Optical reflection and transmission properties of exfoliated graphite from a graphene monolayer to several hundred graphene layers.

The optical reflection contrast and optical transmission contrast of graphitic films on glass ranging in thickness from a monolayer to the limit of bulk graphite have been experimentally measured. For samples with more than 10 graphene layers where optical contrast quantization becomes difficult to observe, atomic force microscopy was used to measure the sample thickness. The visible optical reflection and transmission of thin graphitic films is found to depend strongly on the real component of the optical conductance per graphene layer, and comparatively weakly on the imaginary component of optical conductance. This observation in part explains the significant variation in the refractive index of graphene and graphite reported in the literature to date. Spectroscopic measurements reveal a strong dispersion in the optical conductance of even a 10 layer film, consistent with an imaginary conductance arising from virtual transitions at the band edges of the pi and sigma bands at the M and Gamma points, respectively.