  The intensity ratio between two major Raman bands in graphene is one of the
most important information for physics of graphene and has been believed to
represent various intrinsic properties of graphene without critical assessment
of extrinsic effects. We report a micro Raman spectroscopy study on the Raman
intensity ratio of the 2D band to the G Raman band of graphene varying the
thickness of dielectric layers (SiO_2) underneath it. The ratio is shown to
change by almost 370% when the thickness is varied by 60%. The large variation
in the ratio is well explained by theoretical calculations considering multiple
Raman scattering events at the interfaces. Our analysis shows that the
interference effect is critical in extracting the intrinsic 2D to G intensity
ratio and therefore must be taken into account in extracting various physical
properties of graphene from Raman measurements.
