Lithographic Manufacturing of Zeroth Order Gratings for Innovative Achromatic IR Phase Shifters - Final Report

Direct detection and characterization of faint sources around bright astrophysical objects is very difficult due to the large flux ratio between them. For example, an Earth-like exoplanet is typically about 1010 times fainter than its host star in the visible spectrum, reducing to about 107 in the thermal infrared. Infrared nulling interferometry proposed by Bracewell, in 1978, appears to be one of the most promising techniques to achieve the high angular resolution and high dynamic range required to allow the ambitious detection of the first exobiological tracers on
extrasolar Earth-like planets, if they exist. The nulling interferometry technique consists in adjusting the phases of the beams coming from various telescopes (two in the most simple configuration) to produce a fully destructive interference on the optical axis. The quality of the destructive interference or the so-called null depth (N) relies on the optical component ability to induce a very precise phase shift (e.g. π radians for a two-telescope configuration) and a very low amplitude mismatch over the considered wavelength range. Unfortunately, searching for biomarkers in exoplanet atmospheres requires spectroscopic characterization over large spectral bands. For example, the Darwin infrared space interferometer considered by ESA will operate in a wavelength band between 6 and 18 μm, seeking biosignatures like the O3-H2O-CO2 triplet. The huge flux ratio between the parent star and the planet therefore requires unprecedented highperformance broadband achromatic phase shifters (APS). The performance requirement for the APS components directly comes from the 10-7 planet-star contrast but also from the system architecture and the associate amount of residual stellar leakage.