Graphene plasmons were predicted to possess simultaneous ultrastrong field confinement and very low damping, enabling new classes of devices for deep-subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. Although all of these great prospects require low damping, thus far strong plasmon damping has been 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 Article 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 the dispersion and plasmon damping in real space. We find unprecedentedly low plasmon damping combined with strong field confinement and confirm the high uniformity of this plasmonic medium. The main damping channels are attributed to 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 to the development of graphene nanophotonic and nano-optoelectronic devices.

Graphene hosts a unique electron system in which electron-phonon scattering is extremely weak but electron-electron collisions are sufficiently frequent to provide local equilibrium above liquid nitrogen temperature. Under these conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic phenomena similar to classical liquids. Here we report strong evidence for this long-sought transport regime. In particular, doped graphene exhibits an anomalous (negative) voltage drop near current injection contacts, which is attributed to the formation of submicrometer-size whirlpools in the electron flow. The viscosity of graphene's electron liquid is found to be ≈0.1 m 2 s -1, an order of magnitude larger than that of honey, in agreement with many-body theory. Our work shows a possibility to study electron hydrodynamics using high quality graphene.Collective behavior of many-particle systems that undergo frequent inter-particle collisions has been studied for more than two centuries and is routinely described by the theory of hydrodynamics (1,2). The theory relies only on the conservation of mass, momentum and energy and is highly successful in explaining the response of classical gases and liquids to external perturbations varying slowly in space and time. More recently, it has been shown that hydrodynamics can also be applied to strongly interacting quantum systems including ultra-hot nuclear matter and ultra-cold atomic Fermi gases in the unitarity limit (3-6). In principle, the hydrodynamic approach can also be employed to describe many-electron phenomena in condensed matter physics (7-13). The theory becomes applicable if electron-electron scattering provides the shortest spatial scale in the problem such that ℓ ee ≪ , ℓ where ℓ ee is the electron-electron scattering length, the characteristic sample size, ℓ ≡ F the mean free path, F the Fermi velocity, and the mean free time with respect to momentum-non-conserving collisions such as those involving impurities, phonons, etc. The above 2 inequalities are difficult to meet experimentally. Indeed, at low temperatures ( ) ℓ ee varies approximately as ∝ −2 reaching a micrometer scale at liquid-helium (14), which necessitates the use of ultra-clean systems to satisfy ℓ ee ≪ ℓ. At higher , electron-phonon scattering rapidly reduces ℓ. However, for two-dimensional (2D) systems with dominating acoustic phonon scattering, ℓ decays only as ∝ −1 , slower than ℓ ee , which should in principle allow the hydrodynamic description over a certain temperature range, until other phonon-mediated processes become important. So far, there has been little evidence for hydrodynamic electron transport. An exception is an early work on 2D electron gases in ballistic devices (ℓ ~ ) made from GaAlAs heterostructures (15). They exhibited nonmonotonic changes in differential resistance as a function of a large applied current that was used to increase the electron temperature (making ℓ ee short) while the lattice temperature remained low (allowing long ℓ).The nonmonotonic behavior ...

The graphene photodetector is illustrated in Fig. 1a. A monolayer graphene sheet was encapsulated between two h-BN layers 15 . The h-BN(13 nm)-graphene-h-BN (42 nm) heterostructure is placed on top of a pair of 15-nm-thick AuPd gates, which are laterally separated by a gap of 50 nm. Applying individual voltages to the gates allows for controlling independently the carrier concentrations n 1 and n 2 in the graphene sheet at the left and right sides of the gap.In Fig. 1a we also introduce the concept of THz photocurrent nanoscopy, and its application for GPs mapping. The setup is based on a s-SNOM (Neaspec), where the metal tip is illuminated with the THz beam of a gas laser (SIFIR-50 from Coherent, providing output power in the range of a few 10 mW). Owing to a lightning-rod effect, the incident field is concentrated at the tip apex yielding a THz nanofocus 16 . Once brought into close proximity of the sample, the near fields of the nanofocus induce a current in the graphene sheet, similar to IR photocurrent nanoscopy 14,17 . Recording the current as a function of the tip position yields nanoscale-resolved THz photocurrent images. For the current measurement, the graphene is contacted electrically in a lateral geometry (i.e. metal contacts were fabricated at both sides of the heterostructure, as shown in Fig. 1a). Analogously to s-SNOM 18 and scanning photocurrent nanoscopy 14, 17 , we isolate the near-field contribution to the total photocurrent, I PC , by (i) oscillating the tip vertically at frequency Ω and (ii) demodulating the detector signal at 2Ω. This 3 technical procedure is required because of the background photocurrent generated by the diffraction limited illumination spot. We achieved a spatial resolution of about 50 nm (supplementary information S1), which is an improvement of nearly 4 orders of magnitude compared to diffraction-limited THz imaging. Fig. 1b shows a photocurrent image of the photodetector, recorded at 2.52 THz (λ 0 = 118.8 µm). Choosing graphene charge carrier densities n 1 = 0.77 and n 2 = -0.77x10 12 cm -2 , we generate a sharp pn-junction in the graphene above the gap between the gates.We observe a strong near-field photocurrent, I PC , which is localized to an about 1 µm wide region centred above the gap (central part of Fig. 1b). It can be explained by a photo-thermoelectric effect: due to a variation of the local Seebeck coefficient S in graphene (generated by the carrier density gradient), a local temperature gradient (caused by the THz nanofocus at the tip apex) generates a net charge current 14,17 .Because the variation of the carrier concentration -and thus ΔS -is largest between the two gates, we expect a maximum in the photocurrent at this location. In Fig. 1b, however, we observe a slight decrease of the photocurrent between the gates. We explain it by the reduced near-field intensity when the tip is above the gap, owing to the weaker near-field coupling between the tip and the metal gates. To corroborate the photo-thermoelectric origin of the THz photocurrent, we carrie...

