Bound states in the continuum (BICs) in artificial photonic structures have received considerable attention since they offer unique methods for the extreme field localization and enhancement of light-matter interactions. Usually, the symmetry-protected BICs are located at high symmetric points, while the positions of accidental BICs achieved by tuning the parameters will appear at some points in momentum space. Up to now, to accurately design the position of the accidental BIC in momentum space is still a challenge. Here, we theoretically and experimentally demonstrate an accurately designed accidental BIC in a two-coupled-oscillator system consisting of bilayer gratings, where the optical response of each grating can be described by a single resonator model. By changing the interlayer distance between the gratings to tune the propagation phase shift related to wave vectors, the position of the accidental BIC can be arbitrarily controlled in momentum space. Moreover, we present a general method and rigorous numerical analyses for extracting the polarization vector fields to observe the topological properties of BICs from the polarization-resolved transmission spectra. Finally, an application of the highly efficient second harmonic generation assisted by quasi-BIC is demonstrated. Our work provides a straightforward strategy for manipulating BICs and studying their topological properties in momentum space.

Metasurfaces with spin-selective transmission play an increasingly critical role in realizing optical chiral responses, especially for strong intrinsic chirality, which is limited to complex three-dimensional geometry. In this paper, we propose a planar metasurface capable of generating maximal intrinsic chirality and achieving dual-band spin-selective transmission utilizing dual quasi-bound states in the continuum (quasi-BICs) caused by the structural symmetry breaking. Interestingly, the value of circular dichroism (CD) and the transmittance of two kinds of circular polarization states can be arbitrarily controlled by tuning the asymmetry parameter. Remarkable CD approaching unity with the maximum transmittance up to 0.95 is experimentally achieved in the dual band. Furthermore, assisted by chiral BICs, the application in polarization multiplexed near-field image display is also exhibited. Our work provides a new avenue to flexibly control intrinsic chirality in planar structure and offers an alternative strategy to develop chiral sensing, multiband spin-selective transmission, and high-performance circularly polarized wave detection. The basic principle and design method of our experiments in the microwave regime can be extended to other bands, such as the terahertz and infrared wavelengths.

在玻塑混合定焦安防镜头中塑胶镜片、玻璃镜片、镜框的材料性质差异较大，复杂温度环境下，镜头机械结构热变形与光学消热设计会共同影响像质。为保证镜头复杂温度环境下成像稳定，根据安防镜头?40 ℃~80 ℃的测试温度，对镜头进行光机热一体化分析。在Ansys workbench中建立镜头热结构有限元模型，计算镜头的热弹性变形;使用Zernike多项式拟合镜面的面型变化，将拟合结果导入Zemax中，判断温度载荷对像质的影响。光机热一体化分析结果表明：在极限测试温度载荷下，镜头底座的材料为聚碳酸酯混合20%玻璃纤维时，有效补偿了Zemax中模拟光学系统本身的热离焦量；镜框材料为聚碳酸酯混合30%玻璃纤维时，塑胶镜片所受挤压应变最大为2.36×10^?3 mm；镜框材料为聚碳酸酯混合20%玻璃纤维时，塑胶镜片所受挤压应变最大为0.53×10^?3 mm，第二种镜框材料可以使镜头像质保持稳定。最后通过对镜头高低温法兰焦距测量试验，验证了镜头的温度适应能力和光机热一体化分析的准确性。

二十一世纪以来, 以氮化镓(GaN)和氧化锌(ZnO)为代表的第三代宽禁带(E_g>2.3 eV)半导体材料正成为半导体产业发展的核心支撑材料。由于GaN与ZnO单晶生长难度较大, 成本较高, 常采用外延技术在衬底材料上生长薄膜, 因此寻找理想的衬底材料成为发展的关键。相比于传统的蓝宝石、6H-SiC、GaAs等衬底材料, 铝镁酸钪(ScAlMgO₄)晶体作为一种新型自剥离衬底材料, 因其与GaN、ZnO具有较小的晶格失配(失配率分别为~1.4%和~0.09%)以及合适的热膨胀系数而备受关注。本文从ScAlMgO₄晶体的结构出发, 详细介绍了其独特的三角双锥配位体结构与自然超晶格结构, 这是其热学性质与电学性质的结构基础。此外, ScAlMgO₄晶体沿着c轴的层状结构使其具有自剥离特性, 大大降低了生产成本, 在制备自支撑GaN薄膜方面具有良好的市场应用前景。然而ScAlMgO₄原料合成难度较大, 晶体生长方法单一, 主要为提拉法, 且与日本存在较大的差距, 亟需开发新的高质量、大尺寸ScAlMgO₄晶体的生长方法来打破技术壁垒。

Optical resonators with high quality (Q) factors are paramount for the enhancement of light–matter interactions in engineered photonic structures, but their performance always suffers from the scattering loss caused by fabrication imperfections. Merging bound states in the continuum (BICs) provide us with a nontrivial physical mechanism to overcome this challenge, as they can significantly improve the Q factors of quasi-BICs. However, most of the reported merging BICs are found at Γ point (the center of the Brillouin zone), which intensively limits many potential applications based on angular selectivity. To date, studies on manipulating merging BICs at off-Γ point are always accompanied by the breaking of structural symmetry that inevitably increases process difficulty and structural defects to a certain extent. Here, we propose a scheme to construct merging BICs at almost an arbitrary point in momentum space without breaking symmetry. Enabled by the topological features of BICs, we merge four accidental BICs with one symmetry-protected BIC at the Γ point and merge two accidental BICs with opposite topological charges at the off-Γ point only by changing the periodic constant of a photonic crystal slab. Furthermore, the position of off-Γ merging BICs can be flexibly tuned by the periodic constant and height of the structure simultaneously. Interestingly, it is observed that the movement of BICs occurs in a quasi-flatband with ultra-narrow bandwidth. Therefore, merging BICs in a tiny band provide a mechanism to realize more robust ultrahigh-Q resonances that further improve the optical performance, which is limited by wide-angle illuminations. Finally, as an example of application, effective angle-insensitive second-harmonic generation assisted by different quasi-BICs is numerically demonstrated. Our findings demonstrate momentum-steerable merging BICs in a quasi-flatband, which may expand the application of BICs to the enhancement of frequency-sensitive light–matter interaction with angular selectivity.

Topological systems containing near-field or far-field couplings between unit cells have been widely investigated in quantum and classic systems. Their band structures are well explained with theories based on tight-binding or multiple scattering formalism. However, characteristics of the topology of the bulk bands based on the joint modulation of near-field and far-field couplings are rarely studied. Such hybrid systems are hardly realized in real systems and cannot be described by neither tight-binding nor multiple scattering theories. Here, we propose a hybrid-coupling photonic topological insulator based on a quasi-1D dimerized chain with the coexistence of near-field coupling within the unit cell and far-field coupling among all sites. Both theoretical and experimental results show that topological transition is realized by introducing near-field coupling for given far-field coupling conditions. In addition to closing and reopening the bandgap, the change in near-field coupling modulates the effective mass of photonics in the upper band from positive to negative, leading to an indirect bandgap, which cannot be achieved in conventional dimerized chains with either far-field or near-field coupling only.