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Observation of Radiation-Loss-Based Non-Hermitian Point Gap in Photonic Crystals

Yuto Moritake1,2,*, Nozomi Ogawa1,3, Issei Takeda1,3, Yusuke Ohinata1,3, Takahiro Uemura1,3, Taiki Yoda1, Kenta Takata3,4, Eiichi Kuramochi3,4, Hisashi Sumikura3,4 et al.

Masaya Notomi1,3,4,†

  • *Contact author: moritake@iis.u-tokyo.ac.jp
  • †Contact author: notomi@phys.sci.isct.ac.jp
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Phys. Rev. Lett. 137, 046903 – Published 23 July, 2026

DOI: https://doi.org/10.1103/vsx1-17cl

Abstract

A non-Hermitian point gap (NHPG) is a unique phenomenon in non-Hermitian systems and induces a non-Hermitian skin effect (NHSE). In photonic crystals, NHPG and NHSE have previously been explored mainly through material loss, where the typically low Q factors make direct observation of complex frequencies challenging. Here, we demonstrate the direct experimental observation of an NHPG by using a radiation-loss-based non-Hermitian photonic crystal. Radiation loss can be engineered through structural design, enabling control of the imaginary part of the complex frequency and allowing relatively high Q factors. This approach is compatible with widely used absorption-free silicon-slab photonic crystals. We developed a measurement system that can measure photonic bands along arbitrary lines in k space. Our measurements showed complex-frequency loops of the NHPG in photonic crystals, and the reversal of non-Hermitian topology through the flip of the loop rotation in a complex plane. These results establish radiation-loss engineering as a practical route to directly measuring and controlling complex photonic band structures and bulk point-gap topology in nanophotonic systems without gain media or nonreciprocity.

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Phys. Rev. Lett. 137, 046903– Published 23 July, 2026
  • Received 25 December 2025
  • Revised 12 April 2026
  • Accepted 29 June 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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