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© 2020, The Author(s), under exclusive licence to Springer Nature Limited. Nonlinear nanophotonics leverages engineered nanostructures to funnel light into small volumes and intensify nonlinear optical processes with spectral and spatial control. Owing to its intrinsically large and electrically tunable nonlinear optical response, graphene is an especially promising nanomaterial for nonlinear optoelectronic applications. Here we report on exceptionally strong optical nonlinearities in graphene–insulator–metal heterostructures, which demonstrate an enhancement by three orders of magnitude in the third-harmonic signal compared with that of bare graphene. Furthermore, by increasing the graphene Fermi energy through an external gate voltage, we find that graphene plasmons mediate the optical nonlinearity and modify the third-harmonic signal. Our findings show that graphene–insulator–metal is a promising heterostructure for optically controlled and electrically tunable nano-optoelectronic components.

Graphene is conceivably the most nonlinear optoelectronic material we know. Its nonlinear optical coefficients in the terahertz frequency range surpass those of other materials by many orders of magnitude. Here, we show that the terahertz nonlinearity of graphene, both for ultrashort single-cycle and quasi-monochromatic multicycle input terahertz signals, can be efficiently controlled using electrical gating, with gating voltages as low as a few volts. For example, optimal electrical gating enhances the power conversion efficiency in terahertz third-harmonic generation in graphene by about two orders of magnitude. Our experimental results are in quantitative agreement with a physical model of the graphene nonlinearity, describing the time-dependent thermodynamic balance maintained within the electronic population of graphene during interaction with ultrafast electric fields. Our results can serve as a basis for straightforward and accurate design of devices and applications for efficient electronic signal processing in graphene at ultrahigh frequencies.

© 2017 The Authors. The ability to confine light into tiny spatial dimensions is important for applications such as microscopy, sensing, and nanoscale lasers. Although plasmons offer an appealing avenue to confine light, Landau damping in metals imposes a trade-off between optical field confinement and losses. We show that a graphene-insulator-metal heterostructure can overcome that trade-off, and demonstrate plasmon confinement down to the ultimate limit of the length scale of one atom. This is achieved through far-field excitation of plasmon modes squeezed into an atomically thin hexagonal boron nitride dielectric spacer between graphene and metal rods. A theoretical model that takes into account the nonlocal optical response of both graphene and metal is used to describe the results. These ultraconfined plasmonic modes, addressed with far-field light excitation, enable a route to new regimes of ultrastrong light-matter interactions.

Nonlinear optics is an increasingly important field for scientific and technological applications, owing to its relevance and potential for optical and optoelectronic technologies. Currently, there is an active search for suitable nonlinear material systems with efficient conversion and a small material footprint. Ideally, the material system should allow for chip integration and room-temperature operation. Two-dimensional materials are highly interesting in this regard. Particularly promising is graphene, which has demonstrated an exceptionally large nonlinearity in the terahertz regime. Yet, the light–matter interaction length in two-dimensional materials is inherently minimal, thus limiting the overall nonlinear optical conversion efficiency. Here, we overcome this challenge using a metamaterial platform that combines graphene with a photonic grating structure providing field enhancement. We measure terahertz third-harmonic generation in this metamaterial and obtain an effective third-order nonlinear susceptibility with a magnitude as large as 3 × 10–8 m2/V2, or 21 esu, for a fundamental frequency of 0.7 THz. This nonlinearity is 50 times larger than what we obtain for graphene without grating. Such an enhancement corresponds to a third-harmonic signal with an intensity that is 3 orders of magnitude larger due to the grating. Moreover, we demonstrate a field conversion efficiency for the third harmonic of up to ∼1% using a moderate field strength of ∼30 kV/cm. Finally, we show that harmonics beyond the third are enhanced even more strongly, allowing us to observe signatures of up to the ninth harmonic. Grating-graphene metamaterials thus constitute an outstanding platform for commercially viable, CMOS-compatible, room-temperature, chip-integrated, THz nonlinear conversion applications.

