In this work we report on the preparation of some aqueous graphene oxide (GO) dispersions and the investigation of their nonlinear optical response under visible (532 nm) and infrared (1064 nm), picosecond and nanosecond laser excitation. The GO colloids were prepared under specific and well-defined conditions resulting in finely dispersed heavily oxidized large GO sheets. In all cases, GO colloids were found to present large nonlinear absorption and negligible nonlinear refraction. The physical mechanisms responsible for their nonlinear optical response are discussed. In addition, the so-prepared GO dispersions were found to exhibit large broadband optical power limiting action for both pulse durations, comparable to that of C 60 for visible laser pulses and much superior for infrared ones.

We report on the effect of the degree of oxidation on the broadband non-linear optical response and magnetic behavior of graphene oxide, as well as on a route for obtaining reduced graphene oxide with enhanced optical properties without sacrificing the high dispersibility of the parent graphene oxide. As more sp(3) states evolved with the rise in oxidation degree, it turned out that the sp(2)/sp(3) fraction and sp(2) clustering are crucial parameters for tuning the broadband non-linear optical absorption over a wide range from ps to ns laser pulses for both visible and infrared laser irradiation. This was clearly confirmed by two different approaches, namely by a synthetic route through the gradual oxidation of graphene oxide from 1 to 3 oxidizing cycles, and reversely by in situ reduction of graphene oxide by UV laser irradiation. Furthermore, as the sp(3) states carry localized magnetic moments, ferromagnetic ordering is observed at low temperatures. The magnetization and temperature at which ferromagnetic ordering evolves are found to increase on increasing the oxidation degree. The tuning of non-linear optical and magnetic properties of graphene oxide by oxidation/reduction thus provides an easy way to endow graphene oxide with tunable physical features highly required in both optoelectronics and spintronics applications.

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