  We report a systematic plasmonic study of twisted bilayer graphene (TBLG) -
two graphene layers stacked with a twist angle. Through real-space nanoimaging
of TBLG single crystals with a wide distribution of twist angles, we find that
TBLG supports confined infrared plasmons that are sensitively dependent on the
twist angle. At small twist angles, TBLG has a plasmon wavelength comparable to
that of single-layer graphene. At larger twist angles, the plasmon wavelength
of TBLG increases significantly with apparently lower damping. Further analysis
and modeling indicate that the observed twist-angle dependence of TBLG plasmons
in the Dirac linear regime is mainly due to the Fermi-velocity renormalization,
a direct consequence of interlayer electronic coupling. Our work unveils the
tailored plasmonic characteristics of TBLG and deepens our understanding of the
intriguing nano-optical physics in novel van der Waals coupled two-dimensional
materials.
