Optical bound states in the continuum (BICs) are states supported by a photonic structure that are compatible with free-space radiation, yet become perfectly bound for one specific in-plane momentum and wavelength^1,2. Recently, it was predicted that light radiated by such modes around the BIC momentum–frequency condition should display a vortex in its far-field polarization profile, making the BIC topologically protected³. Here, we study a one-dimensional grating supporting a transverse magnetic mode with a BIC near 700 nm wavelength, verifying the existence of the BIC using reflection measurements, which show a vanishing reflection feature. Using k-space polarimetry, we measure the full polarization state of reflection around the BIC, highlighting the presence of a topological vortex. We use an electromagnetic dipole model to explain the observed BIC through destructive interference between two radiation channels, characteristic of a Friedrich–Wintgen-type BIC⁴. Our findings shed light on the origin of BICs and verify their topological nature.

连续体中的光学束缚态（BIC）是由与自由空间辐射兼容的光子结构所支持的状态，但对于一个特定的面内动量和波长^1,2则完美地束缚。最近，据预测，通过这种模式在BIC动量-频率条件周围辐射的光应在其远场偏振分布中显示出涡旋，从而使BIC在拓扑上受到保护³。在这里，我们研究了一维光栅，该光栅支持在700 nm波长附近具有BIC的横向磁模式，并使用反射测量来验证BIC的存在，该测量显示出消失的反射特征。使用k空间极化法，我们可以测量BIC周围反射的全极化状态，从而突出显示拓扑涡旋的存在。我们使用电磁偶极子模型通过两个辐射通道之间的相消干涉来解释观测到的BIC，这是Friedrich-Wintgen型BIC ^4的特征。我们的发现揭示了BIC的起源，并验证了它们的拓扑性质。