Graphene is the first of the 2D-material family. It is formed by carbon atoms arranged in a honeycomb lattice, which confers it intriguing physical properties that are still being discovered nowadays. A fundamental advantage found in graphene is the ability to gate tune “in-situ“ its optical response from reflective (metallic) to absorptive (lossy dielectric). It is in the reflective conditions when it becomes more interesting since it supports surface plasmon polaritons in the mid-infrared, similar to metals in the near-infrared and visible spectral regions. Surface plasmons in metals are known to be more confined than free space propagating light. But graphene naturally excels in this aspect by offering a confinement factor around 100, which causes light to couple in inefficiently. Several studies on metal plasmonics have shown the possibilities of confining light into tiny spatial dimensions with applications in molecular sensing as an example. Often, metal plasmons are used in the visible and IR regions with moderate confinement. However, Landau damping limits the optical field confinement due to penetration in the material and the consequent losses. In this thesis, it is shown that graphene-insulator-metal hybrid heterostructures can overcome that limitation by efficiently exciting plasmons in unpatterned graphene with vertical confinement down to the ultimate one-atom insulator thickness. It is accomplished by encapsulating graphene with a single layer of h-BN (or thicker oxide layers for the systematic study) and fabricating metallic nano/micro-ribbons on top. The transmission extinction of the samples was measured and compared with theoretical models accounting for material nonlocal permittivity. The ultimate confinement and the validity of the excitation method are confirmed enabling a path towards ultrastrong light-matter interaction. An example application of the aforementioned method to graphene nonlinear optics is also presented. The large intrinsic gra, El grafeno es el primero de la creciente familia de materiales 2D. Está formado por átomos de carbono dispuestos en una red de panal que le confiere propiedades físicas intrigantes que todavía se están descubriendo hoy en día. Una ventaja fundamental que encontramos en el grafeno es la capacidad de modificar ¿in-situ¿ su respuesta óptica de reflectante (metálico) a absorbente (dieléctrico). Es en el primero cuando el grafeno se muestra más interesante, ya que admite plasmones superficiales en el infrarrojo medio, similarlmente a los metales en las regiones espectrales del infrarrojo cercano y el visible. Se sabe que los plasmones superficiales en metales están más confinados que la luz que se propaga libremente. El grafeno sobresale en este aspecto al ofrecer un factor de confinamiento alrededor de 100 de forma natural, con la contrapartida de que la luz se acople de manera muy ineficiente a los plasmones en grafeno. Varios estudios sobre plasmones metálicos han demostrado que las posibilidades de confinar la luz en pequeñas dimensiones espaciales pueden ser aplicadas, por ejemplo, en la detección de biomoléculas. A menudo, los plasmones metálicos se usan en las regiones visibles e IR con un confinamiento moderado. Sin embargo, el amortiguamiento de Landau limita dicho confinamiento del campo electromagnético debido a la penetración de éste en el material y las consiguientes pérdidas. En esta tesis, se muestra que las heteroestructuras híbridas de grafeno-dieléctrico-metal pueden superar esa limitación excitando eficientemente los plasmones en grafeno extendido con confinamiento vertical máximo, hasta el espesor de un solo átomo de material dieléctrico. Tal efecto se logra encapsulando el grafeno con una sola capa de h-BN y fabricando nano/microtiras metálicas sobre éstos. Otros espesores y materiales también fueron estudiados. La extinción en transmisión de las muestras se midió y comparó con modelos teóricos que incluyen la permitividad no local (dependiente tambi, Postprint (published version)

In this paper, we analyze the effects of nonlocality on the optical properties of a system consisting of a thin metallic film separated from a graphene sheet by a hexagonal boron nitride (hBN) layer. We show that nonlocal effects in the metal have a strong impact on the spectrum of the surface plasmon-polaritons on graphene. If the graphene sheet is nanostructured into a periodic grating, we show that the resulting extinction curves can be used to shed light on the importance of nonlocal effects in metals. Therefore graphene surface plasmons emerge as a tool for probing nonlocal effects in metallic nanostructures, including thin metallic films. As a byproduct of our study, we show that nonlocal effects may lead to smaller losses for the graphene plasmons than what is predicted by a local calculation. Finally, we demonstrate that such nonlocal effects can be very well mimicked using a local theory with an effective spacer thickness larger than its actual value.

Treball final de màster oficial fet en col·laboració amb Universitat Autònoma de Barcelona (UAB), Universitat de Barcelona (UB) i Institut de Ciències Fotòniques (ICFO), [ANGLÈS] Graphene is used as a novel, versatile plasmonic material. The most common way to implement resonant light-plasmon coupling is to etch graphene into periodic nanostructures, which is invasive. Here, we study a non-invasive way to engineer graphene plasmon resonances, based on periodic doping profiles. The plasmon resonances are calculated by performing numerical simulations. In addition, we report on simulations of near-field resonant coupling between a dipole and graphene plasmons. Finally, preliminary results on the experimental realization of graphene plasmon resonances are reported. This study demonstrates the potential to exploit graphene plasmons for extreme energy confinement which could lead to strong nonlinear effects., [CASTELLÀ] El grafeno es utilizado como un nuevo y versátil material plasmónico. El proceso más habitual de implementar acoplamiento resonante luz-plasmón es grabando nano-estructuras periódicas en grafeno, un proceso invasivo. Aquí, estudiamos una forma no invasiva de obtener resonancias de plasmones en grafeno, basada en perfiles periódicos de dopaje. Las resonancias de los plasmones han sido realizadas mediante simulaciones numéricas. Además, presentamos simulaciones de acoplamiento resonante de campo cercano entre dipolo y plasmones en grafeno. Finalmente, se presentan resultados preliminares sobre la realización experimental de resonancias de plasmones en grafeno. Este estudio demuestra el potencial de explotar plasmones en grafeno para confinamiento extremo de energía que puede llegar a conducir hacia intensos efectos no lineales., [CATALÀ] El grafè és utilitzat com a un nou i versàtil material plasmonic. El procés més comú d’implementar l‘acoblament ressonant llum-plasmó és gravant nano-estructures periòdiques en grafè, un procés invasiu. Aquí, estudiem una manera no invasiva d’obtenir ressonàncies de plasmons en grafè, basada en perfils periòdics de dopatge. Les ressonàncies dels plasmons han estat calculades mitjançant simulacions numèriques. A més, presentem simulacions d’acoblament ressonant de camp proper entre un dipol i plasmons en grafè. Finalment, són presentats resultats preliminars sobre la realització experimental de ressonàncies de plasmons en grafè. Aquest estudi demostra el potencial d’explotar plasmons en grafè per a confinament extrem d’energia que pot conduir cap a intensos efectes no lineals.