Development of diffraction gratings and grisms engraved by photolithography for NIR space applications

THESEUS mission aims to detect and observe transient sources in the X-ray and Gamma-Ray bands, with a follow-up in the infrared band. The primary objectives are to provide real time trigger and accurate location of GRBs and to discover new high-energy transients. THESEUS InfraRed Telescope (IRT) is, with the Soft X-ray Imager (SXI) and the X-Gamma ray Imaging Spectrometer (XGIS), one on the 3 instruments onboard THESEUS satellite. The IRT (0.7-1.8 mu m) is a 0.7 m class IR telescope with 15x15 arcmin FOV, for fast response, with both imaging and spectroscopy capabilities. The most critical component for the spectroscopic mode is the grism. This grism is a complex component with multifunction properties: (i) fixing the line of sight towards the detector; (ii) selecting the correct spectral band with interferential filter; (iii) compensating for aberrations (both geometrical and chromatic) to reach near diffraction-limit image quality; (iiii) dispersing the wavelengths with a resolving power around 400. Such grisms have been developed by our laboratory for the NISP instrument of the EUCLID mission, with industry partnership from Silios Technologies. These grisms, with curved lines, are manufactured by photolithography. In the context of THESEUS IRT grisms, new R&D activities are mandatory to validate the manufacturability of the gratings since they have a groove density 3 times higher than NISP grisms. In this article, we will present the results we obtained on grating prototypes developed during the phase A of the project. Profilometric measurements of the groove profile and diffraction efficiency measurement are analyzed.