Efficient control of ultrafast optical nonlinearity of reduced graphene oxide by infrared reduction

Simultaneous occurrence of saturable absorption nonlinearity and two-photon absorption nonlinearity in the same medium is well sought for the devices like optical limiter and laser modelocker. Pristine graphene sheet consisting entirely of sp(2)-hybridized carbon atoms has already been identified having large optical nonlinearity. However, graphene oxide (GO), a precursor of graphene having both sp(2) and sp(3)-hybridized carbon atom, is increasingly attracting crossdiscipline researchers for its controllable properties by reduction of oxygen containing groups. In this work, GO has been prepared by modified Hummers method, and it has been further reduced by infrared (IR) radiation. Characterization of reduced graphene oxide (RGO) by means of Raman spectroscopy, X-ray photoelectron spectroscopy, and UV-Visible absorption measurements confirms an efficient reduction with infrared radiation. Here, we report precise control of non-linear optical properties of RGO in femtosecond regime with increased degrees of IR reduction measured by open aperture z-scan technique. Depending on the intensity, both saturable absorption and two-photon absorption effects are found to contribute to the non-linearity of all the samples. Saturation dominates at low intensity (similar to 127GW/cm(2)) while two-photon absorption becomes prominent at higher intensities (from 217GW/cm(2) to 302 GW/cm(2)). The values of two-photon absorption co-efficient (similar to 0.0022-0.0037 cm/GW for GO, and similar to 0.0128-0.0143 cm/GW for RGO) and the saturation intensity (similar to 57GW/cm(2) for GO, and similar to 194GW/cm(2) for RGO) increase with increasing reduction, indicating GO and RGO as novel tunable photonic devices. We have also explained the reason of tunable nonlinear optical properties by using amorphous carbon model. Published by AIP Publishing.