  We predict high-order harmonics in which the polarization within the spectral
bandwidth of each harmonic varies continuously and significantly. For example,
the interaction of counter-rotating circularly-polarized bichromatic drivers
having close central frequencies with isotropic gas leads to the emission of
polarization-fan harmonics which are nearly circularly-polarized in one tail of
the harmonic peak, linear in the center of the peak and nearly circular with
the opposite helicity in the opposite tail. Polarization fan harmonics are
obtained as a result of multiple (at least two) head-on recollisions of
electrons with their parent ions occurring from different angles. The process
can be phase-matched using standard methods (e.g. pressure tuning phase
matching) and maintains the single-atom polarization property through
propagation. These polarization-fan harmonics may be used for exploring
non-repetitive ultrafast chiral phenomena, e.g. dynamics of magnetic domains,
in a single shot
