Hexagonal boron nitride (h-BN) is a natural hyperbolic material, in which the dielectric constants are the same in the basal plane (ε(t) ≡ ε(x) = ε(y)) but have opposite signs (ε(t)ε(z) < 0) in the normal plane (ε(z)). Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons--collective modes that originate from the coupling between photons and electric dipoles in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene with hyperbolic phonon polaritons in h-BN, so that the eigenmodes of the graphene/h-BN heterostructure are hyperbolic plasmon-phonon polaritons. The hyperbolic plasmon-phonon polaritons in graphene/h-BN suffer little from ohmic losses, making their propagation length 1.5-2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon-phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial as the resulting properties of these devices are not present in its constituent elements alone.

The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at subpicosecond timescales. This handicap has prompted the search for alternative plasmonic media 1-3 , with graphene emerging as one of the most capable candidates for infrared wavelengths. Here we visualize and elucidate the properties of non-equilibrium photo-induced plasmons in a high-mobility graphene monolayer 4 . We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks infrared plasmonic response at equilibrium. In combination with static nano-imaging results on plasmon propagation, our infrared pump-probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene.Graphene plasmonics 5-7 has progressed rapidly, propelled by the electrical tunability, high field confinement 8,9 , potentially long lifetimes 10,11 of plasmons and the strong light-matter interactions 12-15 in graphene. An earlier spectroscopic study has reported photoinduced alteration of the plasmonic response of graphene on optical pumping 16 . In this work, we harnessed ultrafast optical pulses to generate mid-infrared (mid-IR) plasmons in a sample that lacks a plasmonic response at equilibrium. We examined the real-space aspects of non-equilibrium plasmon-polariton generation and propagation under femtosecond (fs) photo-excitation using a new ultrafast nano-infrared (IR) technique that fuses realspace plasmon imaging with spectroscopy. We applied this method to investigate high-quality graphene specimens encapsulated in hexagonal boron nitride: hBN/G/hBN 4 .We performed time-resolved broadband nano-IR experiments using antenna-based near-field nanoscopy (see Methods). This set-up (Fig. 1a,b) combines exceptional spatial, spectral and temporal resolution 16-18 , allowing an experimental probe of the dispersion of graphene plasmons under photo-excitation-a feat previously considered technologically infeasible. In our measurements, the metalized tip of an atomic force microscope (AFM) was illuminated by a focused IR probe beam, generating strong evanescent electric fields beneath the tip. These fields possess a wide range of in-plane momenta q and therefore facilitate efficient coupling to graphene plasmons 19 . Such evanescent fields extend ∼20 nm beneath the top surface of our structures, which is sufficient to launch and detect surface plasmons in a graphene microcrystal protected by a thin (10 nm) encapsulating layer of hBN 10 . The tip of the nanoscope acts as a launcher for surface plasmons of wavelength (λ p ) that propagate radially outwards from the tip. On reflection from the sample edge, these plasmons form sta...

We report on nano-infrared (IR) imaging studies of confined plasmon modes inside patterned graphene nanoribbons (GNRs) fabricated with high-quality chemical-vapordeposited (CVD) graphene on Al2O3 substrates. The confined geometry of these ribbons leads to distinct mode patterns and strong field enhancement, both of which evolve systematically with the ribbon width. In addition, spectroscopic nano-imaging in midinfrared 850-1450 cm -1 allowed us to evaluate the effect of the substrate phonons on the plasmon damping. Furthermore, we observed edge plasmons: peculiar one-dimensional modes propagating strictly along the edges of our patterned graphene nanostructures. KeywordsGraphene nanoribbons, CVD graphene, nano-infrared imaging, plasmon-phonon coupling, edge plasmons Main textSurface plasmon polaritons, collective oscillation of charges on the surface of metals or semiconductors, have been harnessed to confine and manipulate electromagnetic energy at the nanometer length scale. 1 In particular, surface plasmons in graphene are collective oscillations of Dirac quasiparticles that reveal high confinement, electrostatic tunability and long lifetimes. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Plasmons in graphene are promising for optoelectronic and nanophotonic applications in a wide frequency range from the terahertz to the infrared (IR) regime. 16,17 One common approach to investigate plasmons is based on nano-structuring of plasmonic media. 12,18 Large area structures comprised of graphene nanoribbons (GNRs) and graphene nano-disks have been extensively investigated by means of various spectroscopies. [12][13][14][15][16][17] These types of structures are of interest in light of practical applications including: surface enhanced IR vibrational spectroscopy 19,20 , modulators 21 , photodetectors 22 and tunable metamaterials 23,24 . Whereas the collective, area-averaged responses of graphene nanotructures are well characterized, the real-space characteristics of confined plasmon modes within these nanostructures remain completely unexplored.In this work, we performed nano-IR imaging on patterned GNRs utilizing an antennabased nanoscope that is connected to both continuous-wave and broadband lasers 25 (Supporting Information). As shown in Figure 1a, the metalized tip of an atomic force microscope (AFM) is illuminated by IR light thus generating strong near fields underneath the tip apex. These fields have a wide range of in-plane momenta q thus facilitating energy transfer and momentum bridging from photons to plasmons. [3][4][5][6][7][8][9][10][11][12] Our GNR samples were fabricated by lithography patterning of high quality CVD-grown graphene single crystals 26 on aluminum oxide (Al2O3) substrates (Supporting Information). As discussed in detail below, the optical phonon of Al2O3 is below  = 1000 cm -1 ( Figure S2), allowing for a wide mid-IR frequency region free from phonons.In Figure 1b, we show the AFM phase image displaying arrays of GNRs with various widths (darker parts correspond to gr...

Moiré patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a crystal of hexagonal boron nitride. As a result of the moiré pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moiré graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moiré superstructures. This novel form of collective modes is likely to be generic to other forms of moiré-forming superlattices, including van der Waals heterostructures.