  The interaction of intense mid-infrared laser fields with atoms and molecules
leads to a range of new opportunities, from the production of bright, coherent
radiation in the soft x-ray range to imaging molecular structures and dynamics
with attosecond temporal and sub-angstrom spatial resolution. However, all
these effects, which rely on laser-driven recollision of an electron removed by
the strong laser field and the parent ion, suffer from the rapidly increasing
role of the magnetic field component of the driving pulse: the associated
Lorentz force pushes the electrons off course in their excursion and suppresses
all recollision-based processes, including high harmonic generation, elastic
and inelastic scattering. Here we show how the use of two non-collinear beams
with opposite circular polarizations produces a forwards ellipticity which can
be used to monitor, control, and cancel the effect of the Lorentz force. This
arrangement can thus be used to re-enable recollision-based phenomena in
regimes beyond the long-wavelength breakdown of the dipole approximation, and
it can be used to observe this breakdown in high-harmonic generation using
currently-available light sources.
