Voltage-Controlled Dielectric Function of Bilayer Graphene

The refractive index and the extinction coefficient are usually inherent (noncontrollable) material characteristics. Recently, it was reported that the reflectivity of graphene in the mid-infrared spectral range can be modified by an external bias. This report attracted much attention, but the controllable frequency/energy range is too narrow for possible applications. In this work, it is demonstrated that the potential of graphene is not limited to mid-infrared wavelengths, but spans a much wider range including the visible spectral range. Here, back-gated bilayer graphene is characterized in air using spectroscopic ellipsometry with a lateral resolution in the micrometer range. By applying a back-gate voltage, the dielectric function can be modified in a broad spectral range, including the visible spectrum. To explain the change in the dielectric function, a simplified phenomenological approach which assumes that the back-gating-induced change in the carrier density of graphene can be described by a modified 2D Drude model is introduced. The trend of increasing values for the dielectric function with increasing sheet charge carrier density is confirmed by theoretical calculations performed in the independent particle picture.