  The investigation of excitons in van-der-Waals heterostructures has led to
profound insights into the interplay of crystal symmetries and fundamental
effects of light-matter coupling. In particular, the polarization selection
rules in undistorted, slightly twisted heterostructures of MoSe$_2$/WSe$_2$
were found to be connected with the Moir\'e superlattice. Here, we report the
emergence of a significant degree of circular polarization of excitons in such
a hetero-structure upon non-resonant driving with a linearly polarized laser.
The effect is present at zero magnetic field, and sensibly reacts on
perpendicularly applied magnetic field. The giant magnitude of polarization,
which cannot be explained by conventional birefringence or optical activity of
the twisted lattice, suggests a kinematic origin arising from an emergent
pyromagnetic symmetry in our structure, which we exploit to gain insight into
the microscopic processes of our device.
