Single-pass high-harmonic generation at 208 MHz repetition rate

Abstract

We report on single-pass high-harmonic generation (HHG) with amplified driving laser pulses at a repetition rate of 20.8 MHz. An Yb:YAG Innoslab amplifier system provides 35 fs pulses with 20 W average power at 1030 nm after external pulse compression. Following tight focusing into a xenon gas jet, we observe the generation of high-harmonic radiation of up to the seventeenth order. Our results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.

Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources

“…The achieved conversion efficiency of up to 6.7 3 10 27 (H23) is close to the typical values (10 26 ) achievable with other laser systems 44 and should be improvable by at least a factor of 2 by further optimizing the experimental conditions. The achieved photon flux is four orders of magnitude larger than that reported in studies on multi-MHz repetition rate HHG directly driven with a laser, i.e., without the additional enhancement cavity 28,29 . Moreover, the flux obtained in H23 is higher than what the state-of-the art enhancement cavities can deliver 8,24,31 and also exceeds the currently used kHz systems 30 .…”

“…This laser system is used to demonstrate transiently phase-matched HHG. More than 10 13 photons s -1 (.50 mW) are achieved in a single harmonic at 27.7 eV, which is an improvement of four orders of magnitude for a directly driven multiMHz repetition rate HHG 28,29 , and it provides a photon flux that is higher than that delivered by standard kHz laser systems 30 and intracavity HHG within that wavelength range 8,24,31 . The use of a lowenergy driving laser with a compact nonlinear compression setup not only has the potential to significantly reduce the complexity of high repetition rate HHG systems but will also enable efficient HHG at 10 MHz repetition rate, a regime so far only addressable with enhancement cavities.…”

“…This requires tighter focusing to achieve the intensities necessary for HHG, which also severely increases the contribution of the geometrical phase-mismatch term (see above). In combination with the reduced focal spot size and generation volume, it has been suggested that the efficiency will be significantly lower than in loose focusing geometries 28 . Indeed, the lack of phase-matching explains the poor efficiency of previous multi-MHz repetition rate sources that do not deliver more than a few 10 8 photons s -1 in a single harmonic 28,29 .…”

“…In combination with the reduced focal spot size and generation volume, it has been suggested that the efficiency will be significantly lower than in loose focusing geometries 28 . Indeed, the lack of phase-matching explains the poor efficiency of previous multi-MHz repetition rate sources that do not deliver more than a few 10 8 photons s -1 in a single harmonic 28,29 . However, investigations of the phase-matching behavior suggest that this can be overcome by properly choosing the experimental conditions, most notably a high-density gas target, to achieve the same pressure-length product as in loose focusing geometries 10,11 .…”

Oscillator-based High-order Harmonic Generation at 4MHz for Applications in Time-of-Flight Photoemission Spectroscopy

“…To significantly increase the repetition rate of HHG, three conceptually different approaches were demonstrated recently. By using high power fiber-based lasers and amplifiers, Vernaleken et al generate harmonics up to the 17th order at a repetition rate of 20.8 MHz [8]. In their setup, the generation of sufficient power to drive the harmonics and the compression of high power laser pulses are demanding.…”