ABSTRACT The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability

to provide a precise and direct link between microwave and optical frequencies1,2. A further advance in frequency comb technology is the generation of frequency combs in the

extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity3,4. Until now, combs produced by this method have lacked sufficient power for

applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely

transition at 63 nanometres, thus confirming the combs’ coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb

spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent

upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending

ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions

in molecules6, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium7,8,9, the development of next-generation ‘nuclear’

clocks10,11,12 and searches for variation of fundamental constants13 using the enhanced sensitivity of highly charged ions14. Access through your institution Buy or subscribe This is a

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FREQUENCY COMBS FOR PRECISION METROLOGY AND ATTOSECOND SCIENCE Article 28 January 2021 NEAR-ULTRAVIOLET PHOTON-COUNTING DUAL-COMB SPECTROSCOPY Article Open access 06 March 2024 NANOSECOND

TIME-RESOLVED DUAL-COMB ABSORPTION SPECTROSCOPY Article 30 October 2023 REFERENCES * Udem, T., Holzwarth, R. & Hänsch, T. W. Optical frequency metrology. _Nature_ 416, 233–237 (2002)

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L. Spectrum and the energy levels of neutral argon, Ar I. _J. Opt. Soc. Am._ 63, 1185–1198 (1973) Article  ADS  CAS  Google Scholar  Download references ACKNOWLEDGEMENTS We thank J. L. Hall

for the use of an iodine-stabilized laser, M. D. Swallows for the assistance with the hydrogen maser frequency transfer, and S. T. Cundiff and A. Foltynowicz for reading a draft of the

manuscript. This research is funded by the DARPA, AFOSR, NIST and NSF. A.C. and T.K.A. are National Research Council postdoctoral fellows. A.R. acknowledges funding from the Alexander von

Humboldt Foundation (Germany). AUTHOR INFORMATION Author notes * Axel Ruehl Present address: Present address: Institute for Lasers, Life and Biophotonics, Vrije Universiteit Amsterdam, De

Boelelaan 1081, 1081HV Amsterdam, The Netherlands., * Arman Cingöz and Dylan C. Yost: These authors contributed equally to this work. AUTHORS AND AFFILIATIONS * Department of Physics, JILA,

National Institute of Standards and Technology and University of Colorado, University of Colorado, Boulder, 80309-0440, Colorado, USA Arman Cingöz, Dylan C. Yost, Thomas K. Allison & Jun

Ye * IMRA America Inc., 1044 Woodridge Avenue, Ann Arbor, 48105, Michigan, USA Axel Ruehl, Martin E. Fermann & Ingmar Hartl Authors * Arman Cingöz View author publications You can also

search for this author inPubMed Google Scholar * Dylan C. Yost View author publications You can also search for this author inPubMed Google Scholar * Thomas K. Allison View author

publications You can also search for this author inPubMed Google Scholar * Axel Ruehl View author publications You can also search for this author inPubMed Google Scholar * Martin E. Fermann

View author publications You can also search for this author inPubMed Google Scholar * Ingmar Hartl View author publications You can also search for this author inPubMed Google Scholar *

Jun Ye View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS A.C., D.C.Y., T.K.A. and J.Y. conceived of, designed and carried out the XUV power

and spectroscopy measurements. A.R., M.E.F. and I.H. designed and built the Yb:fibre laser. All authors discussed the results and worked on the manuscript. CORRESPONDING AUTHORS

Correspondence to Arman Cingöz or Jun Ye. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests. SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION

This file contains Supplementary Figure 1 with legend, Supplementary Text and Data, which includes details on the femtosecond enhancement cavity design as well as the comb tooth number

determination. (PDF 394 kb) POWERPOINT SLIDES POWERPOINT SLIDE FOR FIG. 1 POWERPOINT SLIDE FOR FIG. 2 POWERPOINT SLIDE FOR FIG. 3 POWERPOINT SLIDE FOR FIG. 4 RIGHTS AND PERMISSIONS Reprints

and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Cingöz, A., Yost, D., Allison, T. _et al._ Direct frequency comb spectroscopy in the extreme ultraviolet. _Nature_ 482, 68–71 (2012).

https://doi.org/10.1038/nature10711 Download citation * Received: 09 September 2011 * Accepted: 03 November 2011 * Published: 01 February 2012 * Issue Date: 02 February 2012 * DOI:

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