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<p>Non-Hermitian systems satisfying <i>PT</i>- or anti-<i>PT</i> symmetry have produced many intriguing wave phenomena. Maintaining the Hermitian (non-Hermitian) property and increasing the non-Hermitian (Hermitian) property difference can cause a phase transition from a symmetric phase to a broken phase through the exceptional point (EP). However, it is experimentally difficult to construct a system working at the EP to satisfy both requirements simultaneously. Bound states in the continuum (BICs) are a special lossless wave state despite their coexistence with continuous waves. Deviation from a BIC leads to an intrinsically dissipative quasi-BIC. Here, we propose to attain anti-<i>PT</i> symmetry from a binary quasi-BIC system, where an introduced asymmetry between two quasi-BICs induces an anti-<i>PT</i> phase transition through the EP. Experimentally, we used the thermo-optic effect to accurately control the asymmetry and demonstrated an anti-<i>PT</i> phase transition. We further found that the system can operate at the EP despite fabrication imperfections, evidenced by the slow-light effect with a group index exceeding 40. By harnessing BICs for constructing anti-<i>PT</i> symmetry, our approach opens up a new way for fundamental research and practical applications based on non-Hermitian physics.</p>

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Anti-PT symmetry with bound states in the continuum

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月7日

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  • Ziyao Feng,
  • Long Jin,
  • Xiankai Sun

摘要

Non-Hermitian systems satisfying PT- or anti-PT symmetry have produced many intriguing wave phenomena. Maintaining the Hermitian (non-Hermitian) property and increasing the non-Hermitian (Hermitian) property difference can cause a phase transition from a symmetric phase to a broken phase through the exceptional point (EP). However, it is experimentally difficult to construct a system working at the EP to satisfy both requirements simultaneously. Bound states in the continuum (BICs) are a special lossless wave state despite their coexistence with continuous waves. Deviation from a BIC leads to an intrinsically dissipative quasi-BIC. Here, we propose to attain anti-PT symmetry from a binary quasi-BIC system, where an introduced asymmetry between two quasi-BICs induces an anti-PT phase transition through the EP. Experimentally, we used the thermo-optic effect to accurately control the asymmetry and demonstrated an anti-PT phase transition. We further found that the system can operate at the EP despite fabrication imperfections, evidenced by the slow-light effect with a group index exceeding 40. By harnessing BICs for constructing anti-PT symmetry, our approach opens up a new way for fundamental research and practical applications based on non-Hermitian physics.