Water window soft x-ray source enabled by a 25  W few-cycle 2.2 µm OPCPA at 100  kHz

Abstract

Coherent soft x-ray (SXR) sources enable fundamental studies in the important water window spectral region. Until now, such sources have been limited to repetition rates of 1 kHz or less, which limits count rates and signal-to-noise ratio for a variety of experiments. SXR generation at high repetition rate has remained challenging because of the missing high-power mid-infrared (mid-IR) laser sources to drive the high-harmonic generation (HHG) process. Here we present a mid-IR optical parametric chirped pulse amplifier (OPCPA) centered at a wavelength of 2.2 µm and generating 16.5-fs pulses (2.2 oscillation cycles of the carrier wave) with 25 W of average power and a peak power exceeding 14 GW at 100-kHz pulse repetition rate. This corresponds to the highest reported peak power for high-repetition-rate mid-IR laser systems. The output of this 2.2-µm OPCPA system was used to generate a SXR continuum extending beyond 0.6 keV through HHG in a high-pressure gas cell. MainProgress in laser technology has enabled rapid development in attosecond science which led to many scientific discoveries [1,2]. Further advances in attosecond science are closely linked to high-harmonic generation (HHG) sources [3,4], and therefore to state-of-the-art laser systems to drive the HHG process into new performance frontiers. Specifically, there is currently great interest in scaling HHG sources to parameters beyond those available in conventional Ti:sapphire amplifier driven beamlines, in particular to higher photon energies and higher repetition rates. Photon energies extending up to 1.6 keV were generated at 20 Hz repetition rate [5]. Recently, multiple research groups have developed 1-kHz laser sources capable of producing coherent soft x-ray (SXR) radiation spanning up to the oxygen K-edge at 543 eV [6][7][8]. Such high-photon-energy sources are interesting for a variety of spectroscopic studies since core electrons can be accessed directly. For example, this enables direct probing of biological molecules in aqueous solutions [9], tracking of electronic, vibrational and rotational [10] as well as magnetization dynamics [8]. Furthermore, the high photon energies allow for the shortest probe pulses ever produced [11]. On the other hand, high repetition rates are especially important for applications limited by space-charge effects, such as the investigation of photoemission delays from surfaces [12,13]. The coherent SXR radiation in the above examples is generated via HHG. At a given intensity I and carrier wavelength λ, the maximum energy of the generated photons scales with ~I·λ 2 of the driving laser field [14]. Thus, to obtain a high-energy cut-off without excessive ionization of the target, which would prevent phasematching, mid-IR driving lasers are required. Longer driving wavelengths also give rise to higher phasematching pressures, which increases the number of potential emitters [15]. On the other hand, the singleatom yield drops rapidly with wavelength, with a scaling of around ~λ -5.5 for a fixed energy interval [16]. This ...

Generation of few-cycle multi-millijoule 2.5 μm pulses from a single-stage Cr2+:ZnSe amplifier

“…In summary, we have demonstrated a single-stage Cr 2+ :ZnSe chirped pulse amplifier capable of producing 4 mJ, 44 fs laser centered at 2.5 μm. The optical-to-optical conversion efficiency from the 2.09 μm pump to the 2.5 μm laser is ≈16%, which is much higher than our previously reported OPCPA (<3%) 13 and other state-of-the-art OPCPAs (<10%) 18,29 . The wall-plug efficiency is even higher for the Cr 2+ :ZnSe laser since the power supply of the Ho:YAG laser consumes 2.1 kW of which 0.8 kW is for the chillers whereas the three Nd:YLF lasers for the OPCPA require 5.7 kW to operate (power for chillers not included).…”

Attosecond science based on high harmonic generation from gases and solids

“…Streaking measurements are affected by space charge effects. SWIR OPA/ OPCPA lasers based on high-average-power diode-pumped thin disc/slab Yb lasers 48 provide a promising approach to deliver high-repetition-rate and high-flux attosecond pulses in the water window. This will significantly improve the signal to noise ratio of streaking traces.…”

27 W 2.1 µm OPCPA system for coherent soft X-ray generation operating at 10 kHz

“…High-energy few-cycle MIR drivers are mainly enabled by optical parametric chirped pulse amplifiers (OPCPA) pumped with commercial high-power picosecond lasers around 1 µm [12][13][14][15]. Using such a pump laser, it is possible to obtain MIR pulses from 2 µm to 4 µm via different nonlinear crystals such as BBO [16], BiBO [17], LNO [18], PPLN [19] and KTA [20]. Operating in the 2 µm wavelength region has three main advantages: First, the radiation strength of the single atomic HHG-process increases with higher frequency [21].…”

“…1 which summarizes the 2 µm OPCPA systems reported in the literature [16][17][18]24,25]. The maximum average output powers are limited to about 10 W, and only recently this value was surpassed with a 100 kHz repetition rate OPCPA system [19]. Furthermore, there are currently no 2 µm OPCPA systems with pulse repetition rates between 5 kHz and 20 kHz.…”

“…In Ref. [19], the photorefractive effect in PPLN crystal is mitigated by lowering down the pumping intensity, which is realized with a high-aspect ratio poled aperture crystal (2 × 10.9 mm 2 ) and a flattened pumping beam. This eventually enables a maximum average output power of 25 W. However, power upscaling needs larger apertures which are not available so far.…”