  Flat bands in twisted moire superlattices support a variety of topological
and strongly correlated phenomena along with easily tunable electrical and
optical properties. Here, we demonstrate the existence of tunable, long-lived,
and flat intraband and interband terahertz plasmons in twisted double bilayer
graphene. We show that the interband plasmons originate from the presence of a
Van Hove singularity in the joint density of states and a finite Berry
connection between the pair of bands involved. We find that the gapped
interband plasmon mode has a universal dispersion, and the plasmon gap is
specified by the location of the Van Hove singularity in the joint density of
states. Metallic moire systems support an additional intraband plasmon mode
which becomes flat in the large momentum limit because of the influence of the
interband correlations. We demonstrate that the undamped and flat plasmon modes
in moire systems are highly tunable and can be controlled by varying the
vertical electric field and electron doping, and they persist over a wide range
of twist angles.
