Dielectric metasurfaces play an increasingly important role in enhancing optical nonlinear generations owing to their ability to support strong light-matter interactions based on Mie-type multipolar resonances. Compared to metasurfaces composed of the periodic arrangement of nanoparticles, inverse, so-called, membrane metasurfaces offer unique possibilities for supporting multipolar resonances, while maintaining small unit cell size, large mode volume and high field enhancement for enhancing nonlinear frequency conversion. Here, we theoretically and experimentally investigate the formation of bound states in the continuum (BICs) from silicon dimer-hole membrane metasurfaces. We demonstrate that our BIC-formed resonance features a strong and tailorable electric near-field confinement inside the silicon membrane films. Furthermore, we show that by tuning the gap between the holes, one can open a leaky channel to transform these regular BICs into quasi-BICs, which can be excited directly under normal plane wave incidence. To prove the capabilities of such metasurfaces, we demonstrate the conversion of an infrared image to the visible range, based on the Third-harmonic generation (THG) process with the resonant membrane metasurfaces. Our results suggest a new paradigm for realising efficient nonlinear photonics metadevices and hold promise for extending the applications of nonlinear structuring surfaces to new types of all-optical near-infrared imaging technologies.

We review the physics and some applications of photonic structures designed for the realization of strong nonlinear chiroptical response. We pay much attention to the recent strategy of utilizing different types of optical resonances in metallic and dielectric subwavelength structures and metasurfaces, including surface plasmon resonances, Mie resonances, lattice-guided modes, and bound states in the continuum. We summarize earlier results and discuss more recent developments for achieving large circular dichroism combined with the high efficiency of nonlinear harmonic generation.

Bound states in the continuum (BICs) have exhibited extraordinary properties in photonics for enhanced light-matter interactions that enable appealing applications in nonlinear optics, biosensors, and ultrafast optical switches. The most common strategy to apply BICs in a metasurface is by breaking symmetry of resonators in the uniform array that leaks the otherwise uncoupled mode to free space and exhibits an inverse quadratic relationship between quality factor (Q) and asymmetry. Here, we propose a scheme to further reduce scattering losses and improve the robustness of symmetry-protected BICs by decreasing the radiation density with a hybrid BIC lattice. We observe a significant increase of radiative Q in the hybrid lattice compared to the uniform lattice with a factor larger than 14.6. In the hybrid BIC lattice, modes are transferred to Г point inherited from high symmetric X, Y, and M points in the Brillouin zone that reveal as multiple Fano resonances in the far field and would find applications in hyperspectral sensing. This work initiates a novel and generalized path toward reducing scattering losses and improving the robustness of BICs in terms of lattice engineering that would release the rigid requirements of fabrication accuracy and benefit applications of photonics and optoelectronic devices.

基于连续域内束缚态（BIC）的超表面具备在时间和空间上实现高效光场调控的能力。在时间维度上，BIC理论上可实现无穷长的辐射光子寿命，因此在激光谐振腔、非线性光学、传感等领域展现了极好的应用价值。为深入理解BIC及衍生谐振特性，基于双开口金属谐振器构成的杂化BIC超表面，探讨由于能带折叠引起的折叠谐振与准BIC谐振之间的特性。利用数值模拟、多极子分析和太赫兹光谱实验验证环形偶极子在这两类谐振辐射特性中的重要作用。此外，通过对比两种不同能带折叠方式的超晶格，进一步确认环形偶极子对折叠谐振品质因数的调制特性。深入理解谐振中多极子的散射特性对获取高品质因数谐振腔具有重要作用，为切实推动超表面在太赫兹传感、调制器和非线性相互作用等领域的应用奠定理论基础。

连续域束缚态（BIC）是发生在辐射连续域频率范围内且被完全局域的共振，具有无限大的Q值，光与物质之间产生强相互作用，该现象对新型功能器件的发展至关重要。在太赫兹超表面中引入BIC机制，为定制高Q值共振提供了新的思路。从BIC的分类、形成机制、基本性质等方面对BIC进行了简要描述，重点介绍了BIC在THz超表面中的新应用，如高灵敏度传感、手性增强、光谱编码、近场成像。此外，BIC携带拓扑电荷，由偏振矢量缠绕圈数定义，这样的电荷只能通过改变系统参数来产生或湮灭。BIC的拓扑性质也为拓扑光子学新现象的发现提供了新的可能，BIC现象的研究可以为光学和光子学领域带来更多的发展。

