  The electrodynamics of topological insulators (TIs) is described by modified
Maxwell's equations, which contain additional terms that couple an electric
field to a magnetization and a magnetic field to a polarization of the medium,
such that the coupling coefficient is quantized in odd multiples of $e^2 / 2 h
c $ per surface. Here, we report on the observation of this so-called
topological magnetoelectric (TME) effect. We use monochromatic terahertz (THz)
spectroscopy of TI structures equipped with a semi-transparent gate to
selectively address surface states. In high external magnetic fields, we
observe a universal Faraday rotation angle equal to the fine structure constant
$\alpha = e^2 / \hbar c$ when a linearly polarized THz radiation of a certain
frequency passes through the two surfaces of a strained HgTe 3D TI. These
experiments give insight into axion electrodynamics of TIs and may potentially
be used for a metrological definition of the three basic physical constants.
