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.

Optical bound states in the continuum (BICs) have recently stimulated a research boom, accompanied by demonstrations of abundant exotic phenomena and applications. With ultrahigh quality (Q) factors, optical BICs have powerful abilities to trap light in optical structures from the continuum of propagation waves in free space. Besides the high Q factors enabled by the confined properties, many hidden topological characteristics were discovered in optical BICs. Especially in periodic structures with well-defined wave vectors, optical BICs were discovered to carry topological charges in momentum space, underlying many unique physical properties. Both high Q factors and topological vortex configurations in momentum space enabled by BICs bring new degrees of freedom to modulate light. BICs have enabled many novel discoveries in light–matter interactions and spin–orbit interactions of light, and BIC applications in lasing and sensing have also been well explored with many advantages. In this paper, we review recent developments of optical BICs in periodic structures, including the physical mechanisms of BICs, explored effects enabled by BICs, and applications of BICs. In the outlook part, we provide a perspective on future developments for BICs.

The technological innovation of thin-film lithium niobate (TFLN) is supplanting the traditional lithium niobate industry and generating a vast array of ultra-compact and low-loss optical waveguide devices, providing an unprecedented prospect for chip-scale integrated optics. Because of its unique strong quadratic nonlinearity, TFLN is widely used to create new coherent light, which significantly promotes all-optical signal processes, especially in terms of speed. Herein, we review recent advances in TFLN, review the thorough optimization strategies of all-optical devices with unique characteristics based on TFLN, and discuss the challenges and perspectives of the developed nonlinear devices.

针对裸光纤光栅在声波作用下的应变灵敏度过低导致难以检测到有效反射波长变化的问题，文章提出以光纤光栅纵向传感为模型，并在光栅周围涂上一层厚度远超光纤内径的低杨氏模量圆柱状涂覆层，以增加光纤光栅的应变灵敏度，并用相移光栅反射谱的线性区作为边缘滤波法边缘来提高激光检测的灵敏度，利用传输矩阵法仿真分析光栅反射谱及激光反射率曲线。仿真结果表明，声波纵向传感模型的应变灵敏度要优于横向传感模型，且低杨氏模量的涂覆层可以显著提高光栅的应变灵敏度，在频率低于1.7 kHz的声波作用下，涂有聚氨酯涂覆层的光纤光栅纵向传感灵敏度达到了6.1 nm/MPa，是裸光纤光栅的381倍，结合相移光栅反射谱线性区综合灵敏度提升2 203.6倍，有效地提升了光纤光栅在声波检测方面的灵敏度。