Frequency comb metrology at PHz frequencies: Precision in the extreme ultraviolet

The capability of frequency-comb (FC) lasers to precisely measure optical frequencies is extended to the multiple-PHz domain. This frequency region, which covers the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), was previously not accessible to these devices. Frequency comb generation is shown for 51-85 nm by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. Moreover, Ramsey-like signals with up to 61% contrast are observed when the XUV comb is scanned over transitions in argon, neon and helium, resulting in an 8-fold improved determination of the ground state ionization energy of helium.