  Extreme-ultraviolet (XUV) sources including high-harmonic generation,
free-electron lasers, soft x-ray lasers and laser-driven plasmas are widely
used for applications ranging from femtochemistry and attosecond science to
coherent diffractive imaging and EUV lithography. The bandwidth of the XUV
light emitted by these sources reflects the XUV generation process used. While
light from soft-x-ray lasers and XUV FELs typically has a relatively narrow
bandwidth, plasma sources and HHG sources driven by few-cycle laser pulses emit
broadband XUV pulses. Since these characteristic properties of a given XUV
source impose limitations to applications, techniques enabling modification of
the XUV bandwidth are highly desirable. Here we demonstrate a concept for
efficient spectral compression of a broadband XUV pulse via four-wave mixing
(FWM) in the presence of a broadband near-infrared (NIR) pulse in a krypton gas
jet, exploiting a phase-matching scheme based on closely-spaced resonances. Our
concept provides new possibilities for tailoring the spectral bandwidth of XUV
beams.
