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.

中文翻译：

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动量空间连续体中精确设计的束缚态的观察

人工光子结构中的连续体束缚态（BIC）受到了相当多的关注，因为它们为极端场定位和增强光与物质相互作用提供了独特的方法。通常，受对称性保护的 BIC 位于高对称点，而通过调整参数实现的偶然 BIC 的位置将出现在动量空间中的某些点。到目前为止，准确设计偶然BIC在动量空间中的位置仍然是一个挑战。在这里，我们从理论上和实验上证明了在由双层光栅组成的双耦合振荡器系统中精确设计的偶然 BIC，其中每个光栅的光学响应可以通过单个谐振器模型来描述。通过改变光栅之间的层间距离来调节与波矢量相关的传播相移，可以在动量空间中任意控制偶然BIC的位置。此外，我们提出了一种通用方法和严格的数值分析，用于提取偏振矢量场，以从偏振分辨透射光谱中观察 BIC 的拓扑特性。最后，演示了准 BIC 辅助的高效二次谐波发生的应用。我们的工作为操纵 BIC 和研究它们在动量空间中的拓扑特性提供了一种简单的策略。