Enhanced optical conductivity of bilayer graphene nanoribbons in the terahertz regime

We reveal that there exists a class of graphene structures (a sub-class of bilayer graphene nanoribbons) which has an exceptionally strong optical response in the terahertz (THz) and far infrared (FIR) regime. The peak conductance of THz/FIR active bilayer ribbons is around two orders of magnitude higher than the universal conductance of $\sigma_0=e^2/4\hbar$ observed in graphene sheets. The criterion for the THz/FIR active sub-class is a bilayer graphene nanoribbon with a 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.