image: Figure | Transition from one bound singularity to a two-dimensional exceptional ring. a, Schematic of the transition of one BIC point to an exceptional ring in a designed unit cell. b, c The real and imaginary parts of the calculated eigenmodes in momentum spaces.
Credit: Caihong Zhang et al.
Originating from quantum systems, bound states in the continuum (BICs) and exceptional points (EPs) have recently attracted growing attention in photonics and optics. The rise of metamaterials and metasurfaces enables precise control of electromagnetic waves, providing new possibilities for exploring these singularities. Among them, dielectric metasurfaces stand out due to their low losses and high design flexibility. While BICs and EPs have been extensively studied separately, their interplay remains largely unexplored. In particular, the Friedrich–Wintgen BIC, involving destructive interference between modes, shares features with EPs. Understanding and experimentally realizing the transition from BICs to EPs marks an exciting frontier in non-Hermitian photonics.
In a new paper published in Light: Science & Applications, a research team, from Research Institute of Superconductor Electronics (RISE), Key Laboratory of Optoelectronic Devices and Systems with Extreme Performances of MOE, School of Electronic Science and Engineering, Nanjing University, China, has achieved a major breakthrough in the field of topological physics of non-Hermitian systems, and the relevant results provide key support for application innovation in areas such as terahertz (THz) technology and integrated optics.
The team observed, for the first time, the phenomenon of "transition from a single singularity to a two-dimensional ring" in non-Hermitian systems. By precisely manipulating the incident wavevector angle, they successfully realized the transition from BIC to EP in metasurfaces. This discovery not only verifies the core theory of the interaction between topological singularities but also provides a new perspective for in-depth understanding of the unique physical properties of non-Hermitian systems.
The research also innovatively proposes the use of optical pumping technology to regulate the carrier concentration of silicon materials: by breaking the system's degeneracy, dynamic switching of EP is achieved. Based on this mechanism, the team further developed a terahertz (THz) transmission beam deflector switch, transforming theoretical achievements into functional devices with practical applications.
“The EP derived from BIC has broad application prospects in the field of integrated optics. It can be used to develop key devices such as compact on-chip high-sensitivity sensors and dynamic wavefront controllers, providing a new path for the miniaturization and high-performance development of next-generation optoelectronic equipment.” the scientists forecast.
Journal
Light Science & Applications
Article Title
Photoswitchable exceptional points derived from bound states in the continuum