Development of an XUV Frequency Comb for Precision Spectroscopy of Highly Charged Ions

Highly charged ions (HCI) have a few tightly bound electrons and many interesting properties for probing fundamental physics and developing new frequency standards [1,2]. Many optical transitions of HCI are located in the extreme ultraviolet (XUV) and conventional light sources do not allow to study these transistions with highest precision. For this reason, we are developing an XUV frequency comb by transfering the coherence and stability of a near infrared frequency comb to the XUV by means of high-harmonic generation (HHG) [3–4]. Reaching intensity levels necessary for HHG 1013W/cm2), while operating at high repetition rates (100 MHz) for large comb line spacing, is challenging. Therefore, the laser pulses are first amplified in a rod-type fiber to 70 W and compressed to sub-200 fs in a grating and prism compressor. Afterwards, pulses are resonantly overlapped in an astigmatism-compensated femtosecond enhancement cavity, which is locked to the frequency comb. To achieve high stability and low-noise performance, the cavity is built on a rigid titanium structure with vibrational decoupling from the vacuum pumps. High-harmonics will then be generated in a target gas in the tight focus of the cavity and coupled out of the cavity by minus-first order diffraction from a small-period grating etched into a high-reflective cavity mirror [5]. Mirror degradation due to contamination and hydrocarbon aggregation is prevented by operating the whole cavity under ultra-high vacuum conditions. A differential pumping scheme will enable high target gas pressures in the laser focus without impairing the pressure elsewhere in the chamber [6].