Phase-matched extreme-ultraviolet frequency-comb generation


Laser-driven high-order harmonic generation1,2 provides spatially3 and temporally4 coherent tabletop sources of broadband extreme-ultraviolet (XUV) light. These sources typically operate at low repetition rates, frep 100 kHz, where phase-matched HHG is readily achieved5,6. However, many applications demand the improved counting statistics or frequency-comb precision afforded by high repetition rates, frep > 10 MHz. Unfortunately, at such high frep, phase matching is prevented by steady-state plasma accumulated in the generation volume7,8,9,10,11, strongly limiting the XUV average power. Here, we use high-temperature gas mixtures as the generation medium to increase the gas translational velocity, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at frep = 77 MHz, enabling a record generated power of ~ 2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy12,13 of few-electron atoms and ions for precision tests of fundamental physical laws and constants14,15,16,17,18,19,20.

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Fig. 1: Plasma dynamics in high-repetition-rate high-harmonic generation.
Fig. 2: Phase-matching high-repetition-rate high-harmonic generation.
Fig. 3: Power scaling of high-repetition-rate high-harmonic generation.


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This work was supported by the Air Force Office of Scientific Research grant FA9550-15-1-0111, National Institute of Standards and Technology and the National Science Foundation Physics Frontier Center at JILA (PHY-1734006). C.M.H. was supported by the Swedish Research Council. K.L.C. acknowledges the support of the JILA Visiting Fellows Program.

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All authors contributed to the design, planning and execution of the experiment. G.P., C.M.H., S.B.S., C.B. and J.Y. analysed the data. All authors contributed to the writing of the manuscript.

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Correspondence to Gil Porat or Jun Ye.

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Porat, G., Heyl, C.M., Schoun, S.B. et al. Phase-matched extreme-ultraviolet frequency-comb generation. Nature Photon 12, 387–391 (2018).

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