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  • Letter
  • Open Access

Free extension of topological states via double-zero-index media

Rui Dong1,*, Changhui Shen2,*, Zhijun Che1, Changqing Xu1,†, Yun Lai2,‡, and Ce Shang3,§

  • 1Key Laboratory of State Manipulation and Advanced Materials in Provincial Universities, School of Physics and Technology, Nanjing Normal University, Nanjing 210023, China
  • 2MOE Key Laboratory of Modern Acoustics, National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, and Jiangsu Physical Science Research Center, Nanjing University, Nanjing 210093, China
  • 3Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China

  • *These authors contributed equally to this work.
  • †Contact author: changqing.xu@nnu.edu.cn
  • ‡Contact author: laiyun@nju.edu.cn
  • §Contact author: shangce@aircas.ac.cn
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APS Open Sci. 1, L000002 – Published 29 April, 2026

DOI: https://doi.org/10.1103/w5cy-d8l1

Abstract

Topological states, known for their robustness against disorder, offer promising avenues for disorder-resistant devices. However, their intrinsic spatial confinement at interfaces imposes geometric constraints that limit the scalability of topological functionalities. Here, we propose a strategy to overcome this limitation by using double-zero-index media to expand topological interfaces. Although occupying finite space, these media are optically equivalent to infinitesimal points at their working frequencies, effectively altering the geometry of topological interfaces and breaking conventional bulk-edge correspondence. This strategy enables the spatial expansion of uniform topological states beyond their native interface, offering new possibilities for topological photonic devices. Validated by simulations and microwave experiments, our findings offer a universal framework to break the inherent dimensional limitations of topological states, with implications extending to general wave systems.

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APS Open Sci. 1, L000002– Published 29 April, 2026
Vol. 1
  • Received 29 July 2025
  • Accepted 3 March 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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