Enhanced optical conductivity of bilayer graphene nanoribbons in the terahertz regime.

We reveal that there exists a class of graphene structures (a subclass 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 2 orders of magnitude higher than the universal conductance of sigma(0) = e(2)/4variant Planck's over 2pi observed in graphene sheets. The criterion for the THz/FIR active subclass 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 the potential application of graphene in electronics and photonics.