Extreme nonlinear optical phenomena enabled by plasmons in nanostructured graphene

The remarkably high intrinsic nonlinearity of graphene can be pushed even further when the frequency of impinging light matches that of its long-lived and electrically-tunable plasmon resonances. Through their enhanced... [ view full abstract ]

The remarkably high intrinsic nonlinearity of graphene can be pushed even further when the frequency of impinging light matches that of its long-lived and electrically-tunable plasmon resonances. Through their enhanced absorption cross-sections, plasmons in highly-doped graphene provide the means to concentrate electromagnetic energy on extreme subwavelength scales, generating enormous local electric fields. We explore the regime of extreme nonlinear optics that is enabled by plasmonic near-field enhancement in nanostructured graphene. In particular we focus on (1) the generation of high-order harmonics and (2) transient absorption arising from elevated electronic temperatures in the carbon layer upon ultrafast optical pumping. Our studies of these plasmon-assisted nonlinear phenomena are based on rigorous time-domain simulataions, where graphene structures are described using an atomistic tight-binding description of their electronic states combined with the random-phase approximation.

Our results indicate that high-harmonic generation can be achieved with remarkably low input optical powers by tuning the incident light wavelength to the localized plasmon resonances of ribbons and finite islands, while these resonances can be modulated via electrical gating. In contrast to atomic gases, we observe no cutoff in harmonic order, while a comparison of graphene plasmon-assisted high-harmonic generation to recent measurements in solid-state systems suggest that the yields from bulk semiconductors can be produced by graphene plasmons using 3-4 order of magnitude lower pulse fluence. At the fundamental excitation frequency, a delayed nonlinearity also takes place as a consequence of the strong dependence of the graphene response on the temperature of its conduction electrons. We demonstrate that strong transient modulation of the optical absorption associated with plasmonic excitations in graphene nanostructures can occur when electrons are optically pumped to an elevated temperature. Our results indicate that doped graphene nanostructures hold great promise for efficient higher-harmonic generation and all-optical switching applications in nonlinear nanophotonic devices.

Figure 1 shows (a) a schematic illustration of a doped nanoribbon illuminated by an intense optical pulse emitting high harmonics; (b) the absorption cross-section of a 20-nm wide nanoribbon exhibiting a prominent plasmonic mode; (c) the spectral decomposition of the light emission intensity under illumination by pulses centered at the plasmon resonance frequency.