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Direct frequency comb spectroscopy in the extreme ultraviolet


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 nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb5 to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon 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.

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Figure 1: Intracavity high-harmonic generation.
Figure 2: Power scaling results.
Figure 3: Atomic fluorescence signal.
Figure 4: Absolute frequency determination.


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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).

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Authors and Affiliations



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.

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Correspondence to Arman Cingöz or Jun Ye.

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The authors declare no competing financial interests.

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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)

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Cingöz, A., Yost, D., Allison, T. et al. Direct frequency comb spectroscopy in the extreme ultraviolet. Nature 482, 68–71 (2012).

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