Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.

Highly confined low-loss plasmons in graphene-boron nitride heterostructures

Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering-type scanning near-field optical microscopy (s-SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5-3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm-1 ), and temporal (<10 fs) resolution. The history of s-SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research.

Modern Scattering-Type Scanning Near-Field Optical Microscopy for Advanced Material Research

Graphene offers the possibility for actively controlling plasmon confinement and propagation by tailoring its spatial conductivity. However, implementation of this concept has been hampered because it is difficult to control the conductivity pattern without disturbing the electromagnetic environment of graphene plasmons. Here we demonstrate full electrical control of plasmon reflection/transmission in graphene at electronic boundaries induced by a transparent patterned zinc oxide gate, which is designed to minimize the electromagnetic coupling to graphene in the terahertz range. This approach enables plasmons to be confined to desired regions. Our approach might be applied to various types of plasmonic devices, paving the way for implementing a programmable plasmonic circuit. Controlling spatial conductivity in graphene is important for plasmonic devices, yet conductivity patterning typically changes the electromagnetic environment. Here, teraherz plasmons in graphene are confined to specific regions via a patterned zinc oxide gate, reducing electromagnetic coupling.

/pdf/active-spatial-control-of-terahertz-plasmons-in-graphene-526sqyd1yn.pdf

Active spatial control of terahertz plasmons in graphene

Two-dimensional (2D) metal oxide (MO) nanomaterials, like graphene, possess unique electrical, mechanical, optical and catalytic performances, and have attracted substantial research interest recently. However, it remains a challenge to easily obtain 2D MO nanosheets by a generalized synthetic pathway. Here, we report a general and facile strategy for the synthesis of 2D MO nanosheets induced by nonionic surfactant micelles. Notably, the novel strategy primarily relies on the thermoregulated phase transition of the micelles. The resulting 2D MO nanosheets show high specific surface areas. As a demonstration, Sb2O3 nanosheets synthesized by our method as anodes for sodium-ion batteries (SIBs) have a high reversible capacity of 420 mA h g−1 and a high capacity retention of 99% after 150 cycles at 0.1 A g−1. Mn3O4 nanosheets for supercapacitors have a remarkable specific capacitance of 127 F g−1 at a current density of 0.5 A g−1. Even at a large current density of 5 A g−1 after 10 000 cycles, 96% of the specific capacitance is retained, demonstrating the remarkable performance of these nanosheets for energy storage applications.

A generalized strategy for the synthesis of two-dimensional metal oxide nanosheets based on a thermoregulated phase transition

Hexagonal boron nitride (hBN) supports two types of hyperbolic phonon polaritons (HPPs), whose properties of strong electromagnetic field confinement and low propagation loss have been proposed for various applications in nanophotonics. Conventionally, real-space imaging of HPPs by scattering-type scanning near-field optical microscopy (s-SNOM) with vertical polarization excitation contains both tip and edge launched polariton modes, which leads to hybrid interference fringes. In this work, we symmetrically study the tip and edge excited HPPs in both boron nitride with the natural distribution of boron isotopes (natural hBN) and 11B isotope-enriched boron nitride (99.2% 11B hBN). The intrinsic HPPs excited in 99.2% 11B hBN exhibit a lower damping rate and longer propagation length than that in natural hBN. We experimentally realize a tuning from tip-dominated to edge-dominated excited HPPs by rotating the polarization of incident light. The near-field electric field intensity (NEFI) of edge-excited HPPs Eedge and the angle β (between the hBN edge and the projective direction of the incident electric field on the hBN plane) present a sine function relationship as Eedge ∝ |sin β| under an s-polarized incident light. The NEFI of edge-excited HPPs in 99.2% 11B hBN shows a 10% enhancement compared to natural hBN under the same measurement conditions. Our findings demonstrate an effective approach to reducing phonon polariton damping and manipulating phonon polariton excitation in hBN, which are beneficial for developing HPPs-based nanophotonic applications.

Manipulating phonon polaritons in low loss 11B enriched hexagonal boron nitride with polarization control

The idea of squeezing optical field intensity into nanoscopic dimensions can be achieved through plasmon polaritons, where the prerequisite is to bridge the unmatched momentum of plasmons and free-space photons. Conventionally, complicated subwavelength structures or artificial dipole nanostructures are adopted to impart the necessary momentum for the plasmon excitation. In this work, we show that by using the near-field imaging technique, the plasmon can be launched directly from the edge of graphene lying on the high-κ oxide substrates when illuminated by an infrared light. In addition, we show that such an edge-excited mode can be remarkably tailored by changing the angle between the graphene edge and the incident light field. Further theoretical analysis reveals the strength of the edge-excited mode and its superposition with a tip-excited mode and an edge localized mode. We attribute our findings to the reduced Coulomb scattering and phonon scattering in graphene, allowing the edge-excited mode to be identified. The conceptual edge "antenna" is found to be a very convenient approach to initiate plasmons in two-dimensional systems, which opens up a compelling route for realising nanophotonic applications.

Nano-imaging of an edge-excited plasmon mode in graphene.

Quantum mechanical effects of single particles can affect the collective plasmon behaviors substantially. In this work, the quantum control of plasmon excitation and propagation in graphene is demonstrated by adopting the variable quantum transmission of carriers at Heaviside potential steps as a tuning knob. First, the plasmon reflection is revealed to be tunable within a broad range by varying the ratio γ between the carrier energy and potential height, which originates from the quantum mechanical effect of carrier propagation at potential steps. Moreover, the plasmon excitation by free-space photos can be regulated from fully suppressed to fully launched in graphene potential wells also through adjusting γ, which defines the degrees of the carrier confinement in the potential wells. These discovered quantum plasmon effects offer a unified quantum-mechanical solution toward ultimate control of both plasmon launching and propagating, which are indispensable processes in building plasmon circuitry.

Quantum Control of Graphene Plasmon Excitation and Propagation at Heaviside Potential Steps

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Nano-imaging of an edge-excited plasmon mode in graphene.

Quantum Control of Graphene Plasmon Excitation and Propagation at Heaviside Potential Steps