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Enhanced optical second-harmonic generation from current-biased graphene on the substrates of Si and SiC

We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition (CVD) graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of direct (dc) electric current in graphene. We also find that optical SHG in reflection from a 4-layer-graphene film epitaxially grown on a vicinal SiC(0001) substrate is enhanced 25% by the flow of dc electric current in graphene. Measurements of rotationalanisotropy SHG from both samples revealed that the current-induced SHG varies strongly with the measurement location on graphene along the current flow direction. The enhancement of SHG from the graphene/SiO2/Si(001) sample is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The enhancement of SHG from the graphene/vicinal-SiC(0001) sample is due to the current-associated electric field at the graphene/SiC interface that produces electric field-induced SHG. The functions of the current-induced SHG varying with the measurement location are different for the CVD graphene/SiO2/Si(001) sample and the epitaxial graphene/vicinal-SiC(0001) sample.