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Surface plasmons on a thin film topological insulator

Recently it has been shown that surface plasmons supported by an interface between a 3+1 dimensional topological insulator and a metal or between a 3+1 dimensional topological insulator with residual bulk charge carriers and vacuum have a polarization that is rotated by an angle of order of the fine structure constant alpha compared to their topological trivial counterparts. In this work we generalize this analysis to the more realistic case of thin films, taking into account the effect of multiple reflections. In the symmetric case of a thin film surrounded by the same material on both sides the polarization of both allowed surface plasmon modes is unchanged from the single interface case, even though their dispersion relation is altered by the interactions between the two surface layers. In the general asymmetric case the angle is affected as well. We give a simple analytic expression for the resonance condition that determines the angle and solve it perturbatively for the case of a thin film between two media with almost identical permittivity.