Photonic bound states in the continuum (BICs) are spatially localized modes with infinitely long lifetimes, which exist within a radiation continuum at discrete energy levels. These states have been explored in various systems, including photonic and phononic crystal slabs, metasurfaces, waveguides, and integrated circuits. Robustness and availability of the BICs are important aspects for fully taming the BICs toward practical applications. Here, we propose a generic mechanism to realize BICs that exist by first principles free of fine parameter tuning based on non-Maxwellian double-net metamaterials (DNMs). An ideal warm hydrodynamic double plasma (HDP) fluid model provides a homogenized description of DNMs and explains the robustness of the BICs found herein. In the HDP model, these are standing wave formations made of electron acoustic waves (EAWs), which are pure charge oscillations with vanishing electromagnetic fields. EAW BICs have various advantages, such as (i) frequency-comb-like collection of BICs free from normal resonances; (ii) robustness to symmetry-breaking perturbations and formation of quasi-BICs with an ultrahigh Q-factor even if subject to disorder; and (iii) giving rise to subwavelength microcavity resonators hosting quasi-BIC modes with an ultrahigh Q-factor.

连续域束缚态（BIC）是一种特殊本征态，它与扩展态共存却具有强烈的局域性，不向自由空间辐射能量。自1929年被发现以来，BIC的相关理论与实验都取得了长足发展，成为当前的一个热门研究方向。在BIC研究的众多方向中，光子学是BIC研究中的一个重要平台，在非线性光学、传感与滤波、波导与通信等方向都有应用。本文回顾了BIC的研究历史并系统地介绍了光子学领域中BIC的分类及产生机制；归纳了几种常用的理论分析方法，并讨论了光子学BIC现有的应用以及未来展望。

为了提高品质因子（Quality value，Q）以增强光与物质的耦合作用，本文提出一种结构简单、工艺制备要求低的介质超材料，它可激发对称保护的连续介质束缚态（bound states in the continuum , BICs）。该介质超材料具有四聚孔组成的平面纳米孔板，通过改变纳米孔的位置，可使对称保护BIC转变为对称保护的QBIC，进而诱导出两个高品质因子Q值Fano共振。经计算Fano共振在非对称参数Δ=3 nm时，Q值可达到1×e⁶。随后将QBIC和Fano共振的远场辐射分解为不同多极子分量的贡献，基于散射功率和电场矢量分布可以发现，介质超材料在λ₁出现高Q值Fano共振主要是因为磁四极子和环偶极子的存在，而在λ₂出现高Q值Fano共振主要是因为环偶极子的存在。最后分析计算了纳米孔边长和纳米孔填充材料对两个Fano共振的影响。本文的研究可以为研究制备高Q值光学响应器件提供理论指导。

Although previously reported terahertz absorbers can achieve high-sensitivity refractive index sensing, the resonant peak is too broad, which leads to a low figure of merit (FOM). Transmissive sensors based on bound states in the continuum (BIC) can achieve high FOM, but they have some limitations in high sensitivity. Herein, we propose a periodic triple parallel metal bars structure to obtain high quality, a strong field, and multiple hot spots by the Friedrich–Wintgen BIC. Numerical results show the sensitivity and FOM can reach 1877 GHz/RIU and 665, respectively. Compared to the previously reported transmissive sensors based on BIC, the sensitivity has been greatly improved.

The realization of high-Q resonances in a silicon metasurface with various broken-symmetry blocks is reported. Theoretical analysis reveals that the sharp resonances in the metasurfaces originate from symmetry-protected bound in the continuum (BIC) and the magnetic dipole dominates these peculiar states. A smaller size of the defect in the broken-symmetry block gives rise to the resonance with a larger Q factor. Importantly, this relationship can be tuned by changing the structural parameter, resulting from the modulation of the topological configuration of BICs. Consequently, a Q factor of more than 3,000 can be easily achieved by optimizing dimensions of the nanostructure. At this sharp resonance, the intensity of the third harmonic generation signal in the patterned structure can be 368 times larger than that of the flat silicon film. The proposed strategy and underlying theory can open up new avenues to realize ultrasharp resonances, which may promote the development of the potential meta-devices for nonlinearity, lasing action, and sensing.