  High-order harmonic generation with bicircular fields - the combination of
counter-rotating circularly polarized pulses at different frequencies - results
in a series of short-wavelength XUV harmonics with alternating circular
polarizations, and experiments show that there is an asymmetry in the emission
between the two helicities: a slight one in helium, and a larger one in neon
and argon, where the emission is carried out by p-shell electrons. Here we
analyze this asymmetry by switching to a rotating frame in which the field is
linearly polarized; this induces an effective magnetic field which lowers the
ionization potential of the $p_+$ orbital that co-rotates with the
lower-frequency driver, enhancing its harmonic emission and the overall
helicity of the generated harmonics, while also introducing nontrivial effects
from the transformation to a non-inertial frame in complex time. In addition,
this analysis directly relates the small asymmetry produced by s-shell emission
to the imaginary part of the recollision velocity in the standard
strong-field-approximation formalism.
