Extreme-ultraviolet (XUV) sources including high-harmonic generation (HHG), free-electron lasers (FELs), soft-X-ray and laser-driven plasmas are widely used for applications ranging from femtochemistry attosecond science to coherent diffractive imaging EUV (or XUV) lithography. The bandwidth of the XUV light emitted by these reflects process used. Whereas lasers1 seeded FELs2 typically has a re...