Abstract
Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons-the collective oscillations of electrons in conducting media-offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology. Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.
Highlights
Research in nanophotonics is strongly rooted in the study of noble metal nanostructures supporting plasmons,[1] collective free-electron oscillations that can focus light into atomic length scales for a plethora of applications ranging from biosensing[2] to photochemistry[3] and photovoltaics.[4]
This simple estimate of the graphene nonlinear optical response neglects interband effects, which are predicted to impede this strong anharmonicity,[68] the collective intraband motion associated with graphene plasmons and their ability to intensify the electric field within the graphene plane offer an appealing prescription to further enhance High-harmonic generation (HHG)
We have reviewed some of these possibilities in light of recent realistic theoretical predictions
Summary
Research in nanophotonics is strongly rooted in the study of noble metal nanostructures supporting plasmons,[1] collective free-electron oscillations that can focus light into atomic length scales for a plethora of applications ranging from biosensing[2] to photochemistry[3] and photovoltaics.[4]. Plasmon resonance frequencies in a graphene nanostructure of characteristic size D scale as ωp ∝ EF/D, with typical experimentally observed Fermi energies reaching 1 eV through electrostatic gating.[9] These ideas were initially confirmed by pioneering spectroscopic measurements of electrically tunable THz plasmons in microstructured graphene ribbons,[10,26,27] followed shortly thereafter by mid-IR plasmons in structures with lateral sizes of ∼ hundreds of nanometers In this spectral range, interaction with the 0.2 eV intrinsic optical phonons was originally thought to be detrimental,[28] but their effect was later observed to be rather localized in frequency.[29] Interaction with phonons in surrounding materials has been investigated and shown to produce strong avoided crossing dispersion.[30,31] These advances in the mid-IR spectral range are hard to extrapolate to the more technologically relevant visible and near-IR regimes, several strategies have been suggested,[32] which could benefit from experimental efforts to explore extreme doping[33] and smaller structures.[34]. A reduction in size has the additional benefit that lower doping is needed to reach a given optical frequency, and in turn a lower Fermi energy increases the nonlinear response by engaging electronic motion closer to the Dirac point, where the anharmonicity associated with the conical electronic bands becomes more relevant
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