulses.