  We study the selective excitation at infrared and THz frequencies of optical
and acoustic plasmonic modes supported by thin topological insulators. These
modes are characterized by effective net charge or net spin density,
respectively, and we study their excitation by combining many-body and
electromagnetic calculations. We first show that non-locality can significantly
modify the plasmonic response: it changes the energy of propagating plasmons up
to tens of percent. We then discuss how, by changing the distance between a
dipolar source and a semi-infinite 10 nm thin film, it is possible to control
the excitation of acoustic and optical propagating plasmons, which can
propagate over a distance of several plasmonic wavelengths. Furthermore, we
consider 10 nm thin TI nanodisks and study the excitation of acoustic and
optical localized plasmon modes by a point dipole source and plane wave
illumination, respectively. The resonant plasmonic modes appear at frequencies
that strongly depends on the size of the disk, and that can be potentially
tuned by applying electrostatic gating to modify the Fermi Energy of the
conductive 2-dimensional layer that forms at the interfaces of the TI. We
observe a spectral shift from ~29 $\mu$m to ~34 $\mu$m by changing the Fermi
Energy from 250meV to 350meV. Last, the electromagnetic energy of these
plasmonics modes can be confined to very small regions, of effective volume
~(120 nm)^3 for the smaller disk considered, much less than the free-space
wavelength cubed $\lambda$^3 ~(35000 nm)^3. The strong confinement is desirable
for achieving very efficient coupling with nearby systems. Our detailed study
thus shows that thin topological insulators are a promising system to control
both the spin and charge oscillations associated with the plasmonic resonances,
with possible applications to fast, compact and electrically-controlled
spintronics devices.
