Thin-Film Achromatic Phase Shifters for Nulling Interferometry: Design Approach

Nulling interferometry in the thermal IR is the most promising technique for direct detection of Earth-like exoplanets. This technique requires a p phase shifter for the parent star of the planet to be completely extinguished by destructive interference. We investigate how thin films can be used to design p achromatic phase shifters. The design approach that we propose works on reflection and can be carried out by two steps, namely, the design of a mirror and an antireflection structure with no constraint on the phase properties of the thin film stacks. Phase-shift accuracy is derived analytically, and a numerical example illustrates this concept.