Exotic ultrafast optical nonlinearity in reduced graphene oxide via comprehensive dual beam approach

摘要

Using an optical pump and a time delayed white light super continuum probe, delay dependent switching is achieved between saturation absorption (SA) and reverse saturation absorption (RSA) above a threshold pump intensity in reduced graphene oxide (RGO2). RGO2 is obtained using photo-thermal reduction and chemical reduction respectively. The wavelength regime which experience switching can be varied by changing the degree of reduction. At 415 nm pump, the threshold intensity to obtain switching property decreases to 9 GW/cm~2 for RGO2 from 18 GW/cm~2 in graphene oxide(GO) and the tunability range shifts from 471-526 nm for as grown GO to 519-623 nm in maximally reduced RGO2.Though the saturation intensity of intrinsic non-degenerate two photon absorption (nd-TPA) is found to be lower in GO (4.3 GW/cm2) than RGO2 (18.2 GW/cm~2), nd-TPA coefficient increases from 0.0015 cm/GW (GO) to 0.0026 cm/GW (RGO2) with increasing reduction. Decay dynamics of scattering processes show faster relaxation of electron in RGO than in GO. Results are accounted using a model band diagram based on amorphous-carbon model.