  An optical vortex (OV) is a beam with spiral wave front and screw phase
dislocation. This kind of beams is attracting rising interest in various
fields. Here we theoretically proposed and experimentally realized a novel but
easy approach to generate optical vortices. We leverage the inherent
topological vortex structures of polarization around bound states in the
continuum (BIC) in the momentum space of two dimensional periodic structures,
e.g. photonic crystal slabs, to induce Pancharatnam-Berry phases to the beams.
This new class of OV generators operates in the momentum space, meaning that
there is no real-space center of structure. Thus, not only the fabrication but
also the practical alignment would be greatly simplified. Any even order of OV,
which is actually a quasi-non-diffractive high-order quasi-Bessel beam, at any
desired working wavelength could be achieved in principle. The proposed
approach expands the application of bound states in the continuum and
topological photonics.
