Polaritons—hybrid light–matter excitations—enable nanoscale control of light. Particularly large polariton field confinement and long lifetimes can be found in graphene and materials consisting of two-dimensional layers bound by weak van der Waals forces1,2 (vdW materials). These polaritons can be tuned by electric fields3,4 or by material thickness5, leading to applications including nanolasers6, tunable infrared and terahertz detectors7, and molecular sensors8. Polaritons with anisotropic propagation along the surface of vdW materials have been predicted, caused by in-plane anisotropic structural and electronic properties9. In such materials, elliptic and hyperbolic in-plane polariton dispersion can be expected (for example, plasmon polaritons in black phosphorus9), the latter leading to an enhanced density of optical states and ray-like directional propagation along the surface. However, observation of anisotropic polariton propagation in natural materials has so far remained elusive. Here we report anisotropic polariton propagation along the surface of α-MoO3, a natural vdW material. By infrared nano-imaging and nano-spectroscopy of semiconducting α-MoO3 flakes and disks, we visualize and verify phonon polaritons with elliptic and hyperbolic in-plane dispersion, and with wavelengths (up to 60 times smaller than the corresponding photon wavelengths) comparable to those of graphene plasmon polaritons and boron nitride phonon polaritons3,4,5. From signal oscillations in real-space images we measure polariton amplitude lifetimes of 8 picoseconds, which is more than ten times larger than that of graphene plasmon polaritons at room temperature10. They are also a factor of about four larger than the best values so far reported for phonon polaritons in isotopically engineered boron nitride11 and for graphene plasmon polaritons at low temperatures12. In-plane anisotropic and ultra-low-loss polaritons in vdW materials could enable directional and strong light–matter interactions, nanoscale directional energy transfer and integrated flat optics in applications ranging from bio-sensing to quantum nanophotonics.
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All the data are available in the online version of the paper. The data that support the findings of this study are available from the corresponding authors on reasonable request.
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We thank S. C. Dhanabalan and J. S. Ponraj for their efforts in the early stages of this project. We thank M. H. Lu, L. Liu, C. W. Qiu and L. Wang for suggestions, and H. Yan and Q. Xing for their assistance with micro-FTIR measurements. We thank Quantum Design China (Beijing laboratory) for technical support of some s-SNOM measurements. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). We acknowledge support from the National Natural Science Foundation of China (grant numbers 51222208, 51290273, 51601131, 61604102, 51702219 and 91433107), the Youth 973 programme (2015CB932700), the National Key Research and Development Program (2016YFA0201900), ARC (DP140101501, IH150100006, FT150100450 and CE170100039), the Natural Science Foundation of Jiangsu Province (BK20150053), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Collaborative Innovation Center of Suzhou Nano Science and Technology, and the Spanish Ministry of Economy, Industry and Competitiveness (national projects MAT2015-65525-R, FIS2014-60195-JIN, MAT2017-88358-C3-3-R, MAT2014-53432-C5-4-R, and the project MDM-2016-0618 of the Maria de Maeztu Units of Excellence Programme). Q.B. acknowledges support from the Australian Research Council (ARC) Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET). P.A.-G. acknowledges support from the European Research Council under Starting Grant 715496, 2DNANOPTICA. J.M.-S. acknowledges support through the Clarín Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND grant (PA-18-ACB17-29). P.L. acknowledges support from a Marie Sklodowska-Cuire individual fellowship (SGPCM-705960).
Nature thanks A. Chaves and the other anonymous reviewer(s) for their contribution to the peer review of this work.