Ioachim Pupeza and Maximilian Högner from the Field-Resolved Infrared Metrology Research Group have co-authored the Review Article “Extreme-ultraviolet frequency combs for precision metrology and attosecond science” in Nature Photonics, surveying the coming-of-age of cavity-enhanced high-order harmonic generation for precision time- and frequency-domain metrology.

Femtosecond modelocked lasers emitting trains of coherent visible/infrared pulses have steadily advanced our understanding of basic processes in nature. For example, optical clocks employ frequency-comb techniques for the most precise measurement of time, permitting the search for minuscule drifts of natural constants. Furthermore, the generation of optical-field-synchronised extreme-ultraviolet attosecond bursts affords real-time measurements of fundamental electron dynamics. In a passive optical resonator – or enhancement cavity – the pulse train of a visible/infrared modelocked laser can be resonantly enhanced, providing the intensities necessary for high-order harmonic generation in gases at repetition rates of several tens of MHz. Thus, these coherent radiation sources uniquely combine broadband vacuum- and extreme-ultraviolet spectral coverage with pulse repetition rates and coherence properties akin to those of modelocked lasers.

The paper reviews the milestones of the development of this technology, as well as recent applications and prospects, including precision frequency-comb spectroscopy of electronic and potentially nuclear transitions, and low-space-charge attosecond-temporal-resolution photoelectron spectroscopy with nearly 100% temporal detection duty cycle.

Picture: Christian Hackenberger