  Optical frequency combs have had a remarkable impact on precision
spectroscopy. Enabling this technology in the x-ray domain is expected to
result in wide-ranging applications, such as stringent tests of astrophysical
models and quantum electrodynamics, a more sensitive search for the variability
of fundamental constants, and precision studies of nuclear structure.
Ultraprecise x-ray atomic clocks may also be envisaged. In this work, an x-ray
pulse-shaping method is put forward to generate a comb in the absorption
spectrum of an ultrashort high-frequency pulse. The method employs an
optical-frequency-comb laser, manipulating the system's dipole response to
imprint a comb on an excited transition with a high photon energy. The
described scheme provides higher comb frequencies and requires lower
optical-comb peak intensities than currently explored methods, preserves the
overall width of the optical comb, and may be implemented by presently
available x-ray technology.
