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Limits to XUV frequency combs?

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By measuring the tiny phase fluctuations between consecutive pulses from an infrared laser frequency comb we pose stringent limits to the extension of its spectroscopic and metrological applications towards the extreme ultraviolet. Just appeared in PRL.

The invention of laser frequency combs, whose spectrum consists of narrow and precisely equidistant lines, has revolutionized the field of optical spectroscopy. High-order harmonic up-conversion of infrared combs promises to extend a similar revolution to the extreme ultraviolet, with fundamental tests of quantum electrodynamics and the development of novel ‘nuclear’ clocks.

However, while fast feedback loops allow infrared combs to exhibit ultranarrow spectral ‘teeth’ by correcting the phase drifts accumulating over many successive laser pulses, the same is not possible in the extreme ultraviolet. In this paper, we show that the pulse-to-pulse phase jitter of the driving infrared laser and the impossibility of controlling this phase noise on sufficiently short timescales are responsible for the dramatic difference (more than nine orders of magnitude) in the frequency resolution between state-of-the-art frequency combs in the infrared and extreme ultraviolet regions.