  We discuss plasmonic excitations in a thin slab of a topological insulators.
In the limit of no hybridization of the surface states and same electronic
density of the two layers, the electrostatic coupling between the top and
bottom layers leads to optical and acoustic plasmons which are purely charge
and spin collective oscillations. We then argue that a recent experiment on the
plasmonic excitations of Bi2Se3 [Di Pietro et al, Nat. Nanotechnol. 8, 556
(2013)] must be explained by including the charge response of the
two-dimensional electron gas of the depletion layer underneath the two
surfaces. We also present an analytic formula to fit their data.
