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


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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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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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.

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

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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).

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