  We have experimentally studied the optical refractive index of few-layer
graphene through reflection spectroscopy at visible wavelengths. A laser
scanning microscope (LSM) with a coherent supercontinuum laser source measured
the reflectivity of an exfoliated graphene flake on a Si/SiO2 substrate,
containing monolayer, bilayer and trilayer areas, as the wavelength of the
laser was varied from 545nm to 710nm. The complex refractive index of few-layer
graphene, n-ik, was extracted from the reflectivity contrast to the bare
substrate and the Fresnel reflection theory. Since the SiO2 thickness enters to
the modeling as a parameter, it was precisely measured at the location of the
sample. It was found that a common constant optical index cannot explain the
wavelength-dependent reflectivity data for single-, double- and three-layer
graphene simultaneously, but rather each individual few-layer graphene
possesses a unique optical index whose complex values were precisely and
accurately determined from the experimental data.
