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Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal

Nature Nanotechnology volume 18, pages 64–70 (2023)Cite this article

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Abstract

Various optical crystals possess permittivity components of opposite signs along different principal directions in the mid-infrared regime, exhibiting exotic anisotropic phonon resonances. Such materials with hyperbolic polaritons—hybrid light–matter quasiparticles with open isofrequency contours—feature large-momenta optical modes and wave confinement that make them promising for nanophotonic on-chip technologies. So far, hyperbolic polaritons have been observed and characterized in crystals with high symmetry including hexagonal (boron nitride), trigonal (calcite) and orthorhombic (α-MoO3 or α-V2O5) crystals, where they obey certain propagation patterns. However, lower-symmetry materials such as monoclinic crystals were recently demonstrated to offer richer opportunities for polaritonic phenomena. Here, using scanning near-field optical microscopy, we report the direct real-space nanoscale imaging of symmetry-broken hyperbolic phonon polaritons in monoclinic CdWO4 crystals, and showcase inherently asymmetric polariton excitation and propagation associated with the nanoscale shear phenomena. We also introduce a quantitative theoretical model to describe these polaritons that leads to schemes to enhance crystal asymmetry via the damping loss of phonon modes. Ultimately, our findings show that polaritonic nanophotonics is attainable using natural materials with low symmetry, favouring a versatile and general way to manipulate light at the nanoscale.

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Fig. 1: Hyperbolic polaritons in LSCs.
Fig. 2: Near-field observation of polaritons in LSCs.
Fig. 3: Near-field images of polaritons in low-symmetry CdWO4 crystals.
Fig. 4: Symmetry-broken nature of hyperbolic polaritons in LSCs.

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Acknowledgements

C.-W.Q. acknowledges support from the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP award NRF CRP22-2019-0006) and from the grant (A-0005947−16-00) from Advanced Research and Technology Innovation Centre (ARTIC). Q.D. acknowledges support from the National Natural Science Foundation of China (grant no. 51925203). D.H. acknowledges support from the National Natural Science Foundation of China (grant no. 52072083). J.D.C. acknowledges financial support from the US National Science Foundation (grant no. 2128240). J.W. acknowledges the Advanced Manufacturing and Engineering Young Individual Research Grant (AME YIRG grant no. A2084c170) and SERC Central Research Fund (CRF). P.L. acknowledges support from the National Natural Science Foundation of China (grant no. 62075070). W.M. acknowledges support from the Fundamental Research Funds for the Central Universities, HUST (grant no. 2022JYCXJJ009). A.A. and X.N. were supported by the Air Force Office of Scientific Research, the Office of Naval Research and the Simons Foundation.

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Author notes

  1. These authors contributed equally: Guangwei Hu, Weiliang Ma, Debo Hu.

Authors and Affiliations

  1. Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore

    Guangwei Hu, Chunqi Zheng, Kaipeng Liu & Cheng-Wei Qiu

  2. Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, China

    Weiliang Ma, Xinliang Zhang & Peining Li

  3. CAS Key Laboratory of Nanophotonic Materials and Devices, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, China

    Debo Hu & Qing Dai

  4. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore, Singapore

    Jing Wu

  5. NUS Graduate School, National University of Singapore, Singapore, Singapore

    Chunqi Zheng & Cheng-Wei Qiu

  6. Ministry of Industry and Information Technology Key Lab of Micro-Nano Optoelectronic Information System, Harbin Institute of Technology, Shenzhen, China

    Xudong Zhang

  7. Photonics Initiative, Advanced Science Research Center, City University of New York, New York, NY, USA

    Xiang Ni & Andrea Alù

  8. Institute of Physics, Chinese Academy of Sciences, Beijing, China

    Jianing Chen

  9. Department of Mechanical Engineering, Vanderbilt University, Nashville, TN, USA

    Joshua D. Caldwell

  10. Fritz Haber Institute of the Max Planck Society, Berlin, Germany

    Alexander Paarmann

  11. Physics Program, Graduate Center, City University of New York, New York, NY, USA

    Andrea Alù

Authors
  1. Guangwei Hu
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Contributions

C.-W.Q, P.L. and A.A. conceived the idea. G.H. and W.M. performed the theory, simulation and design, with substantial input from C.-W.Q. and A.A. D.H. measured the sample with supervision by P.L. and Q.D. X.D.Z. fabricated the Au nanoantenna. G.H., W.M., X.L.Z., J.C., A.P., Q.D., A.A., P.L. and C.W. analysed the data, with input from all the authors. G.H., W.M. and D.H. wrote the manuscript with substantial contributions from all the others. C.-W.Q. oversaw the project.

Corresponding authors

Correspondence to Qing Dai, Andrea Alù, Peining Li or Cheng-Wei Qiu.

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Supplementary Sections 1–3 and Figs. 1–9.

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Hu, G., Ma, W., Hu, D. et al. Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal. Nat. Nanotechnol. 18, 64–70 (2023). https://doi.org/10.1038/s41565-022-01264-4

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