Many atomic and molecular systems of fundamental interest possess resonance frequencies in the extreme ultraviolet where laser technology is limited and radiation sources have traditionally lacked long-term phase coherence. Recent breakthroughs in XUV frequency comb technology have demonstrated spectroscopy with unprecedented resolution at the MHz-level, but even higher resolutions are desired for future applications in precision measurement. By characterizing heterodyne beats between two XUV comb sources, we demonstrate the capability for sub-Hz spectral resolution. This corresponds to coherence times > 1 s at photon energies up to 20 eV, more than 6 orders of magnitude longer than previously reported. This work establishes the ability of creating highly phase stable radiation in the XUV with performance rivaling that of visible light. Further, by direct sampling of the phase of the XUV 1 arXiv:1404.3779v2 [physics.atom-ph]

We demonstrate an efficient 102-MW peak power, 103-W average power, Kerr-lens mode-locked thin-disk laser (TDL) oscillator generating 52-fs pulses at 17.1-MHz repetition rate. The TDL is based on an Yb:YAG disk and operates in the strongly self-phase-modulation (SPM) broadened regime. In this regime, the spectral bandwidth of the oscillating pulse exceeds the available gain bandwidth by generating additional frequency components via SPM in the Kerr medium inside the laser cavity. At an optical-to-optical efficiency of 26%, our oscillator delivers a more than six times higher average power compared to any 50-fs-class laser oscillator. Compared to previous 100-W-class high-power laser oscillators, we reach this performance in a more than two times shorter pulse duration at a comparable optical-to-optical efficiency. Our TDL delivers the highest peak power of any ultrafast laser oscillator. The short pulse duration combined with high average power and peak power makes the presented TDL oscillator an attractive source for high field science and nonlinear optics.

We demonstrate a compact extreme ultraviolet (XUV) source based on high-harmonic generation (HHG) driven directly inside the cavity of a mode-locked thin-disk laser oscillator. The laser is directly diode-pumped at a power of only 51 W and operates at a wavelength of 1034 nm and a 17.35 MHz repetition rate. We drive HHG in a high-pressure xenon gas jet with an intracavity peak intensity of 2.8×10 W/cm and 320 W of intracavity average power. Despite the high-pressure gas jet, the laser operates at high stability. We detect harmonics up to the 17th order (60.8 nm, 20.4 eV) and estimate a flux of 2.6×10 photons/s for the 11th harmonic (94 nm, 13.2 eV). Due to the power scalability of the thin-disk concept, this class of compact XUV sources has the potential to become a versatile tool for areas such as attosecond science, XUV spectroscopy, and high-resolution imaging.

We demonstrate the first Kerr lens mode-locked Yb:CaGdAlO (Yb:CALGO) thin-disk laser oscillator. It generates pulses with a duration of 30 fs at a central wavelength of 1048 nm and a repetition rate of 124 MHz. The laser emits the shortest pulses generated by a thin-disk laser oscillator, equal to the shortest pulse duration obtained by Yb-doped bulk oscillators. The average output power is currently limited to 150 mW by the low gain and limited disk quality. We expect that more suitable Yb:CALGO disks will enable substantially higher power levels with similar pulse durations.