Direct observation of an irradiance dependent nonlinear refraction in CVD single layer graphene

The nonlinear optical properties of single layer graphene are studied via Z-scan. The measurements are carried out at 1045 nm with a 360 femtosecond highly stable Yb doped mode-locked fiber laser. Under laser illumination single-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase-shift due to nonlinear refraction. The nonlinear refraction measurements were carried out from 0.5 to 8 GW/cm² revealing an expected irradiance-dependent nonlinear refraction. An effective nonlinear refractive index coefficient ñI is used to describe this and discriminate it from the conventional n₂ coefficient usually measured by Z-scan. The saturation level of the nonlinear phase-shift where the nonlinear response becomes irradiance dependent is deduced from the experimental data.