Experimental observation of a polarization vortex at an optical bound state in the continuum

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 protected(3). 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.