Nonlinear Graphene Plasmonics

As the accessibility of nanostructured materials offered by modern nanofabrication techniques continues to increase, so does the interest in mastering nonlinear optics on subwavelength scales. Applications for nonlinear optical nanomaterials include optical microscopy, biological imaging/detection, and signal conversion in nanoscale photonic devices. However, nanostructured materials inherently possess small volumes with which optical fields can interact appreciably, severely limiting the efficiencies of nonlinear optical processes. The intense near-field enhancement associated with localized plasmons partially alleviates this situation, and is responsible for the highest recorded nonlinear frequency conversion efficiencies per volume observed in noble metal nanoparticles [1].

Compared to noble metals, doped graphene nanostructures support plasmon excitations that couple more strongly with light, possess longer lifetimes, and are electrically tunable [2]. A strong intrinsic nonlinearity in graphene has also been predicted due to its anharmonic charge-carrier dispersion relation, while recent four-wave mixing experiments already confirm a large third-order response [3]. Here we show that small graphene nanoislands can be electrically tuned to yield nonlinear frequency conversion with efficiencies surpassing those of much larger noble metal nanoparticles by several orders of magnitude. This extraordinary behaviour extends over the visible and near-infrared for islands consisting of hundreds of carbon atoms doped with moderate carrier densities. The optical response of these nanoislands is modelled using rigorous quantum mechanical simulations, taking into account significant contributions from nonlocal and finite-size effects. We find that the polarizabilities for graphene nanoislands corresponding to various nonlinear optical processes, including second- and third-harmonic generation, sum and difference frequency generation, and four-wave mixing, are strongly enhanced when the fundamental and/or converted frequencies are resonant with electrically tunable plasmons.

[1] M. Kauranen and A. V. Zayats, Nat. Photon. 6, 737 (2012).
[2] F. J. García de Abajo, ACS Photon. 1, 135 (2014).
[3] E. Hendry, P. J. Hale, J. Moger, A. K. Savchenko, and S. A. Mikhailov, Phys. Rev. Lett. 105, 097401 (2010).

joel.cox@icfo.es