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Description
The extreme ultraviolet (XUV) frequency comb is an indispensable tool for extending optical frequency metrology into the unexplored wavelength range below 200 nm. With XUV frequency combs, precision spectroscopy for fundamental physics, optical clocks and laser cooling can be extended into the XUV regime for the first time. This includes applications in the spectroscopy of hydrogen-like ions such as highly charged ions, nuclear excitations of Th$^+$, and our planned experiment on the spectroscopy of the He$^+$ 1S-2S transition for testing quantum electrodynamics [1,2].
Historically, the generation of XUV lasers has been a daunting task, requiring either particle accelerators or high harmonic generation (HHG) techniques with peak intensities on the order of $10^{13} - 10^{15}$ W/cm$^2$. Our current approach involves boosting the average power of an infrared frequency comb to 8 kW before HHG converts it into an XUV frequency comb. While effective, this method introduces complexity and requires special measures to protect optical components from the rigors of high-power lasers.
To overcome this challenge, this work explores a novel path: the use of low repetition rate optical frequency combs for frequency conversion [3]. By reducing the repetition rate while increasing the peak power, we could perform HHG at a lower average power, effectively simplifying the laser system. The technique relies on pulse picking using an acousto-optic modulator (AOM) that selects pulses at specific intervals, effectively reducing the repetition rate. The carrier-envelope offset (CEO) frequency can be preserved after pulse picking, as long as certain synchronicity requirements are met [3].
This research focuses on the pulse-picking of an infrared frequency comb generated by a Kerr-lens mode-locked Yb:KYW oscillator with a repetition rate of 40 MHz. Pulse-picking with an AOM, which reduces the repetition rate to 40 kHz, and subsequent amplification with multiple stages of diode-pumped double-pass Yb:LuAG amplifiers allows the pulse energy to be increased to 4.2 $\mu$J, with future plans to further increase the pulse energy to 50 - 100 $\mu$J. Notably, the pulse characteristics of this frequency comb, including 1.3 ps pulse duration and 1.5 nm spectral width (measured by frequency-resolved optical gating), remain consistent with our cavity-enhanced HHG system developed at 40 MHz repetition rate and 8 kW circulating power [1]. The main difference is the significantly reduced repetition rate, which results in a three order of magnitude reduction in average power. The results demonstrate the feasibility of generating XUV frequency combs using HHG with infrared average powers as low as 10 W. The primary limitation is the transition linewidth of the spectroscopic target, which cannot exceed 20 kHz. However, this can be overcome by using two low repetition rate frequency combs and performing XUV dual comb spectroscopy on the target.
This innovative approach offers a tantalizing prospect: XUV frequency combs that do not require high average powers or particle accelerators. It opens new horizons for XUV frequency comb spectroscopy, making it more accessible to researchers across disciplines. The allure of exploring the XUV spectrum without significant barriers is likely to capture the interest of scientists and researchers, fostering exciting opportunities for scientific exploration.
[1] J. Moreno, et. al., Eur. Phys. J. D 77, 67 (2023)
[2] F. Schmid, et. al., APL Photonics 9, 026105 (2024)
[3] F. Canella, et. al., Optica 11, 1-9 (2024)
