2025/12/17 by Anonymous, Yuto Moritake, Nozomi Ogawa +18 · 1 voice
Engineering · Physics and Astronomy · #Band gap #Photonic Crystal and Fiber Optics #Photonic Crystals and Applications #Photonic bandgap #Photonic crystal #Photonics #Point (geometry) #Quantum Mechanics and Non-Hermitian Physics #Yablonovite #physics.optics
paper · pdf · doi:10.1103/vsx1-17cl
arxiv published 2025/12/17 · arxiv updated 2025/12/17 · openalex publication_date 2026/06/29 · openalex created_date 2026/06/30 · openalex updated_date 2026/08/06
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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>Q</a:mi> </a:mrow> </a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>Q</c:mi> </c:math> 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 <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>k</e:mi> </e:math> 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.