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 transparent 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 non-interacting massless Dirac Fermions. This simple result breaks down at lower photon energies, where both spectral and sample-to-sample variations were observed. This "non-universal" behavior is explained by including the effects of doping and finite temperature, as well as contributions from intraband transitions. 
Kin Fai Mak, Matthew Y. Sfeir, Yang Wu, Chun Hung Lui, James A.   Misewich and Tony F. Heinz