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 ${\mathrm{SiO}}_2$ substrate. For photon energies above 0.5 eV, graphene yielded a spectrally flat optical absorbance of $(2.3\pm 0.2)\%$. 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.