Gratings for future spectro-imagers in space and ground-based telescopes: from lithography techniques to metasurfaces

In next generation instrumentation for Universe and Earth Observation, new instrument concepts include often non planar gratings. We propose two new technologies using lithography techniques and metasurfaces, for designing and realizing convex blazed gratings for high throughput spectrographs. Lithographic gratings have been fabricated by lithography, angular Ar ion etching, transfer of the blazed grating from a flat surface onto a convex substrate, and finally etched by RIE into the substrate. Efficiency close to 90% on the 1st diffraction order at 700nm has been obtained and a low straylight level has been measured on the silver coated device. Metasurface gratings have been designed using topology optimization based on finite element modeling of Maxwell’s equations. Single and multi-wavelengths optimization leads to gratings with much higher performance than the classical triangular blazed gratings, on one wavelength and on a large 400-1500nm wavelength range. First gratings with Nb2O5, 200nm thick and 220-70nm wide features (pillars and holes) on a silver layer have been realized. Performances characterization including diffraction efficiency and straylight level are under way.