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Graphene plasmons promise unique possibilities for controlling light in nanoscale devices and for merging optics with electronics. We developed a versatile platform technology based on resonant optical antennas and conductivity patterns for launching and control of propagating graphene plasmons, an essential step for the development of graphene plasmonic circuits. We launched and focused infrared graphene plasmons with geometrically tailored antennas and observed how they refracted when passing through a two-dimensional conductivity pattern, here a prism-shaped bilayer. To that end, we directly mapped the graphene plasmon wavefronts by means of an imaging method that will be useful in testing future design concepts for nanoscale graphene plasmonic circuits and devices.

Enhanced light-matter interactions are the basis of surface-enhanced infrared absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanoresonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high-quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hexagonal boron nitride (h-BN). In this work we use, for the first time, phonon-polariton-resonant h-BN ribbons for SEIRA spectroscopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.

Correspondence to: r.hillenbrand@nanogune.eu Metasurfaces with strongly anisotropic optical properties can support deep subwavelength-scale confined electromagnetic waves (polaritons) that promise opportunities for controlling light in photonic and optoelectronic applications. We develop a mid-infrared hyperbolic metasurface by nanostructuring a thin layer of hexagonal boron nitride supporting deep subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic dispersion. By applying an infrared nanoimaging technique, we visualize the concave (anomalous) wavefronts of a diverging polariton beam, which represent a landmark feature of hyperbolic polaritons. The results illustrate how near-field microscopy can be applied to reveal the exotic wavefronts of polaritons in anisotropic materials, and demonstrate that nanostructured van der Waals materials can form a highly variable and compact platform for hyperbolic infrared metasurface devices and circuits.Optical metasurfaces are thin layers with engineered optical properties (described by the effective permittivities in the two lateral directions), which are obtained by lateral structuring of the layers (1-3). Applications include flat lenses, high-efficiency holograms, generation of optical vortex beams and manipulation of polarization state of light (1-5). With metallic metasurfaces one can also control the properties of surface plasmon polaritons (SPPs, electromagnetic waves arising from the coupling of light with charge oscillations in the metasurface) propagating along the metasurface. The near-field enhancement and confinement provided by SPPs is another effective means for controlling the phase and polarization of transmitted light, or the thermal radiation

Surface plasmon modes supported by graphene ribbon waveguides are studied and classified. The properties of both modes with the field concentration within the ribbon area (waveguiding modes) and on the edges (edge modes) are discussed. The waveguide and edge modes are shown to be separated from each other by a gap in wave numbers. The even-parity hybridized edge mode results to be the fundamental electromagnetic mode of the ribbon, possessing also the lowest losses. All of the plasmonic modes in the ribbons have an optimum frequency, at which the absorption losses are minimum, due to compromise between the plasmon confinement and the decrease of the group velocity close to the modal cutoff. The presented results show that the diffraction limit in graphene ribbons can be surpassed.

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...

The extraordinary properties of graphene make it a very promising material for optoelectronics. However, basic attributes of the electromagnetic field in graphene are still unexplored. Here we report on the in-plane fields radiated by a nanoemitter lying on a graphene sheet in terahertz regime, which present a rich dependence on frequency, distance to the source, and orientation of the dipole moment. The field pattern is mainly composed of a core region, dominated by surface plasmons, where the electric field can be several orders of magnitude larger than in vacuum, and an outer region where the field is virtually the same as what it would be in vacuum.