  We investigate the optical properties of an ultrathin film of a topological
insulator in the presence of an in-plane magnetic field. We show that due to
the combination of the overlap between the surface states of the two layers and
the magnetic field, the optical conductivity can show strong anisotropy. This
leads to the effective optical activity of the ultrathin film by influencing
the circularly polarized incident light. Intriguingly, for a range of magnetic
fields, the reflected and transmitted lights exhibit elliptic character. Even
for certain values almost linear polarizations are obtained, indicating that
the thin film can act as a polaroid in reflection. All these features are
discussed in the context of the time reversal symmetry breaking as one of key
ingredients for the optical activity.
