Milliwatt-class high harmonic generation with an high average power short wavelength fiber laser

Table-top extreme ultraviolet (XUV) light sources with laser like properties are of interest in a vast variety of applications. To achieve such light pulses with femtosecond pulse durations, high harmonic generation (HHG) is an established technique. It delivers ultrashort pulses and a wide range of photon energies, suitable for the study of core level transitions, highly excited states, or transitions in highly-charged ions [1], as well as for experiments in solid state physics such as photoelectron spectroscopy [2]. To achieve short acquisition times as well as acceptable signal to noise ratios, a high repetition rate and high photon flux is beneficial. Therefore, average power scalable fiber-based lasers are an ideal driver [3]. Since the HHG single atom response scales with ∼λ
^−5.5
[4] and phase matching conditions are more beneficial for a shorter driving wavelength, a much higher overall conversion efficiency can be achieved for short wavelength drivers [5]. By using the second harmonic of an infrared laser for HHG we demonstrate the highest average power high harmonic source to date. This cascaded frequency conversion yields a record high average power of (832 + 204) μW, contained in a single harmonic line at 21.7 eV with a relative energy bandwidth of ΔΕ/E ≈ 10
⁻²
, shown in Fig. 1a) [6].