How to Generate XUV Frequency Combs Without Enhancement Resonators?

Extreme ultraviolet (XUV) frequency combs will revolutionize optical frequency metrology, enabling precision measurements in the wavelength range below 200 nm that was previously inaccessible. With XUV frequency combs, precision spectroscopy in the extreme ultraviolet regime can be performed, such as for highly charged ions, nuclear excitations of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">229</sup>Th, and our planned experiment on the spectroscopy of the He+ 1S-2S transition for testing quantum electrodynamics [1], [2]. Generating these frequency combs at repetition rate exceeding 10 MHz involves cavity-enhanced high harmonic generation (CE-HHG) at peak intensities on the order of 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> – 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> W/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>, which in our current implementation requires complex laser systems with average powers approaching 10 kW.