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Abstract

Optical frequency combs have had a remarkable impact on precision spectroscopy1, 2, 3. 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 electrodynamics4, a more sensitive search for the variability of fundamental constants5, and precision studies of nuclear structure6. Ultraprecise X-ray atomic clocks may also be envisaged7. In this work, an X-ray pulse-shaping method is proposed 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 methods8, 9, 10, preserves the overall width of the optical comb, and may be implemented using currently available X-ray technology11.