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Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons

Nature Materials volume 22, pages 860–866 (2023)Cite this article

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Abstract

Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)—which exist in much broader spectral ranges than phonon polaritons—has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag2Te platelets. The hybridization of the PPs with their mirror image—by placing the platelets above a Au layer—increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

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Fig. 1: Real-space nanoimaging of in-plane anisotropic THz PPs on Ag2Te platelets.
Fig. 2: Real-space nanoimaging of in-plane anisotropic THz APPs on a Ag2Te/SiO2/Au heterostructure.
Fig. 3: Relationship between anisotropic APP propagation, platelet geometry and crystal lattice structure.
Fig. 4: PP dispersions and isofrequency curves.

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Data that support the results of this work are available from the corresponding authors upon reasonable request.

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Acknowledgements

The work was financially supported by the Spanish Ministry of Science and Innovation under the María de Maeztu Units of Excellence Program (CEX2020-001038-M/MCIN/AEI/10.13039/501100011033) (R.H., A.C., L.E.H. and E.A.); Projects PID2021-123949OB-I00 (R.H.), PID2019-109905GB-C21 (M.G.V. and I.E.), RTI2018-094861-B-100 (L.E.H.), PID2019-107432GB-I00 (J.A.) and PID2019-107338RB-C61 (E.A.) funded by MCIN/AEI/10.13039/501100011033 and by ‘ERDF—A Way of Making Europe’; the National Natural Science Foundation of China (NSFC) (52225207 and 11934005) and the Shanghai Pilot Program for Basic Research—Fudan University 21TQ1400100 (21TQ006) (F.X.X.); NSFC grant no. 61988102 and the Science and Technology Commission of Shanghai Municipality (nos. 23010503400 and 23ZR1443500) (S.C.); the Czech Science Foundation GACR under the Junior Star grant no. 23-05119M (A.K.); the European Research Council (ERC) under grant agreement no. 101020833 (M.G.V.); the German Research Foundation (DFG) under project nos. 467576442 (I.N.) and GA 3314/1-1–FOR 5249 (QUAST) (M.G.V.); the Gipuzkoa Council (Spain) in the frame of the Gipuzkoa Fellows Program (B.M.-G.); and the University groups of the Basque Government (IT1526-22) (J.A.).

Author information

Author notes

  1. These authors contributed equally: S. Chen, P. L. Leng.

Authors and Affiliations

  1. Terahertz Technology Innovation Research Institute, National Basic Science Center—Terahertz Science and Technology Frontier, Terahertz Precision Biomedical Discipline 111 Project, Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, Shanghai, China

    S. Chen

  2. CIC nanoGUNE BRTA, Donostia-San Sebastián, Spain

    S. Chen, E. Modin, E. Vicentini, B. Martín-García, M. Barra-Burillo, I. Niehues, C. Maciel Escudero, L. E. Hueso, E. Artacho & A. Chuvilin

  3. State Key Laboratory of Surface Physics and Department of Physics, Institute for Nanoelectronic Devices and Quantum Computing, Zhangjiang Fudan International Innovation Center, Fudan University, Shanghai, China

    P. L. Leng, X. Y. Xie & F. X. Xiu

  4. Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic

    A. Konečná

  5. Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic

    A. Konečná

  6. Materials Physics Center, CSIC-UPV/EHU, Donostia-San Sebastián, Spain

    M. Gutierrez-Amigo, C. Maciel Escudero, J. Aizpurua & I. Errea

  7. Departamento de Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV/EHU), Bilbao, Spain

    M. Gutierrez-Amigo

  8. IKERBASQUE, Basque Foundation for Science, Bilbao, Spain

    B. Martín-García, L. E. Hueso, E. Artacho, A. Chuvilin & R. Hillenbrand

  9. Theory of Condensed Matter, Cavendish Laboratory, University of Cambridge, Cambridge, UK

    E. Artacho

  10. Donostia International Physics Centre (DIPC), Donostia-San Sebastián, Spain

    E. Artacho, J. Aizpurua, I. Errea & M. G. Vergniory

  11. Departamento de Física Aplicada, Escuela de Ingeniería de Gipuzkoa, Universidad del País Vasco (UPV/EHU), Donostia-San Sebastián, Spain

    I. Errea

  12. Max Planck for Chemical Physics of Solids, Dresden, Germany

    M. G. Vergniory

  13. CIC nanoGUNE BRTA and Department of Electricity and Electronics, UPV/EHU, Donostia-San Sebastián, Spain

    R. Hillenbrand

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  1. S. Chen
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Contributions

R.H. and S.C. conceived the study. P.L.L. and X.Y.X. fabricated the Ag2Te platelets and performed the electrical transport and Hall measurements under the supervision of F.X.X. S.C. performed the THz s-SNOM imaging and related data analysis. A.K. developed the theoretical description of polariton modes and performed the dispersion fitting. E.V. performed the infrared PhP interferometry and related data analysis. A.C. fabricated the Ag2Te disc. E.M. and A.C. performed the STEM analysis. M.G. performed the ab initio calculations under the supervision of I.E. and M.G.V. B.M.-G. participated in the crystal structure characterization and discussions. M.B.-B. and I.N. participated in the sample preparation. C.M.E., E.A., L.E.H. and J.A. participated in the theory discussions. R.H., S.C. and A.K. wrote the manuscript with input from all the authors. R.H. supervised the work.

Corresponding authors

Correspondence to F. X. Xiu or R. Hillenbrand.

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Competing interests

R.H. was a co-founder of Neaspec GmbH, which now is a part of Attocube AG, a company producing s-SNOM systems, such as the one used in this study. The remaining authors declare no competing interests.

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Nature Materials thanks Joshua Caldwell, Ido Kaminer and Sang-Hyun Oh for their contribution to the peer review of this work.

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Supplementary Notes 1–12 and Figs. 1–13.

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Chen, S., Leng, P.L., Konečná, A. et al. Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons. Nat. Mater. 22, 860–866 (2023). https://doi.org/10.1038/s41563-023-01547-8

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