Nanostructured blazed gratings for high performance spectrographs

The state-of-the-art spectroscopy instruments are designed to collect as much light as possible, especially for astronomy applications. In this context, a blazed sub-wavelength disperser (called a metasurface) is a promising alternative to the widely used sawtooth blazed gratings with a metal coating. In order to find the optimal opto-geometric characteristics of such a device, topology optimization based on Finite Element modelling of Maxwell's equations is used. This paper deals with the application of this powerful optimization process, using different materials and introducing the manufacturability constraints. The reflection averaged on the [400,1500]nm wavelength range can reach 80% with a broadband optimization on silica. It is 28% higher than that of the sawtooth blazed grating (which reaches 52%) in absolute terms, and 54% in relative terms. First samples of metasurface gratings have been manufactured.