Bound states in the continuum (BICs) enable perfect wave localization and significantly enhance light–matter interactions although systems are optically open. Those trapped modes without the leaky-wave radiation in an open continuum are important in numerous applications, including optical nonlinearity, light emitters, and nano-sensors.
BICs in common reciprocal media are bounded by the time-reversal symmetry, rendering difficulties to manipulate the freedom related to leaky directions. For example, an emitter in dielectric cores must radiate symmetrically in both forward and backward directions, due to the open channels coupled to the radiation continuum (Fig. 1a). Thus, asymmetric leaky mode and light harvesting are not available in those configurations. Breaking the reciprocity and developing the unidirectional BICs would be of great significance to allow more exotic light-matter interactions.
To address this problem, the research group led by Prof. Yong-Zhe Zhang from the Beijing University of Technology, cooperating with Prof. Cheng-Wei Qiu from the National University of Singapore, proposed an approach to use the emerging low-dimensional quantum materials of magnetic Weyl semimetal (MWS), which is naturally endowed with a magneto-optical plasmonic response. By covering the dielectric core by paired MWS with antiparallel magnetism, the unidirectional and nonreciprocal BIC could be formed, where the radiation of leaky modes from a dielectric core could only be supported in one direction.
The relevant research results are published in Photonics Research, Volume. 10, Issue 8, 2022 (Chen Zhao, Guangwei Hu, Yang Chen, Qing Zhang, Yongzhe Zhang and Cheng-Wei Qiu. Unidirectional bound states in the continuum in Weyl semimetal nanostructures[J]. Photonics Research, 2022, 10 (8): 1828-1838).
Those results are theoretically developed, suggesting that the interference of the Fabry-Perot modes in the system and magnetic epsilon-near-zero (ENZ) resonance of MWS would close the leaky channel despite it is within the radiation continuum, i.e. Re (β/k0) ≤ 1. Such important result depends deeply on infrared magneto-plasmonic response of MWS, as, for example, perfect destructive interference only holds true for forward leaky mode at ~3 μm but not valid for backward ones.
Therefore, a perfect unidirectional BIC could exist (Fig. 1c) with diverging quality factor (Fig. 1d). Moreover, a symmetry-protected BIC at Γ point in such nonreciprocal system is achieved as the system is under (rotation of π about y axis followed by time reversal) and symmetry (up–down mirror symmetry of the x–z plane), as shown in Fig. 1b.
Fig. 1 (a), the usual setup with symmetric radiation of leaky mode. (b), the unidirectional leaky wave enabled by unidirectional BIC in antiparallel-magnetism configuration. (c), the reflection map with incident wave. Here, kx is the x component of incident wave vector. (d), the quality factor of leaky mode in forward directions and at Γ point. Here, quality factor is defined as the ratio of real and imaginary part of eigenvector
Due to the flexibility of magneto-optical properties of MWSs via varying the Fermi level, the propagation constant and resonant frequency of unidirectional BICs can be dynamically tuned, offering a pathway of tunable BICs for practical applications, spanning a broad frequency range, such as leaky wave antenna, efficient energy delivering and nonreciprocal energy harvesting. This technology could largely impact the infrared technologies as it allows both new fundamental optical physics for wave control and the extreme light-matter interactions.
Future work can further study the evolution and topological nature of BIC in nonreciprocal system. It may also be possible to design the asymmetric structures to further leverage the symmetry breaking for more exotic nonreciprocal devices.