Strong Anomalous Optical Dispersion of Graphene: Complex Refractive   Index Measured by Picometrology
  We apply spinning-disc picometrology to measure the complex refractive index of graphene on thermal oxide on silicon. The refractive index varies from n = 2.4-1.0i at 532 nm to n = 3.0-1.4i at 633 nm at room temperature. The dispersion is five times stronger than bulk graphite (2.67-1.34i to 2.73-1.42i from 532 nm to 633 nm). 
Xuefeng Wang, Yong P. Chen and David D. Nolte