Abstract
We report the generation of coherent water-window soft x-ray harmonics in a neon-filled semi-infinite gas cell driven by a femtosecond multi-mJ mid-infrared optical parametric chirped-pulse amplification (OPCPA) system at a 1 kHz repetition rate. The cutoff energy was extended to ∼450 eV with a 2.1 μm driver wavelength and a photon flux of photons/s/1% bandwidth was obtained at 350 eV. A comparable photon flux of photons/s/1% bandwidth was observed at the nitrogen K-edge of 410 eV. This is the first demonstration of water-window harmonic generation up to the nitrogen K-edge from a kHz OPCPA system. Finally, this system is suitable for time-resolved soft x-ray near-edge absorption spectroscopy. Further scaling of the driving pulse's energy and repetition rate is feasible due to the availability of high-power picosecond Yb-doped pump laser technologies, thereby enabling ultrafast, tabletop water-window x-ray imaging.
Highlights
Recent progress in ultrafast high-peak-power laser systems has allowed for significant advancements in the field of nonlinear optics
Our experimental setup consisted of a custom, home-made multi-mJ 1 kHz 2.1 μm optical parametric chirped-pulse amplification (OPCPA) system discussed at length in [17, 18] driving a semi-infinite gas cell as a HHG target, which is the combination of a gas cell and a hollow capillary, followed by an x-ray filter assembly, a grazing-incidence toroidal mirror and a 1 m radius of curvature Rowland circle spectrometer (McPherson Model 248/310) for measuring the high-harmonic spectra
We observed that the location of the focus was deep into the capillary, so that the maximum intensity of the beam occurred within 1–2 mm of the vacuum region at the output. It seems this condition helps to minimize the reabsorption of soft x-ray harmonics and the the plasma defocusing of the pulses as they propagate through the gas
Summary
Recent progress in ultrafast high-peak-power laser systems has allowed for significant advancements in the field of nonlinear optics. Generation (HHG), in which an atom is tunnel-ionized at high laser intensities of 1013–1014 W cm−2 and the electron is accelerated by the rapidly oscillating optical field and eventually recombines with its parent ion [1]. This process can be used to generate coherent photons in energy ranges, extending into the soft x-ray regime, that are not otherwise achievable from a tabletop system, resulting in coherent short-wavelength harmonic emission over multiple half cycles of the driving laser field.
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