  Strong-field ionization and rescattering beyond the long-wavelength limit of
the dipole approximation is studied with elliptically polarized mid-IR pulses.
We have measured the full three-dimensional photoelectron momentum
distributions (3D PMDs) with velocity map imaging and tomographic
reconstruction. The ellipticity-dependent 3D-PMD measurements revealed an
unexpected sharp, thin line-shaped ridge structure in the polarization plane
for low momentum photoelectrons. With classical trajectory Monte Carlo (CTMC)
simulations and analytical methods we identified the associated ionization
dynamics for this sharp ridge to be due to Coulomb focusing of slow
recollisions of electrons with a momentum approaching zero. This ridge is
another example of the many different ways how the Coulomb field of the parent
ion influences the different parts of the momentum space of the ionized
electron wave packet. Building on this new understanding of the PMD, we extend
our studies on the role played by the magnetic field component of the laser
beam when operating beyond the long-wavelength limit of the dipole
approximation. In this regime, we find that the PMD exhibits an
ellipticity-dependent asymmetry along the beam propagation direction: the peak
of the projection of the PMD onto the beam propagation axis is shifted from
negative to positive values with increasing ellipticity. This turnover occurs
rapidly once the ellipticity exceeds $\sim$0.1. We identify the sharp, thin
line-shaped ridge structure in the polarization plane as the origin of the
ellipticity-dependent PMD asymmetry in the beam propagation direction. These
results yield fundamental insights into strong-field ionization processes, and
should increase the precision of the emerging applications relying on this
technique, including time-resolved holography and molecular imaging.
