XUV frequency comb operation in an astigmatism-compensated enhancement cavity
J. Nauta, J.-H. Oelmann, A. M. Borodin, Alexander Ackermann, Patrick Knauer, I. S. Muhammad, Ronja Pappenberger, Thomas Pfeifer, Crespo López Urrutia, J.
Abstract
We have developed an extreme ultraviolet (XUV) frequency comb for performing ultra-high precision spectroscopy on the many XUV transitions found in highly charged ions (HCI). Femtosecond pulses from a 100 MHz phase-stabilized near-infrared frequency comb are amplified and then fed into a femtosecond enhancement cavity (fsEC) inside an ultra-high vacuum chamber. The low-dispersion fsEC coherently superposes several hundred incident pulses and, with a single cylindrical optical element, fully compensates astigmatism at the w0 = 15 µm waist cavity focus. With a gas jet installed there, intensities reaching ∼ 10<sup>14</sup> W/cm<sup>2</sup> generate coherent high harmonics with a comb spectrum at 100 MHz rate. We couple out of the fsEC harmonics from the 7th up to the 35th (42 eV; 30 nm) to be used in upcoming experiments on HCI frequency metrology.