A geometric-phase-based phase-knife mask for stellar nulling

Exoplanets can potentially be detected very close to stars with single-mode cross-aperture nulling interferometry, a photonic nulling technique that relies on the inability of an anti-symmetric stellar point-spread function to couple to a single-mode fiber’s symmetric mode. We prepared an asymmetric field distribution using a flat geometric-phase-based pupil-plane phase-knife mask made of a planar liquid crystal polymer layer with orthogonal optical axes on opposite sides of a pupil bisector. Our mask yielded an on-axis laboratory null depth of 2.2 x 10-5. Mask modifications are described that make use of additional layers and more phase regions to spectrally and spatially broaden the null, thereby promising reduced stellar leakage. Considerations of such modified masks have also led a topological correspondence between separated aperture nullers, cross-aperture nullers and phase coronagraphs.