Abstract
Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments. The observed strong nonlinear response ascribed to the refractive part of graphene’s electronic third-order susceptibility χ(3) cannot, however, be explained using the relatively modest χ(3) value theoretically predicted for the 2D material. Here we solve this long-standing paradox and demonstrate that, rather than χ(3)-based refraction, a complex phenomenon which we call saturable photoexcited-carrier refraction is at the heart of nonlinear-optical interactions in graphene such as self-phase modulation. Saturable photoexcited-carrier refraction is found to enable self-phase modulation of picosecond optical pulses with exponential-like bandwidth growth along graphene-covered waveguides. Our theory allows explanation of these extraordinary experimental results both qualitatively and quantitatively. It also supports the graphene nonlinearities measured in previous self-phase modulation and self-(de)focusing (Z-scan) experiments. This work signifies a paradigm shift in the understanding of 2D-material nonlinearities and finally enables their full exploitation in next-generation nonlinear-optical devices.
Highlights
Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments
Over the past several years numerous experiments have been carried out to study the response of graphene in parametric nonlinear-optical processes like four-wave mixing[1,2,3,4,5], third-harmonic generation[6,7], self-(de)focusing[8,9,10,11,12], and self-phase modulation (SPM)[13,14,15]
We investigate if the underlying physics can be explained by chirping effects induced by photoexcited free carriers in the graphene
Summary
Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments. Both the negative sign of the experimentally observed nonlinearity and its large quantitative discrepancy as compared to the theoretically predicted response have unclear origins at this point This holds for graphene experiments with freespace optical excitation[1,6,7,8,9,10,11,12,13] and for the cases where graphene on a photonic chip is examined using waveguided excitation beams[2,3,4,13,14]. In case of graphene-covered photonic chips, the core of the on-chip waveguides should preferably be made of SiO2 for the larger part, since the nonlinearity of SiO2 is orders of magnitude smaller than that of other materials typically employed for photonic chip fabrication (Si, InP, Si3N4,...)[21,27] and because its dielectric nature provides electrical isolation of the graphene
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