Chiral geometric-phase metasurface for Bloch surface wave out-coupling in free space

Vortex beams (VBs) represent an important instance of structured light carrying orbital angular momentum (OAM), which is attracting particular interest from many application fields, from free space optical communications to sensing and imaging. As OAM radiation from a single emitter is concerned, a precise positioning of the source in the center of axis-symmetric, possibly resonating diffractive structures is often required. In addition, efficient free-space outcoupling of light with pure OAM and polarization states remains a difficult task. Here, we propose a dielectric multilayer platform decorated with chiral gold metasurfaces able to provide polarization-selective diffraction of Bloch surface waves sustained by the multilayer. We demonstrate that the generation of well-defined OAM occurs only when Bloch surface waves (BSWs) are coupled (in this case, from an external coherent source). The metasurface chirality is shown to be polarization-selective so that at least 74% is out-coupled in free space with a specific circular polarization, determined by design. In perspective, this work suggests new opportunities for the generation of free-space OAM single photons as an alternative to plasmonic structures recently proposed.