XUV frequency comb operation in an astigmatism-compensated enhancement cavity

TL;DR

This paper reports the development of an XUV frequency comb using an astigmatism-compensated enhancement cavity, enabling high-precision spectroscopy of highly charged ions with coherent high harmonics at 100 MHz repetition rate.

Contribution

It introduces a novel XUV frequency comb system with astigmatism compensation in an enhancement cavity for high-precision spectroscopy applications.

Findings

Generated coherent high harmonics up to the 35th order (42 eV)

Achieved intensities of ~10^{14} W/cm^2 in the cavity

Coupled out a stable comb spectrum at 100 MHz rate

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 $w_0=15\mu$m waist cavity focus. With a gas jet installed there, intensities reaching $\sim10^{14}$ W/cm$^2$ 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.