Abstract

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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Data availability

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.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

  1. 1.

    Basov, D., Fogler, M. & de Abajo, F. G. Polaritons in van der Waals materials. Science 354, aag1992 (2016).

  2. 2.

    Low, T. et al. Polaritons in layered two-dimensional materials. Nat. Mater. 16, 182–194 (2017).

  3. 3.

    Fei, Z. et al. Gate-tuning of graphene plasmons revealed by infrared nano-imaging. Nature 487, 82–85 (2012).

  4. 4.

    Chen, J. et al. Optical nano-imaging of gate-tunable graphene plasmons. Nature 487, 77–81 (2012).

  5. 5.

    Dai, S. et al. Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride. Science 343, 1125–1129 (2014).

  6. 6.

    Chakraborty, S. et al. Gain modulation by graphene plasmons in aperiodic lattice lasers. Science 351, 246 (2016).

  7. 7.

    Cai, X. et al. Plasmon-enhanced terahertz photodetection in graphene. Nano Lett. 15, 4295–4302 (2015).

  8. 8.

    Rodrigo, D. et al. Mid-infrared plasmonic biosensing with graphene. Science 349, 165–168 (2015).

  9. 9.

    Low, T. et al. Plasmons and screening in monolayer and multilayer black phosphorus. Phys. Rev. Lett. 113, 106802 (2014).

  10. 10.

    Woessner, A. et al. Highly confined low-loss plasmons in graphene–boron nitride heterostructures. Nat. Mater. 14, 421–425 (2015).

  11. 11.

    Giles, A. J. et al. Ultralow-loss polaritons in isotopically pure boron nitride. Nat. Mater. 17, 134–139 (2018).

  12. 12.

    Ni, G. X. et al. Fundamental limits to graphene plasmonics. Nature 557, 530–533 (2018).

  13. 13.

    Hoffman, A. J. et al. Negative refraction in semiconductor metamaterials. Nat. Mater. 6, 946–950 (2007).

  14. 14.

    Liu, Z., Lee, H., Xiong, Y., Sun, C. & Zhang, X. Far-field optical hyperlens magnifying sub-diffraction-limited objects. Science 315, 1686 (2007).

  15. 15.

    Podolskiy, V. A. & Narimanov, E. E. Strongly anisotropic waveguide as a nonmagnetic left-handed system. Phys. Rev. B 71, 201101 (2005).

  16. 16.

    Cortes, C. L., Newman, W., Molesky, S. & Jacob, Z. Quantum nanophotonics using hyperbolic metamaterials. J. Opt. 14, 063001 (2012).

  17. 17.

    Takayama, O., Bogdanov, A. A. & Lavrinenko, A. V. Photonic surface waves on metamaterial interfaces. J. Phys. Condens. Matter 29, 463001 (2017).

  18. 18.

    Caldwell, J. D. et al. Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride. Nat. Commun. 5, 5221 (2014).

  19. 19.

    Yoxall, E. et al. Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity. Nat. Photon. 9, 674–678 (2015).

  20. 20.

    Li, P. et al. Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing. Nat. Commun. 6, 7507 (2015).

  21. 21.

    Dai, S. et al. Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material. Nat. Commun. 6, 6963 (2015).

  22. 22.

    Gomez-Diaz, J. S., Tymchenko, M. & Alù, A. Hyperbolic plasmons and topological transitions over uniaxial metasurfaces. Phys. Rev. Lett. 114, 233901 (2015).

  23. 23.

    Li, P. et al. Infrared hyperbolic metasurface based on nanostructured van der Waals materials. Science 359, 892–896 (2018).

  24. 24.

    Song, J. C. W. & Rudner, M. S. Fermi arc plasmons in Weyl semimetals. Phys. Rev. B 96, 205443 (2017).

  25. 25.

    Mazor, Y. & Steinberg, B. Z. Longitudinal chirality, enhanced nonreciprocity, and nanoscale planar one-way plasmonic guiding. Phys. Rev. B 86, 045120 (2012).

  26. 26.

    Kildishev, A. V., Boltasseva, A. & Shalaev, V. M. Planar photonics with metasurfaces. Science 339, 1232009 (2013).

  27. 27.

    Zheng, Z. et al. Highly confined and tunable hyperbolic phonon polaritons in van der Waals semiconducting transition metal oxides. Adv. Mater. 30, 1705318 (2018).

  28. 28.

    de Castro, I. A. et al. Molybdenum oxides — from fundamentals to functionality. Adv. Mater. 29, 1701619 (2017).

  29. 29.

    Lajaunie, L., Boucher, F., Dessapt, R. & Moreau, P. Strong anisotropic influence of local-field effects on the dielectric response of α-MoO3. Phys. Rev. B 88, 115141 (2013).

  30. 30.

    Py, M. A., Schmid, P. E. & Vallin, J. T. Raman scattering and structural properties of MoO3. Nuovo Cimento B 38, 271–279 (1977).

  31. 31.

    Caldwell, J. D. et al. Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons. Nanophotonics 4, 44–68 (2015).

  32. 32.

    Dai, S. et al. Efficiency of launching highly confined polaritons by infrared light incident on a hyperbolic material. Nano Lett. 17, 5285–5290 (2017).

  33. 33.

    Hu, F. et al. Imaging the localized plasmon resonance modes in graphene nanoribbons. Nano Lett. 17, 5423–5428 (2017).

  34. 34.

