  We reveal that there exists a class of graphene structures (a sub-class of
bilayer graphene nanoribbons) which has unusually strong optical response in
the terahertz (THz) and far infrared (FIR) regime. The peak conductance of
terahertz/FIR active bilayer ribbons is around two orders of magnitude higher
than the universal conductance of $e^2/4\hbar$ observed in graphene sheets. The
criterion for the terahertz/FIR active sub-class is a bilayer graphene
nanoribbon with one-dimensional massless Dirac Fermion energy dispersion near
the $\Gamma$ point. Our results overcome a significant obstacle that hinders
potential application of graphene in electronics and photonics.
