  The electronic properties of twisted bilayer graphene (TBG) can be
dramatically different from those of a single graphene layer, in particular
when the two layers are rotated relative to each other by a small angle. TBG
has recently attracted a great deal of interest, sparked by the discovery of
correlated insulating and superconducting states, for twist angle $\theta$
close to a so-called 'magic angle' $\approx 1.1{\deg}$. In this work, we
unveil, via near-field optical microscopy, a collective plasmon mode in
charge-neutral TBG near the magic angle, which is dramatically different from
the ordinary single-layer graphene intraband plasmon. In selected regions of
our samples, we find a gapped collective mode with linear dispersion, akin to
the bulk magnetoplasmons of a two-dimensional (2D) electron gas. We interpret
these as interband plasmons and associate those with the optical transitions
between quasi-localized states originating from the moir\'e superlattice.
Surprisingly, we find a higher plasmon group velocity than expected, which
implies an enhanced strength of the corresponding optical transition. This
points to a weaker interlayer coupling in the AA regions. These intriguing
optical properties offer new insights, complementary to other techniques, on
the carrier dynamics in this novel quantum electron system.