    Ocelic, N., Huber, A. & Hillenbrand, R. Pseudoheterodyne detection for background-free near-field spectroscopy. Appl. Phys. Lett. 89, 101124 (2006).

  35. 35.

    Huth, F., Schnell, M., Wittborn, J., Ocelic, N. & Hillenbrand, R. Infrared-spectroscopic nanoimaging with a thermal source. Nat. Mater. 10, 352–356 (2011).

  36. 36.

    Nikitin, A. Y. in World Scientific Handbook of Metamaterials and Plasmonics 307–338 (World Scientific Series in Nanoscience and Nanotechnology, World Scientific, Singapore, 2017).

  37. 37.

    Tilley, D. R. Surface Polaritons: Electromagnetic Waves at Surfaces and Interfaces (North-Holland Publishing Co., Amsterdam, 1982).

Download references

Acknowledgements

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).

Reviewer information

Nature thanks A. Chaves and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Author notes

  1. These authors contributed equally: W. Ma, P. Alonso-González, S. Li

Affiliations

  1. Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-based Functional Materials and Devices, and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, China

    • Weiliang Ma
    • , Shaojuan Li
    • , Jian Yuan
    • , Shuit-Tong Lee
    •  & Qiaoliang Bao
  2. Departamento de Física, Universidad de Oviedo, Oviedo, Spain

    • Pablo Alonso-González
    • , Javier Martín-Sánchez
    •  & Javier Taboada-Gutiérrez
  3. Donostia International Physics Center (DIPC), Donostia-San Sebastián, Spain

    • Alexey Y. Nikitin
  4. IKERBASQUE, Basque Foundation for Science, Bilbao, Spain

    • Alexey Y. Nikitin
    •  & Rainer Hillenbrand
  5. CIC nanoGUNE, Donostia-San Sebastián, Spain

    • Iban Amenabar
    • , Peining Li
    • , Saül Vélez
    • , Christopher Tollan
    •  & Rainer Hillenbrand
  6. Department of Materials, ETH Zürich, Zürich, Switzerland

    • Saül Vélez
  7. Department of Materials Science and Engineering, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria, Australia

    • Zhigao Dai
    • , Yupeng Zhang
    •  & Qiaoliang Bao
  8. Functional Materials and Microsystems Research Group and MicroNano Research Facility, RMIT University, Melbourne, Australia

    • Sharath Sriram
  9. School of Chemical Engineering, University of New South Wales (UNSW), Kensington, New South Wales, Australia

    • Kourosh Kalantar-Zadeh
  10. CIC nanoGUNE and UPV/EHU, Donostia-San Sebastián, Spain

    • Rainer Hillenbrand

Authors

  1. Search for Weiliang Ma in:

  2. Search for Pablo Alonso-González in:

  3. Search for Shaojuan Li in:

  4. Search for Alexey Y. Nikitin in:

  5. Search for Jian Yuan in:

  6. Search for Javier Martín-Sánchez in:

  7. Search for Javier Taboada-Gutiérrez in:

  8. Search for Iban Amenabar in:

  9. Search for Peining Li in:

  10. Search for Saül Vélez in:

  11. Search for Christopher Tollan in:

  12. Search for Zhigao Dai in:

  13. Search for Yupeng Zhang in:

  14. Search for Sharath Sriram in:

  15. Search for Kourosh Kalantar-Zadeh in:

  16. Search for Shuit-Tong Lee in:

  17. Search for Rainer Hillenbrand in:

  18. Search for Qiaoliang Bao in:

Contributions

W.M., P.A.-G. and S.L. contributed equally to this work. Q.B. conceived the initial measurements on α-MoO3. R.H., P.A.-G. and Q.B. supervised the project. W.M. and P.A.-G. carried out the near-field imaging experiments with the help of I.A., J.M.-S., J.T.-G., Z.D. and P.L. J.Y. carried out the far-field experiments. W.M., P.A.-G., A.Y.N., S.L., R.H. and Q.B. participated in data analysis and co-wrote the manuscript. A.Y.N. suggested the model and supervised the development of the theory. J.M.-S., J.T.-G. and P.A.-G. carried out the simulations. Y.J., S.S., Y.Z. and K.K.-Z. contributed to the material synthesis. S.V., C.T., Z.D. and Y.Z. contributed to sample fabrication.

Competing interests

R.H. is cofounder of Neaspec GmbH, a company producing scattering-type near-field scanning optical microscope systems, such as the one used in this study. The remaining authors declare no competing financial interests.

Corresponding authors

Correspondence to Pablo Alonso-González or Rainer Hillenbrand or Qiaoliang Bao.

Supplementary information

  1. Supplementary Information

    This file contains Supplementary Text and Data, Supplementary Figures 1–31, Supplementary Tables 1–11 and Supplementary References. Details of the IR reflectivity of α-MoO3 crystals through FTIR spectroscopy, structural characterizations by SEM, Raman spectra and HRTEM, structure and optical anisotropy analysis, and notes for the fabrication of the α-MoO3 disk are provided. Derivation process and detailed analysis of the phase velocities and group velocities of α-MoO3 PhPs, edge and tip-launched PhPs in α-MoO3, numerical simulations of s-SNOM polariton interferometry, the polariton dispersion along a thin anisotropic sheet, lifetimes and their dependence on the flake thickness, the relationship between group velocity and lifetime are also contained.

About this article

Publication history

Received

Accepted

Published

DOI

https://doi.org/10.1038/s41586-018-0618-9

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.