Ghost hyperbolic surface polaritons in bulk anisotropic crystals

Weiliang Ma, Guangwei Hu, Debo Hu, Runkun Chen, Tian Sun, Xinliang Zhang (), Qing Dai (), Ying Zeng, Andrea Alù (), Cheng-Wei Qiu () and Peining Li ()
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Weiliang Ma: Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology
Guangwei Hu: National University of Singapore
Debo Hu: National Center for Nanoscience and Technology
Runkun Chen: Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology
Tian Sun: Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology
Xinliang Zhang: Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology
Qing Dai: National Center for Nanoscience and Technology
Ying Zeng: Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology
Andrea Alù: City University of New York
Cheng-Wei Qiu: National University of Singapore
Peining Li: Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology

Nature, 2021, vol. 596, issue 7872, 362-366

Abstract: Abstract Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1–10. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell’s equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

Date: 2021
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DOI: 10.1038/s41586-021-03755-1

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