Variable Groove Depth Gratings and their Applications in Soft X-ray Monochromators

A Variable Groove Depth (VGD) grating is a laminar grating where the depth of the groove profile changes continuously along the grating lines. When illuminated by a narrow beam a few mm wide, they allow to adapt the depth of the groove profile the wavelength in use by simply translating the grating perpendicular to the dispersion plane. We show theoretically and experimentally that by varying the groove depth, either flux or harmonic contamination can be optimized. Low second harmonic contamination in the order of 5% at any energy should be obtained on SOLEIL monochromators equipped with VGD gratings