Materials subjected to a magnetic field exhibit the Hall effect, a phenomenon studied and understood in fine detail. Here we report a qualitative breach of this classical behavior in electron systems with high viscosity. The viscous fluid in graphene is found to respond to non-quantizing magnetic fields by producing an electric field opposite to that generated by the classical Hall effect. The viscous contribution is large and identified by studying local voltages that arise in the vicinity of current-injecting contacts. We analyze the anomaly over a wide range of temperatures and carrier densities and extract the Hall viscosity, a dissipationless transport coefficient that was long identified theoretically but remained elusive in experiment. Good agreement with theory suggests further opportunities for studying electron magnetohydrodynamics.

Hydrodynamics is a general description for the flow of a fluid, and is expected to hold even for fundamental particles such as electrons when inter-particle interactions dominate. While various aspects of electron hydrodynamics were revealed in recent experiments, the fundamental spatial structure of hydrodynamic electrons, the Poiseuille flow profile, has remained elusive. In this work we provide the first real-space imaging of Poiseuille flow of an electronic fluid, as well as visualization of its evolution from ballistic flow. Utilizing a scanning nanotube single electron transistor, we image the Hall voltage of electronic flow through channels of high-mobility graphene. We find that the profile of the Hall field across the channel is a key physical quantity for distinguishing ballistic from hydrodynamic flow. We image the transition from flat, ballistic field profiles at low temperature into parabolic field profiles at elevated temperatures, which is the hallmark of Poiseuille flow. The curvature of the imaged profiles is qualitatively reproduced by Boltzmann calculations, which allow us to create a 'phase diagram' that characterizes the electron flow regimes. Our results provide long-sought, direct confirmation of Poiseuille flow in the solid state, and enable a new approach for exploring the rich physics of interacting electrons in real space.

Plasmons, collective oscillations of electron systems, can efficiently couple light and electric current, and thus can be used to create sub-wavelength photodetectors, radiation mixers, and on-chip spectrometers. Despite considerable effort, it has proven challenging to implement plasmonic devices operating at terahertz frequencies. The material capable to meet this challenge is graphene as it supports long-lived electrically tunable plasmons. Here we demonstrate plasmon-assisted resonant detection of terahertz radiation by antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot cavities and rectifying elements. By varying the plasmon velocity using gate voltage, we tune our detectors between multiple resonant modes and exploit this functionality to measure plasmon wavelength and lifetime in bilayer graphene as well as to probe collective modes in its moiré minibands. Our devices offer a convenient tool for further plasmonic research that is often exceedingly difficult under non-ambient conditions (e.g. cryogenic temperatures) and promise a viable route for various photonic applications.

We demonstrate that the plasmon frequency and Drude weight of the electron liquid in a doped graphene sheet are strongly renormalized by electron-electron interactions even in the longwavelength limit. This effect is not captured by the Random Phase Approximation (RPA), commonly used to describe electron fluids and is due to coupling between the center of mass motion and the pseudospin degree of freedom of the graphene's massless Dirac fermions. Making use of diagrammatic perturbation theory to first order in the electron-electron interaction, we show that this coupling enhances both the plasmon frequency and the Drude weight relative to the RPA value. We also show that interactions are responsible for a significant enhancement of the optical conductivity at frequencies just above the absorption threshold. Our predictions can be checked by far-infrared spectroscopy or inelastic light scattering.

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities, and superior electrical and optoelectronic properties [1][2][3][4][5][6][7]. Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting [8][9][10][11][12][13][14][15][16] require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons [17][18][19] in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times, for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism through distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.Owing to its large in-plane thermal conductivity, graphene has been suggested as material for the thermal management of nanoscale devices [8]. At the same time, graphene is well-known for its ability to convert incident light into electrical heat, i.e. hot electrons that can be used to generate photocurrent, with applications in photodetection, data communication and light harvesting [10,20,21]. Understanding, and ultimately controlling, heat flow in graphene-van der Waals heterostructures is therefore of paramount importance. For example, a short cooling time of graphene hot carriers is advantageous for thermal management and for high switching rates of photodetectors (PDs) for data communication, whereas a long cooling time is favorable for photodetection sensitivity [10,20,21]. Of particular relevance are heterostructure devices that contain high-quality graphene encapsulated by layered materials, such as hexagonal BN (hBN) and MoS 2 , which have the potential to crucially improve the performance of electronic and optoelectronic devices [1,2]. * Electronic address: Correspondence: klaas-jan.tielrooij@icfo.eu, frank.koppens@icfo.eu ‡ Equal contribution.A number of cooling pathways for graphene carriers have been proposed, involving among others strongly coupled optical phonons [22][23][24], acoustic phonons [25][26][27][28], substrate phonons [29] and plasmons [30] (see also Appendix 1). Here, using several experimental approaches, we show that cooling in graphene encapsulated by hBN is governed by out-of-plane coupling of graphene electrons to special polar phonon modes that occur in layered materials (LMs): hyperbolic phonon polaritons, where xx zz < 0, with xx and zz the permittivity parallel and perpendicular to the LM plane. Owing to this property, these materials carry deep sub-wavelength, raylike optical phonon polaritons. For hBN, within the two Rest...