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 ...

We compare the generation of high-order harmonics in the water window (283-543 eV) with 0.8-µm and 2.2-µm few-cycle lasers at a pulse repetition rate of 100 kHz. Using conventional phase matching with the 2.2-µm driver and what we attribute to nonadiabatic self-phase-matching with the 0.8-µm driver, photons up to 0.6 keV (2 nm) are generated in both cases. Special attention is paid to the understanding of the generation mechanism with the 0.8-µm laser amplifier system. We use the same beamline and pump laser for both drivers, which allows for a direct flux comparison at the two driving wavelengths. For photon energies around 280 eV, a 10-100 times higher flux is obtained from the 2.2-µm versus the 0.8-µm laser system in helium and neon. The crossover at which the 2.2-µm yields a higher flux compared to the 0.8-µm driver is found to be as high as 0.2 keV. Our study supports the common approach of using long-wavelength lasers in a phase-matched regime for efficient generation of water-window harmonics, but also shows that the more widespread 0.8-µm wavelength can be used to generate water-window harmonics with an efficiency close to the one of a less common 2.2-µm source.

We present a near-infrared optical parametric chirped-pulse amplifier (OPCPA) and soft X-ray (SXR) highharmonic generation system. The OPCPA produces few-cycle pulses at a center wavelength of 800 nm and operates at a high repetition rate of 100 kHz. It is seeded by fully programmable amplitude and phase controlled ultra-broadband pulses from a Ti:sapphire oscillator. The output from the OPCPA system was compressed to near-transform-limited 9.3-fs pulses. High-power operation up to an average power of 35 W was achieved, and a fully characterized pulse compression was recorded for a power level of 22.5 W, demonstrating pulses with a peak power greater than 21 GW. We demonstrate that at such high repetition rates, spatiotemporally flattened pump pulses can be achieved through a cascaded second-harmonic generation approach with an efficiency of more than 70%, providing a compelling OPCPA architecture for power-scaling ultra-broadband systems in the near-infrared. The output of this 800-nm OPCPA system was used to generate SXR radiation reaching 190 eV photon energy through high-harmonic generation in helium.

High harmonic generation (HHG) in atomic gases is generally assumed to originate from photoelectrons that are not perturbed by neighboring particles. In this paper, we study theoretically and experimentally the regime where this approximation breaks down. At high laser intensities, we experimentally find that producing soft x-rays beyond this single-collision condition leads to a strong reduction of the coherent HHG response and appearance of incoherent radiation. We generalize our results to phase-matched HHG with mid-infrared drivers, and determine that a minimum pulse energy is needed to simultaneously phase match the HHG process and keep photoelectrons unperturbed by surrounding particles. Therefore, while previous research showed that HHG efficiency is independent of the driving pulse energy if other experimental parameters are scaled accordingly, we find that this rule no longer applies for high photon energies. Our study thus provides important guidelines for the laser parameters needed for the generation of high flux soft x-ray high harmonics.

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