Silicon metasurfaces for EO imaging spectrometer

The CO2Image mission, projected by the German Aerospace Center (DLR), features an imaging spectrometer with an aperture of F/2, high spatial resolution of 50 m on ground as well as high spectral resolution of 1.3 nm in a range of 1.95 µm to 2.4 µm. The opto-mechanical design utilizes a Three-Mirror-Anastigmat configuration with a plane transmission grating as the core wavelength dispersing element. The grating has a footprint of approximately 9 inches, and stringent requirements for wave-front error (WFE) and optical throughput of larger than 70% throughout the entire spectral bandwidth. Here, we explore the adaptation of dielectric optical metasurfaces to develop and manufacture a transmission grating that meets the required optical performance while being compatible with binary lithographic methods such as electron beam lithography and deep reactive ion etc -medium gratings, however it relies on a rather unusual material system, i.e. amorphous silicon on glass. Due to silicon's high refractive index, the necessary depth of the microstructure is significantly reduced, but this also tightens manufacturing tolerances. Our latest grating design features an arrangement of 10 silicon pillars that gradually increase in size within one grating period, achieving overall diffraction efficiencies of 75%, peaking at 90%. Eventually, we will present test results from our first manufactured small-sized development samples. Although the theoretical optical performance limits could not be achieved, we present a systematic analysis for the reasons and discuss the way and required efforts onto the way for full-size flight model elements.