  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.
