Hexagonal boron nitride (h-BN) is a natural hyperbolic material1, in which the dielectric constants are the same in the basal plane (εt ≡ εx = εy) but have opposite signs (εtεz < 0) in the normal plane (εz)1,2,3,4. Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons1,2,5—collective modes that originate from the coupling between photons and electric dipoles6 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-BN1,2,7. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure8 composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene9,10,11,12,13 with hyperbolic phonon polaritons in h-BN1,2, 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 metamaterial14 as the resulting properties of these devices are not present in its constituent elements alone.
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Work at the University of California, San Diego (UCSD), on optical phenomena in vdW materials is supported by DOE-BES DE-FG02-00ER45799 and the Moore Foundation. Research at UCSD on metamaterials and the development of nano-infrared instrumentation is supported by the Air Force Office of Scientific Research (AFOSR), the University of California Office of The President and the Office of Naval Research. P.J-H. acknowledges support from the AFOSR (grant no. FA9550-11-1-0225).
F.K. is one of the cofounders of Neaspec and Lasnix, producer of the s-SNOM and infrared source used in this work. All other authors declare no competing financial interests.
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Dai, S., Ma, Q., Liu, M. et al. Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial. Nature Nanotech 10, 682–686 (2015). https://doi.org/10.1038/nnano.2015.131
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