  Collective plasma excitations in moir\'e flat bands display unique properties
reflecting strong electron-electron interactions and unusual carrier dynamics
in these systems. Unlike the conventional two-dimensional plasmon modes,
dispersing as $\sqrt{k}$ at low frequencies and plunging into particle-hole
continuum at higher frequencies, the moir\'e plasmons pierce through the
flat-band continuum and acquire a strong over-the-band character. Due to the
complex structure of the moir\'e superlattice unit cell, the over-the-band
plasmons feature several distinct branches connected through zone folding in
the superlattice Brillouin zone. Using a toy Hubbard model for the correlated
insulating order in a flat band, we predict that these high-frequency modes
become strongly dipole-active upon the system undergoing charge ordering, with
the low-frequency modes gapped out within the correlated insulator gap. Strong
dipole moments and sensitivity to charge order make these modes readily
accessible by optical measurements, providing a convenient diagnostic of the
correlated states.
