On the contribution of gratings with laterally graded groove depths to the design and performances of SOLEIL soft X-ray monochromators

The so called varied groove depth (VGD) etching technology, developed by Horiba-Jobin-Yvon, which consists in laterally grading the groove depth of a lamellar grating, is exploited in the monochromators of the seven soft X-ray beamlines of SOLEIL. With a VGD profile a single grating can perform in more than a decade wide energy band and some control is given over the harmonic content. It also requires a specific, but easily achieved design of the beamline optics to match the lateral footprint of the beam to the depth variation. Performances predicted by computation with electromagnetic propagation code are supported by flux measurements of the two first harmonics in the experimental chamber of a photoemission beamline, and by measurements of grating efficiencies on SOLEIL etrology beamline.