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A frequency comb in the extreme ultraviolet


Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency ‘combs’ (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies1,2 and enabled optical atomic clocks3. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses4, and have been used in a recently demonstrated ‘oscilloscope’ for light waves5. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments6. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.

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Figure 1: The XUV laser set-up.
Figure 2: Normalized spectrum of the pump laser (black) and the circulating pulse in the enhancement resonator (red) when locked.
Figure 3: Harmonic spectrum obtained with the resonator locked and the xenon jet on.
Figure 4: Frequency comb coherence.
Figure 5: Generated power versus the fundamental intensity I for three spectral features.


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We thank A. Apolonski and M. Yu. Ivanov for discussions, and E. Seres and J. Seres for lending us the XUV monochromator.

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Correspondence to Christoph Gohle.

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Gohle, C., Udem, T., Herrmann, M. et al. A frequency comb in the extreme ultraviolet. Nature 436, 234–237 (2005).

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