Development of silicon immersed grating for METIS on E-ELT

Abstract

We have developed the technology to manufacture an immersed grating in silicon for the Mid-infrared E-ELT Imager and Spectrograph, METIS. We show that we can meet the required diffraction-limited performance at a resolution of 100000 for the L and M spectral bands. Compared to a conventional grating, the immersed grating drastically reduces the beam diameter and thereby the size of the spectrometer optics. As diffraction takes place inside the high-index medium, the optical path difference and angular dispersion are boosted proportionally, thereby allowing a smaller grating area and a smaller spectrometer size. The METIS immersed grating is produced on a 150 mm industry standard for wafers and replaces a classical 400 mm echelle. Our approach provides both a feasible path for the production of a grating with high efficiency and low stray light and improves the feasibility of the surrounding spectrometer optics. In this contribution we describe and compare the classical-grating solution for the spectrometer with our novel immersed-grating based design. Furthermore, we discuss the production route for the immersed grating that is based on our long-standing experience for space-based immersed gratings. We use standard techniques from the semiconductor industry to define grating grooves with nanometer accuracy and sub-nanometer roughness. We then use optical manufacturing techniques to combine the wafer and a prism into the final immersed grating. Results of development of the critical technology steps will be discussed.