English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

The operation of a frequency comb at extreme ultraviolet (XUV) wavelengthsbased on pair-wise amplification and nonlinear upconversion to the 15thharmonic of pulses from a frequency comb laser in the near-infrared range isreported. Following a first account of the experiment [Kandula et al., Phys.Rev. Lett. 105, 063001 (2010)], an extensive review is given of thedemonstration that the resulting spectrum at 51 nm is fully phase coherent andcan be applied to precision metrology. The pulses are used in a scheme ofdirect-frequency-comb excitation of helium atoms from the ground state to the1s4p and 1s5p 1P_1 states. Laser ionization by auxiliary 1064 nm pulses is usedto detect the excited state population, resulting in a cosine-like signal as afunction of the repetition rate of the frequency comb with a modulationcontrast of up to 55%. Analysis of the visibility of this comb structure yieldsan estimated timing jitter between the two upconverted comb laser pulses of 50attoseconds, whch indicates that extension to even shorter wavelengths shouldbe feasible. The helium metrology investigation results in transitionfrequencies of 5740806993(10) MHz and 5814248672(6) MHz for excitation of the1s4p and 1s5p 1P_1 states, respectively. This constitutes the first absolutefrequency measurement in the XUV, attaining unprecedented accuracy in thiswindowless part of the electromagnetic spectrum. From the measured transitionfrequencies an eight-fold improved 4He ionization energy of 5945204212(6) MHzis derived. Also a new value for the 4He ground state Lamb shift is found of41247(6) MHz. This experimental value is in agreement with recent theoreticalcalculations up to order m\alpha^6 and m^2/(M\alpha^5), but with a six timeshigher precision, therewith providing a stringent test of quantumelectrodynamics in bound two-electron systems.

XUV frequency comb metrology on the ground state of helium