  A striking feature of 3 dimensional (3D) topological insulators (TIs) is the
theoretically expected topological magneto-electric (TME) effect, which gives
rise to additional terms in Maxwell's laws of electromagnetism with an
universal quantized coefficient proportional to half-integer multiples of the
fine structure constant $\alpha$. In an ideal scenario one therefore expects
also quantized contributions in the magneto-optical response of TIs. We review
this premise by taking into account the trivial dielectric background of the TI
bulk and potential host substrates, and the often present contribution of
itinerant bulk carriers. We show that (i) one obtains a non-universal
magneto-optical response whenever there is impedance mismatch between different
layers and (ii) that the detectable signals due to the TME rapidly approach
vanishingly small values as the impedance mismatch is detuned from zero. We
demonstrate that it is methodologically impossible to deduce the existence of a
TME exclusively from an optical experiment in the thin film limit of 3D TIs at
high magnetic fields.
