  In the phenomenon of electromagnetically induced transparency1 (EIT) of a
three-level atomic system, the linear susceptibility at the dipole-allowed
transition is canceled through destructive interference of the direct
transition and an indirect transition pathway involving a meta-stable level,
enabled by optical pumping. EIT not only leads to light transmission at
otherwise opaque atomic transition frequencies, but also results in the slowing
of light group velocity and enhanced optical nonlinearity. In this letter, we
report an analogous behavior, denoted as phonon-induced transparency (PIT), in
AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the
plasmon excitation is suppressed in a narrow window due to the coupling with
the infrared active {\Gamma}-point optical phonon, whose function here is
similar to that of the meta-stable level in EIT of atomic systems. We further
show that PIT in bilayer graphene is actively tunable by electrostatic gating,
and estimate a maximum slow light factor of around 500 at the phonon frequency
of 1580 cm-1, based on the measured spectra. Our demonstration opens an avenue
for the exploration of few-photon non-linear optics and slow light in this
novel two-dimensional material, without external optical pumping and at room
temperature.
