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Finite frequency magnetoelectric response of three dimensional Topological Insulators

Topological insulators with a time reversal symmetry breaking perturbation near the surface present a magnetoelectric response that is quantized when the frequency of the probing fields is much smaller than the surface gap induced by the perturbation. In this work we describe the intrinsic finite frequency magnetoelectric response of topological insulators for frequencies of the order and larger than the surface gap, including the experimentally relevant case where the system is metallic. This response affects physical observable quantities and will give rise to new finite frequency phenomena of intrinsic topological origin.