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.

We describe ultraresolution microscopy far beyond the classical Abbe diffraction limit of one half wavelength (λ/2), and also beyond the practical limit (ca. λ/10) of aperture–based scanning near–field optical microscopy (SNOM). The ‘apertureless’ SNOM discussed here uses light scattering from a sharp tip (hence scattering–type or s–SNOM) and has no λ–related resolution limit. Rather, its resolution is approximately equal to the radius a of the probing tip (for commercial tips, a<20 nm) so that 10 nm is obtained in the visible (λ/60). A resolution of λ/500 has been obtained in the mid–infrared at λ=10 μm. The advantage of infrared, terahertz and even microwave illumination is that specific excitations can be exploited to yield specific contrast, e.g. the molecular vibration offering a spectroscopic fingerprint to identify chemical composition. S–SNOM can routinely acquire simultaneous amplitude and phase images to obtain information on refractive and absorptive properties. Plasmon– or phonon–resonant materials can be highlighted by their particularly high near–field signal level. Furthermore, s–SNOM can map the characteristic optical eigenfields of small, optically resonant particles. Lastly, we describe theoretical modelling that explains and predicts s–SNOM contrast on the basis of the local dielectric function.