  We propose a method to monochromatize multiple orders of high harmonics by
using a proper designed multilayer mirror. Multilayer mirrors designed by our
concept realize the perfect extraction of a single-order harmonic from
multiple-order harmonic beam, and exhibit broadband tenability and high
reflectivity in the soft-x-ray region. Furthermore, the proposed
monochromatization method can preserve the femtosecond to attosecond pulse
duration for the reflected beam. This device is very useful for ultrafast soft
x-ray experiments that require high-order harmonic beams, such as
femtosecond/attosecond, time-resolved, pump-probe spectroscopy.
