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. 2020 Feb 14;11:883. doi: 10.1038/s41467-020-14721-2

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — a Energy scheme of helium with relevant levels and interaction pathways. b Real part of the downshifted WP signal, with the delay incremented in 2 fs steps. Temporal overlap region of seed pulses is indicated (gray area). Each data point corresponds to only 640 consecutive FEL shots, no additional data filtering was applied. c Data points taken with delay increment of 500 as (blue) compared to downshifted oscillation frequency of the excited transition (black). For comparison, the gray curve shows the theoretical rapid oscillation, that would be obtained without rotating frame detection (T = 174 as, see also inset). Due to rotating frame detection, we get a frequency downshift by a factor of ≈ 52, transferring the attosecond beats to the femtosecond regime. d Fourier transform (absolute value) of the signal, showing the He 1s → 4p resonance. The upper energy scale shows the downshifted frequency range obtained by rotating frame detection, the lower energy scale shows corresponding absolute energy values.