Nonlinear Thermoplasmonics\nin Graphene Nanostructures

The linear electronic dispersion\nrelation of graphene endows the\natomically thin carbon layer with a large intrinsic optical nonlinearity,\nwith regard to both parametric and photothermal processes. While plasmons\nin graphene nanostructures can further enhance nonlinear optical phenomena,\nboosting resonances to the technologically relevant mid- and near-infrared\n(IR) spectral regimes necessitates patterning on ∼10 nm length\nscales, for which quantum finite-size effects play a crucial role.\nHere we show that thermoplasmons in narrow graphene nanoribbons can\nbe activated at mid- and near-IR frequencies with moderate absorbed\nenergy density, and furthermore can drive substantial third-harmonic\ngeneration and optical Kerr nonlinearities. Our findings suggest that\nphotothermal excitation by ultrashort optical pulses offers a promising\napproach to enable nonlinear plasmonic phenomena in nanostructured\ngraphene that avoids potentially invasive electrical gating schemes\nand excessive charge carrier doping levels.