Nonlinear plasmonics in atomically-thin materials

Nanoscale nonlinear optics has received a recent stimulus with the isolation of graphene and other atomically-thin crystals, which combine a large electro-optical response with strong intrinsic optical nonlinearities. In particular, the conical electronic dispersion of graphene boosts its nonlinear response through both intraand interband transitions [1,2], which are predicted to be further increased by coupling to plasmons—the collective oscillations of electrons in conducting media—sustained by highly doped graphene nanostructures [3,4]. Also, transition-metal dichalcogenides such as MoS2 are observed to produce efficient harmonic generation [5]. Here we study the plasmon-assisted nonlinear optical response of graphene and other atomically-thin materials. Atomistic simulations provide an accurate description of such phenomena in graphene nanostructures, both in perturbative (weak field) and non-perturbative (strong field) regimes, although their computational cost is prohibitive for large systems [3]. In the perturbative regime, this limitation can be overcome by exploiting an eigenmode decomposition of the optical field in the framework of classical electrodynamics [4], yielding an analytical prescription that can be used to quantify the nonlinear optical response in 2D nanostructures. For strong external fields, the light-intensity threshold for extreme nonlinear phenomena such as saturable absorption and higher-order harmonic generation is dramatically reduced by graphene plasmons [2,3]. In this non-perturbative regime, incoherent plasmonassisted electron heating compliments the intrinsically-large nonlinear absorption. We anticipate that these findings will elucidate the role of coherent and incoherent nonlinearities for future studies and applications of nonlinear plasmonics in atomically-thin materials.