Extreme ultraviolet frequency comb metrology

TL;DR

This paper demonstrates the transfer of frequency comb precision to the extreme ultraviolet region near 51 nm, enabling highly accurate measurements of helium ionization energy and challenging quantum electrodynamics calculations.

Contribution

The authors generated a coherent XUV frequency comb from a near-infrared laser and used it to measure helium ionization energy with unprecedented accuracy.

Findings

Achieved a 4He ionization energy measurement with nearly an order of magnitude improved accuracy.

Demonstrated phase coherence of XUV frequency comb using helium as a phase detector.

Observed stable Ramsey-like fringes indicating high coherence in the XUV frequency comb.

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 accessable to these devices. A frequency comb at XUV wavelengths near 51 nm is generated by amplification and coherent upconversion 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 4He ionization energy at h 5945204212(6) MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system.