Phys. Rev. Lett. 115, 113004 (2015)

If you're on the hunt for vortex patterns, you might not think to look at ionized helium. But Jean Marcel Ngoko Djiokap and colleagues have suggested that ultrafast lasers could stir up beautiful helical vortices in the electron momentum distribution of ionized helium atoms. The trick, they say, is to use two delayed attosecond laser pulses of opposite circular polarization.

Ngoko Djiokap et al. solved the two-electron time-dependent Schrödinger equation numerically and plotted the photoelectron momentum distributions for various time delays between the laser pulses, relative phases and polarization directions. They found vortex patterns akin to the spiral interference fringes observed in experiments with optical beams carrying orbital angular momentum. The photoelectron momentum distribution vortices were shown to be two-arm Fermat spirals — their handedness determined by the combination of the two directions of the circular polarization of the laser beams.

Seeing these vortices in experiment would certainly be rewarding, not only aesthetically, as they might also find applications in attosecond science.