Maxwell's field coupled nonminimally to quadratic torsion and induced axion field

A nonminimal interaction is considered between the electromagnetic field $F$ and a possible torsion field $T^\alpha$ of spacetime. We derive the complete class of parity conserving supplementary Lagrangians that are {\it quadratic} in torsion. They are of the type $\sim F^2 T^2$. From them we extract the corresponding electromagnetic axion field that is induced by the torsion. In the background of a Friedmann universe with torsion, we study the effect of the axion field on the propagation of light. It doesn't touch the light cone structure. However, it induces an optical activity into spacetime.