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 wavelength1,2. 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.

Springer Nature
Nature Photon.
Resonant Nanophotonics

Doeleman, H.M, Monticone, F, den Hollander, W, Alù, A, & Koenderink, A.F. (2018). Experimental observation of a polarization vortex at an optical bound state in the continuum. Nature Photon., 12, 397–401. doi:10.1038/s41566-018-0177-5