Plasmon modes in the magnetically doped Single Layer and Multilayers of Helical Metals

We study the plasmon excitations and the electromagnetic response of the magnetically doped single layer and multilayer of ``helical metals'', which emerge at the surfaces of topological insulators. For the single layer case, we find a ``spin-plasmon'' mode with the rotating spin texture due to the combination of the spin-momentum locking of ``helical metals'' and the Hall response from magnetization. For the multilayers case, we investigate the electromagnetic response due to the plasmon excitations, including the Faraday rotation for the light propagating normal to the helical metal layers and an additional optical mode with the frequency within the conventional plasmon gap for the light propagating along the helical metal layers.