Bendable grating for monochromatization in the extreme-ultraviolet

Broadband light monochromatization in the extreme ultraviolet requires typically the use of diffraction gratings at grazing incidence, where the grating is rotated and eventually translated to perform the wavelength scanning. Several configurations have been proposed, using plane or concave gratings with uniform or variable line spacing. We propose here a low-cost experimental setup in which a thin plane diffraction grating is bended to an almost cylindrical shape to perform at the same time the spectral selection and the focalization of the selected spectral component. In such a way, the number of optical elements needed to realize the monochromator is reduced to two: the grating and the focusing mirror. A bendable grating has been tested in the 13-50 eV region, very good focal properties have been measured with very low residual aberrations. Furthermore, the shape of the bended surface has been measured using a wavefront sensor in order to confirm the effectiveness of the bending technique. The proposed solution can be used to achieve monochromatization both in large-scale facilities such as FELs or synchrotrons, but also in table-top setups, such as those exploiting high-order harmonic generation, that are often used for the preparatory phase of experiments to be later performed at FEL facilities.