  Vortex coronagraphs are an attractive solution for imaging exoplanets with
future space telescopes due to their relatively high throughput, large spectral
bandwidth, and low sensitivity to low-order aberrations compared to other
coronagraphs with similar inner working angles. Most of the vortex coronagraph
mask development for space applications has focused on generating a
polychromatic, vectorial, optical vortex using multiple layers of liquid
crystal polymers. While this approach has been the most successful thus far,
current fabrication processes achieve retardance errors of 0.1-1.0$^\circ$,
which causes a nonnegligible fraction of the starlight to leak through the
coronagraph. Circular polarizers are typically used to reject the stellar
leakage reducing the throughput by a factor of two. Vector vortex masks also
complicate wavefront control because they imprint conjugated phase ramps on the
orthogonal circular polarization components, which may need to be split in
order to properly sense and suppress the starlight. Scalar vortex masks can
potentially circumvent these limitations by applying the same phase shift to
all incident light regardless of the polarization state and thus have the
potential to significantly improve the performance of vortex coronagraphs. We
present scalar vortex coronagraph designs that make use of focal plane masks
with multiple layers of dielectrics that (a) produce phase patterns that are
relatively friendly to standard manufacturing processes and (b) achieve
sufficient broadband starlight suppression, in theory, for imaging Earth-like
planets with future space telescopes.
