Dynamic wavefront rotation in the attosecond lighthouse

Attosecond pulses propagating in different directions, generated in a rotating wavefront of a driving laser field, can provide a source of multiple isolated attosecond pulses. Clear spatial separation of the attosecond pulses is attained if the divergence of the individual attosecond pulse is smaller than their angular separation, which is limited by the bandwidth of the driving laser pulse. Here we demonstrate both experimentally and numerically that an additional wavefront rotation is imposed during the propagation of the driving laser pulse in a highly ionizing medium. This dynamic wavefront rotation enables the generation of the isolated attosecond pulse even in the case when the conditions derived from a linear diffraction theory do not permit the angular separation. The described nonlinear phenomenon has its roots in the half-cycle ionization events, and may open up new ways to study strong field processes in highly ionizing media.