Graphene has very good optical properties, with an absorptivity of about 2.3% over a wide range of wavelengths, and appears almost transparent. Within the thickness of several layers of graphene, each increase in thickness increases the absorption rate by 2.3%. Large-area graphene films also have excellent optical characteristics, and their optical characteristics change with the thickness of graphene. This is the unusual low-energy electronic structure of single-layer graphene. When voltage is applied to the double-gate double-layer graphene field effect transistor at room temperature, the band gap of graphene can be adjusted between 0 and 0.25eV. By applying a magnetic field, the optical response of graphene nanoribbons can be tuned to the terahertz range.
When the intensity of incident light exceeds a certain threshold, the absorption of graphene will reach saturation. These characteristics make graphene suitable for passive mode-locked lasers. This unique absorption may become saturated when the input light intensity exceeds a threshold value, which is called the saturation effect. Graphene can be saturated and easily visible under strong excitation in the near-infrared region due to global optical absorption and zero band gap. Because of this special property, graphene is widely used in ultrafast photonics. The optical response of the graphene / graphene oxide layer can be tuned electrically. Under more intense laser illumination, graphene may possess an optically nonlinear Kerr effect with a non-linear phase shift.