Cross-aperture nulling with a phase-knife mask based on geometric phase

Single-mode cross-aperture nulling interferometry can potentially be used to observe exoplanets at smaller distances from their host stars than classical coronagraphy. This approach makes use of the fact that an anti-symmetric input field cannot couple to the symmetric mode of a single-mode fiber. Pupil-plane phase masks can be used to make the input stellar field anti-symmetric, with the simplest such pupil mask being a phase-knife mask that introduces a relative phase shift of  radians across a pupil bisector. A simple means of providing the needed phase pattern is to make use of geometric phase, which can be implemented by means of a liquid-crystal polymer layer with orthogonally oriented optical axes on opposite sides of the pupil bisector. A prototype phase-knife mask of this type has demonstrated a narrowband null depth of 2.2  10-5 in the laboratory, a level that is sufficient for potentially detecting Hot Jupiters with large ground-based telescopes. Somewhat more complex spatial mask patterns as well as multi-layer masks, can be used to further mitigate stellar leakage due to finite stellar diameters, pointing errors and bandwidth. Moreover, as null depths improve to longer wavelengths, near-infrared cross-aperture nulling may be able to serve as a long-wavelength complement to visible wavelength coronagraphy on exoplanet missions such as the Habitable Worlds Observatory.