Thin film approach for π achromatic phase shifters

Direct detection and characterization of Earth-like planets orbiting nearby stars are possible with nulling interferometry and coronagraphy in the thermal infrared. With these techniques, the star is extinguished by interference, with a ratio greater than 10(5) to allow planet detection. Generally, these techniques require a pi phase shifter. In this paper, we investigate how thin film coatings can be used to design pi achromatic phase shifters. The design, which provides a pi phase shift upon reflection, is basically formed with a high reflectance mirror and an antireflection structure that can be designed without any constraint on their own phase properties. The principle of the design is detailed and numerical examples illustrates this concept.