Research progress in the phenomenon of exceptional point by passive non-Hermitian metasurfaces
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摘要:
在非厄密系统中，调节系统的增益或损耗可以使系统状态从PT对称向PT对称破缺转变，转变过程中存在一个特殊的状态转变点，使得系统本征值和本征态同时简并，该点称为奇异点。奇异点结合超构表面产生了许多有趣的光学现象：不对称传输、拓扑相位、非厄密趋肤效应等。然而引入增益的有源超构表面在实验上很难实现，因此利用系统损耗构建虚拟增益的无源超构表面成为非厄密研究的有力武器。本文将从无源非厄密超构表面奇异点的理论模型、研究进展、具体应用和实验设计4个方面进行综述，并对该领域未来的发展方向进行展望。
Abstract:In non-Hermitian systems, controlling the gain or loss of the system can enable the system state to transition from PT-symmetry to broken PT-symmetry. This transition leads to a special point known as the exceptional point, where the system eigenvalues and eigenstates become simultaneously degenerate. When combined with metasurfaces, the exceptional point leads to various intriguing optical phenomena, such as asymmetric transmission, exceptional topological phase, and the non-Hermitian skinning effect. However, active metasurfaces introducing gains are difficult to realize experimentally. Therefore, designing passive metasurfaces using equivalent gains through loss becomes a powerful tool in non-Hermitian research. In this paper, we review the theoretical models, research progress, specific applications, and experimental design in the study of the exceptional point on passive non-Hermitian metasurfaces and look forward to the future direction of this field.
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Key words:
- exceptional point /
- non-hermitian /
- metasurfaces /
- passive systems
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图 1 无源非厄密超构表面的光学性质。（a）双原子PT对称超构表面；（b）系统从PT对称到PT对称破缺转变过程中本征态的变化[20]。（c）拓扑相位[23]。（d）复空间中微扰结构与奇异点相离，相切，包围
Figure 1. Optical properties of passive non-hermitian metasurfaces. (a) Biatomic PT symmetric metasurfaces. (b) Changes in eigenstates during the transition of the system from PT symmetry to broken PT symmetry[20]. (c) Exceptional topological phase[23]. (d) Perturbative structures in complex space separated from EP, tangent to EP, and surrounded by EP
图 2 自由空间光传输的非厄密超构表面器件。（a）石墨烯金属谐振环超构表面示意图。（b）奇异点反射率图[28]。（c）超构光栅示意图。（d）偏振光反射率和复系数A的关系[29]。（e）超构薄膜示意图。（f）光吸收[30]。
Figure 2. Passive non-hermitian metasurface devices for free-space optical transmission. (a) Schematic diagram of graphene metal resonant ring metasurface. (b) Exceptional point reflectance map[28]. (c) Metasurface grating. (d) Relationship between polarized light reflectance and complex coefficient A[29]. (e) Metasurface thin films. (f) Light absorption[30].
图 4 基于无源非厄密超构表面的奇异光学现象和器件设计。（a）入射方向敏感的不对称传输[36]。（b）奇异拓扑相位。（c）基于拓扑相位的角度不对称全息[37]。（d）奇异点结构实现角度敏感脉冲传输[39]。（e）彩色矢量全息[43]。
Figure 4. Exceptional optical phenomena and device design based on passive non-Hermitian metasurfaces. (a) Asymmetric transmission sensitive to incidence direction[36]. (b) Exceptional topological phase. (c) Angular asymmetric holography based on exceptional topological phase[37]. (d) Angle-sensitive pulse transmission realized with EP structure[39]. (e) Color vector holographic[43]
图 6 复杂奇异光学现象。（a）气隙中的费米弧[57]。（b）三阶奇异点[60]。（c）周期边界的能带谱。（d）开放边界的能带谱[62]。（e）Floquet拓扑光子晶格模型。（f）趋肤效应[67]
Figure 6. Complex exceptional optical phenomena. (a) Fermi arcs in the air gap[57]. (b) Third-order EP[60]. (c) Energy band spectrum at periodic boundary. (d) Energy band spectrum with open boundary[62]. (e) Floquet topological photonic lattice models. (f) Skin effect[67]
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