Diffraction Efficiency of Reflective Metallic Gratings Operating in the THz Range

Far infrared spectrometers have a prominent role to play in future space missions devoted to unveiling the obscure universe. Diffraction gratings are the heart of these instruments that, when operating in the range of a few terahertz, shall cover a wide spectral range providing the highest resolution and efficiency, generally for only a single polarization, within the specified spectral range. This contribution describes the optimization of the diffraction efficiency of the first diffractive order for a blazed metallic grating working at a large angle of incidence. Since the grating period is of the same order of magnitude as the wavelength, it is necessary to approach the design by solving Maxwell's equations rigorously using computational electromagnetism. We have applied the rigorous coupled-wave analysis and the finite element methods to simulate the performance of the grating. This analysis provides relevant hints for the selection of the period, and the blaze and slant angles of the grating. A metallic blazed grating has been fabricated on aluminum and its topography has been characterized to determine the discrepancies with respect to the nominal values. The fabricated profile has been simulated to obtain the expected results in efficiency in comparison with the ideal case. This analysis provides guidance for improved manufacturing processes and experimental verification.