Chiral Phenomena Related to Bound States in Continuum in Photonics (Invited)

Significance The light-matter interaction at the nanoscale is crucial for the development of miniaturized optoelectronic devices. These devices often encounter energy leakage and loss, stimulating researchers to explore non-Hermitian photonics. An exceptional point, a specific optical degenerate state with identical momentum and energy has emerged as a research hotspot in this field. In recent years, the research on physical mechanisms of phenomena like parity-time symmetry, geometric phase, asymmetric scattering, and bound states in the continuum (BICs) has all revolved around exceptional points. These unique physical mechanisms are expected to inject new energy into the advancement of fields such as quantum computing, advanced materials, and low-power optoelectronic devices. We primarily focus on the chiral phenomena associated with the BIC mode, a concept originating from quantum mechanics and first proposed by von Neumann and Wigner in 1929. They identified a unique solution to the Schrodinger equation: a spatially localized electronic state with zero linewidth and positive energy, despite existing within the continuum spectrum of the radiation. Theoretically, BICs are non- radiating solutions to the wave equation and can manifest in various systems such as acoustics and fluids. However, it was not until 2008 that this concept was introduced into optics by Borisov et al. Subsequently, Plotnik et al. utilized a single-mode optical waveguide array to achieve an initial experimental observation of BICs. In 2013, researchers from MIT detected optical BICs in periodic photonic crystal slabs, boosting further exploration of BIC modes in planar artificial nanostructures. Optical BICs not only squeeze light fields and enhance resonance Q-factors in real space but also exhibit diverse polarization topological properties in momentum space. By adjusting the interaction between BIC modes, individuals can precisely manipulate the distribution, polarization, and emission of the light fields. Over the past decade, owing to the easily fabricated metasurface platform with numerous degrees of freedom, optical BICs have rapidly evolved as a novel approach for controlling light fields in nanophotonics. Additionally, artificial nanostructures can offer chiroptical responses surpassing those of natural materials, with the involved intricate physical mechanisms catching significant attention. The momentum space characteristics of optical BICs provide fresh theoretical insights and design strategies for enhancing the chiroptical response of chiral metasurfaces. The ideal BIC mode is completely decoupled from the free space. By breaking the symmetry of the system, the topology charge of the ideal BIC in momentum space splits into two circularly polarized states, which enables precise control of the radiation process to maximize the chiroptical response. In approximately 2020, research teams from the Russian Academy of Science and the City University of New York independently verified that high-Q quasi-BIC resonances can manipulate the wavefront of circularly polarized light and optimize the chiroptical response of metasurfaces. Both studies utilized periodic chiral metasurfaces with dual tuning parameters, and it was easy to break the symmetry within and out of the structural plane by simultaneously controlling the two parameters. This transformation converted the ideal BIC (with an infinite Q-factor) into chiral quasi-BIC. This highlights that the often- disregarded longitudinal dimension of metasurfaces, particularly symmetry, plays a crucial role in their interaction with circularly polarized light. However, the experimental validation was hindered until 2022 due to constraints in fabricating multi- layer metasurfaces. Our team overcame this obstacle by employing a tilted etching scheme to break the out-of- plane symmetry and observe chiral quasi-BIC in the visible spectrum. Over the last decade, BICs have been identified in different photonic structures, particularly in the metasurfaces platform, which leads to numerous fascinating phenomena. By thoroughly investigating the properties of BICs in both real and momentum spaces, it is possible to reveal clearer physical mechanisms behind various intricate chiroptical phenomena. Progress The concept of BIC has been around for almost a century, with well- established basic theories and various property studies. We begin by briefly outlining the concept and characteristics of BIC (Fig. 1), and then discuss the topic of chiral quasi-BIC (Fig. 2). Subsequently, we explore the applications of chiral BIC and other chiroptical phenomena related to BIC. In Fig. 3, we summarize the methods for creating chiral BIC by breaking the structural symmetry. Figure. 4 illustrates instances of chiral BIC resulting from the disruption of the individual dimension symmetry of nanostructures (including the breaking of in-plane or out-of-plane symmetry), while Fig. 5 presents research on intrinsic chirality induced by slant-perturbation metasurfaces that completely break both in-plane and out-of-plane symmetries. In addition to the tilted etching method, out-of-plane symmetry can also be disrupted by grayscale electron beam lithography and multi- step nanofabrication methods (Fig. 6). We have included Table 1 to compare the specific features of chiral BIC nanodevices currently yielded in the laboratory. Furthermore, we present examples of other chiroptical phenomena related to BICs in Figs. 7. 9, corresponding to nonlinear circular dichroism, vortex beam generation, superchiral field enhancement, and the optical spin Hall effect, respectively. Finally, we address the current challenges and potential applications in this field. Conclusions and Prospects In photonics, BIC initially caught attention for its exceptional high- Q resonance and later was extensively studied due to its unique momentum space polarization characteristics. The high-Q BIC can significantly enhance the performance of applications that rely on strong light-matter interaction, with lowered laser thresholds and improved nonlinear conversion efficiency. Meanwhile, the polarization characteristics of BICs in momentum space have greatly expanded the application fields, thus achieving polarization conversion and enhancing chiral light-matter interaction. Furthermore, more attention is paid to exploring new applications and mechanisms of BIC in combination with novel materials or special photonic structural systems. Although research on BICs in nanostructures has rapidly developed from theory to experimental stages, it still faces many challenges. In terms of sample design, there is an urgent need to explore rapid design schemes using artificial intelligence or inverse design methods. In sample fabrication, tasks such as improving the fabrication precision, implementing double- layer metasurfaces, and incorporating active semiconductor materials are very difficult. In terms of sample characterization, the extreme high-Q resonances make it difficult to measure the physical properties of BICs. Generally, although the biggest challenge in this field is from sample fabrication, combining sophisticated fabrication steps with reasonable sample design can accelerate the development of BIC- assisted photonic devices.