Geometric-phase-based phase-knife mask for stellar nulling and coronagraphy

Exoplanets can be detected very close to stars using single-mode cross-aperture nulling interferometry, a photonic technique that relies on the inability of an anti-symmetric stellar point-spread function to couple to the symmetric mode of a single-mode fiber. We prepared an asymmetric field distribution from a laboratory point source using a flat geometric-phase-based pupil-plane phase-knife mask comprised of a planar liquid crystal polymer layer with orthogonal optical axes on opposite sides of a linear pupil bisector. Our mask yielded an on-axis laboratory point-source rejection (i.e., an interferometric "null depth") of 2.2 × 10^-5. Potential mask modifications to better reject starlight are described that incorporate additional phase regions to spatially broaden the rejection area, and additional layers to spectrally broaden the rejection. Also discussed is a topological correspondence between the spatial configurations of separated-aperture nullers, cross-aperture nullers and full-aperture phase coronagraphs.