  We present a theoretical study of the intrinsic plasmonic properties of
twisted bilayer graphene (TBG) as a function of the twist angle $\theta$ (and
other microscopic parameters such as temperature and filling factor). Our
calculations, which rely on the random phase approximation, take into account
four crucially important effects, which are treated on equal footing: i) the
layer-pseudospin degree of freedom, ii) spatial non-locality of the
density-density response function, iii) crystalline local field effects, and
iv) Hartree self-consistency. We show that the plasmonic spectrum of TBG
displays a smooth transition from a strongly-coupled regime (at twist angles
$\theta \lesssim 2^{\circ}$), where the low-energy spectrum is dominated by a
weakly dispersive intra-band plasmon, to a weakly-coupled regime (for twist
angles $\theta \gtrsim 2^{\circ}$) where an acoustic plasmon clearly emerges.
This crossover offers the possibility of realizing tunable mid-infrared
sub-wavelength cavities, whose vacuum fluctuations may be used to manipulate
the ground state of strongly correlated electron systems.
