  Chiral nanophotonic devices are promising candidates for chiral molecules
sensing, polarization diverse nanophotonics and display technologies. Active
chiral nanophotonic devices, where the optical chirality can be controlled by
an external stimulus has triggered great research interest. However, efficient
modulation of the optical chirality has been challenging. Here, we demonstrate
switching of the extrinsic chirality by applied magnetic fields in a
magneto-plasmonic metasurface device based on a magneto-optical oxide material,
Ce1Y2Fe5O12 (Ce:YIG). Thanks to the low optical loss and strong magneto-optical
effect of Ce:YIG, we experimentally demonstrated a giant and continuous
far-field circular dichroism (CD) modulation by applied magnetic fields from
-0.65{\deg} to +1.9{\deg} at 950 nm wavelength under glancing incident
conditions. The far field CD modulation is due to both magneto-optical circular
dichroism and near-field modulation of the superchiral fields by applied
magnetic fields. Finally, we demonstrate magnetic field tunable chiral imaging
in millimeter-scale magneto-plasmonic metasurfaces fabricated using
self-assembly. Our results provide a new way for achieving planar integrated,
large-scale and active chiral metasurfaces for polarization diverse
nanophotonics.
