  Twisted bilayer graphene with tiny rotation angles have drawn significant
attention due to the observation of the unconventional superconducting and
correlated insulating behaviors. In this paper, we employ a full tight-binding
model to investigate collective excitations in twisted bilayer graphene near
magic angle. The polarization function is obtained from the tight-binding
propagation method without diagonalization of the Hamiltonian matrix. With the
atomic relaxation considered in the simulation, damped and undamped interband
plasmon modes are discovered near magic angle under both room temperature and
superconductivity transition temperature. In particular, an undamped plasmon
mode in narrow bands can be directly probed in magic angle twisted bilayer
graphene at superconductivity transition temperature. The undamped plasmon mode
is tunable with angles and gradually fades away with both temperature and
chemical potential. In practice, the flat bands in twisted bilayer graphene can
be detected by exploring the collective plasmons from the measured energy loss
function.
