Bound states in the continuum (BICs) are modes that, although energy and momentum conservation allow coupling to far-field radiation, do not show any radiation loss. As such, energy can theoretically be stored in the mode for infinite time. Such states have been shown to exist for e.g. photonic and acoustic waves, and show great promise for applications including lasing, (bio)sensing and filtering. Despite intense research, the mechanism behind these states and their robustness is still poorly understood.
Recently it was proposed theoretically that BICs occur at points where the far-field polarization of the radiated waves shows a vortex, i.e. points where the polarization is undefined . Due to the integer winding number associated to such vortices, the modes should be topologically protected against disorder. In this work, we verify this claim experimentally. We fabricate a SiN grating and use reflection measurements to show that it supports an optical BIC around 700 nm wavelength. We then perform polarimetry measurements in a Fourier reflection microscopy scheme to map the far-field polarization at every angle and wavelength, demonstrating the existence of a vortex at the BIC. We use a simple dipole model to characterize the BIC as a Friedrich-Wintgen type, arising from the interference between two electromagnetic dipoles induced in the grating. Our method can be used to characterize the polarization structure of any leaky photonic mode, including those supporting polarization vortices of arbitrary winding numbers.
 Zhen, B., et al. (2014). Physical review letters, 113(25), 257401.