Abstract

Terahertz (THz) fields are widely used for sensing, communication and quality control1. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs)2,3. These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating4,5,6,7,8. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement9,10,11, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates12. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM)6,7, the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far—only GPs with nearly free-space wavelengths have been observed13. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically14 rather than optically6,7. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp ≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.

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Acknowledgements

We thank C. Crespo for technical assistance with the THz laser. R.H., P.A-G. and A.N. acknowledge support from the Spanish Ministry of Economy and Competitiveness (national projects MAT2015-65525-R, FIS2014-60195-JIN and MAT2014-53432-C5-4-R, respectively). F.H.L.K. acknowledges support from the Fundacio Cellex Barcelona, the ERC Career integration grant (294056, GRANOP), the ERC starting grant (307806, CarbonLight), the Government of Catalonia through the SGR grant (2014-SGR-1535), the Mineco grants Ramón y Cajal (RYC-2012-12281) and Plan Nacional (FIS2013-47161-P) and project GRASP (FP7-ICT-2013-613024-GRASP). R.H., F.H.L.K., A.P. and M.P. acknowledge support by the EC under the Graphene Flagship (contract no. CNECT-ICT-604391). Y.G. and J.H. acknowledge support from the US Office of Naval Research (N00014-13-1- 0662).

Author information

Author notes

    • Alexey Y. Nikitin
    • , Yuanda Gao
    •  & Achim Woessner

    These authors contributed equally to this work

Affiliations

  1. CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain

    • Pablo Alonso-González
    • , Alexey Y. Nikitin
    • , Wenjing Yan
    • , Saül Vélez
    • , Félix Casanova
    •  & Luis E. Hueso
  2. Departamento de Física, Universidad de Oviedo, 33007 Oviedo, Spain

    • Pablo Alonso-González
  3. IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain

    • Alexey Y. Nikitin
    • , Félix Casanova
    • , Luis E. Hueso
    •  & Rainer Hillenbrand
  4. Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA

    • Yuanda Gao
    •  & James Hone
  5. ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain

    • Achim Woessner
    • , Mark B. Lundeberg
    •  & Frank H. L. Koppens
  6. Radboud University, Institute for Molecules and Materials, 6525 AJ Nijmegen, The Netherlands

    • Alessandro Principi
  7. Department of Physics, Imperial College London, London SW7 2AZ, UK

    • Nicolò Forcellini
  8. Neaspec GmbH, 82152 Martinsried, Germany

    • Andreas. J. Huber
  9. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

    • Kenji Watanabe
    •  & Takashi Taniguchi
  10. Istituto Italiano di Tecnologia, Graphene labs, Via Morego 30, 16163 Genova, Italy

    • Marco Polini
  11. ICREA – Institució Catalana de Recerça i Estudis Avancats, 08010 Barcelona, Spain

    • Frank H. L. Koppens
  12. CIC NanoGUNE and UPV/EHU, 20018 Donostia-San Sebastián, Spain

    • Rainer Hillenbrand

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Contributions

P.A-G., A.J.H. and R.H. implemented the THz photocurrent near-field microscope. P.A-G. and A.W. performed the photocurrent nanoscopy experiments. A.Y.N. and A.W. performed the simulations of the GP near-field distributions and GP dispersion. Y.G. fabricated the h-BN/graphene/h-BN photodetector devices. A.P., N.F. and M.P. developed the analytic model for the acoustic GP dispersion and the theory of plasmon damping. W.Y. and S.V. fabricated photocurrent devices based on exfoliated and CVD-grown graphene. K.W. and T.T. synthesized the h-BN. F.C., L.E.H. and J.H. coordinated the fabrication of the different photocurrent samples. P.A-G., A.Y.N., A.W., M.B.L., M.P., F.H.L.M. and R.H. analysed the data. P.A-G., A.Y.N. and R.H. wrote the manuscript with input from all authors. All authors contributed to the scientific discussion and manuscript revisions.

Competing interests

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

Corresponding authors

Correspondence to Pablo Alonso-González or Rainer Hillenbrand.

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DOI

https://doi.org/10.1038/nnano.2016.185