Optical reflectivity and transmission measurements over photon energies between 0.2 and 1.2 eV were performed on single-crystal graphene samples on a substrate. For photon energies above 0.5 eV, graphene yielded a spectrally flat optical absorbance of . This result is in agreement with a constant absorbance of , or a sheet conductivity of , 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.
- Received 28 June 2008
©2008 American Physical Society