Measurement of the Optical Conductivity of Graphene

Optical reflectivity and transmission measurements over photon energies between 0.2 and 1.2 eV were performed on single-crystal graphene samples on a SiO2 substrate. For photon energies above 0.5 eV, graphene yielded a spectrally flat optical absorbance of (2.3 +/- 0.2)%. This result is in agreement with a constant absorbance of pi alpha, or a sheet conductivity of pi e(2)/2h, predicted within a model of noninteracting massless Dirac fermions. This simple result breaks down at lower photon energies, where both spectral and sample-to-sample variations were observed. This nonuniversal behavior is explained by including the effects of doping and finite temperature, as well as contributions from intraband transitions.