title: Extreme ultraviolet frequency comb metrology.
abstract: The remarkable precision of frequency-comb (FC) lasers is transferred to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), a frequency region previously not accessible to these devices. A frequency comb at XUV wavelengths near 51 nm is generated by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. The phase coherence of the source in the XUV is demonstrated using helium atoms as a ruler and phase detector. Signals in the form of stable Ramsey-like fringes with high contrast are observed when the FC laser is scanned over P states of helium, from which the absolute transition frequency in the XUV can be extracted. This procedure yields a (4)He ionization energy at h×5 945 204 212(6)  MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system.
authors.affiliation: Institute for Lasers, Life and Biophotonics Amsterdam, VU University, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands.
authors.forename: Dominik Z
authors.initials: DZ
authors.lastname: Kandula
authors.forename: Christoph
authors.initials: C
authors.lastname: Gohle
authors.forename: Tjeerd J
authors.initials: TJ
authors.lastname: Pinkert
authors.forename: Wim
authors.initials: W
authors.lastname: Ubachs
authors.forename: Kjeld S E
authors.initials: KS
authors.lastname: Eikema