Extreme Ultraviolet Frequency Comb Metrology

TLDR

Frequency‑comb lasers achieve remarkable precision, now extended to the extreme ultraviolet (XUV) region (<100 nm) which was previously inaccessible. An XUV frequency comb near 51 nm is produced by amplifying and coherently up‑converting two pulses from a near‑infrared femtosecond comb laser. Phase coherence of the XUV comb is confirmed by observing high‑contrast Ramsey‑like fringes in helium, enabling extraction of the absolute transition frequency and yielding a (4)He ionization energy of h×5 945 204 212(6) MHz, improving accuracy by nearly an order of magnitude and challenging QED calculations.

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.