Thin \theta-films optics

A Chern-Simons theory in 3D is accomplished by the so-called $\ensuremath{\theta}$ term in the action $(\ensuremath{\theta}/2)\ensuremath{\int}F\ensuremath{\wedge}F$, which contributes only to observable effects on the boundaries of such a system. When electromagnetic radiation interacts with the system, the wave is reflected and its polarization is rotated at the interface, even when both the $\ensuremath{\theta}$ system and the environment are pure vacuum. These topics have been studied extensively. Here, we investigate the optical properties of a thin $\ensuremath{\theta}$ film, where multiple internal reflections could interfere coherently. The cases of pure vacuum and a material with magnetoelectric properties are analyzed. It is found that the film reflectance is enhanced compared to ordinary non-$\ensuremath{\theta}$ systems and the interplay between magnetoelectric properties and the $\ensuremath{\theta}$ parameter yield film opacity and polarization properties which could be interesting in the case of topological insulators, among other topological systems.