Zhou Chaobiao, Pu Tianyao, Huang Jing, Fan Menghui, Huang Lujun
College of Mechanical and Electronic Engineering, Guizhou Minzu University, Guiyang 550025, China.
School of Engineering and Information Technology, University of New South Wales, Canberra 2602, Australia.
Nanomaterials (Basel). 2021 Dec 25;12(1):54. doi: 10.3390/nano12010054.
Bound states in the continuum (BICs) correspond to a particular leaky mode with an infinitely large quality-factor (Q-factor) located within the continuum spectrum. To date, most of the research work reported focuses on the BIC-enhanced light matter interaction due to its extreme near-field confinement. Little attention has been paid to the scattering properties of the BIC mode. In this work, we numerically study the far-field radiation manipulation of BICs by exploring multipole interference. By simply breaking the symmetry of the silicon metasurface, an ideal BIC is converted to a quasi-BIC with a finite Q-factor, which is manifested by the Fano resonance in the transmission spectrum. We found that both the intensity and directionality of the far-field radiation pattern can not only be tuned by the asymmetric parameters but can also experience huge changes around the resonance. Even for the same structure, two quasi-BICs show a different radiation pattern evolution when the asymmetric structure parameter increases. It can be found that far-field radiation from one BIC evolves from electric-quadrupole-dominant radiation to toroidal-dipole-dominant radiation, whereas the other one shows electric-dipole-like radiation due to the interference of the magnetic dipole and electric quadrupole with the increasing asymmetric parameters. The result may find applications in high-directionality nonlinear optical devices and semiconductor lasers by using a quasi-BIC-based metasurface.
连续谱中的束缚态（BICs）对应于位于连续谱内具有无限大品质因数（Q 因子）的特定泄漏模式。迄今为止，大多数报道的研究工作都集中在 BIC 增强的光与物质相互作用上，因为其具有极强的近场限制。而对 BIC 模式的散射特性关注较少。在这项工作中，我们通过探索多极干涉，对 BICs 的远场辐射操控进行了数值研究。通过简单地打破硅超表面的对称性，一个理想的 BIC 被转换为具有有限 Q 因子的准 BIC，这在透射谱中表现为法诺共振。我们发现，远场辐射图案的强度和方向性不仅可以通过不对称参数进行调节，而且在共振附近会经历巨大变化。即使对于相同的结构，当不对称结构参数增加时，两个准 BIC 也会呈现出不同的辐射图案演化。可以发现，一个 BIC 的远场辐射从以电四极子为主的辐射演变为以环形偶极子为主的辐射，而另一个由于磁偶极子和电四极子的干涉，随着不对称参数的增加呈现出类似电偶极子的辐射。该结果可能在基于准 BIC 的超表面的高方向性非线性光学器件和半导体激光器中找到应用。
