We report the generation of coherent water-window soft x-ray harmonics in a neon-filled semiinfinite\ngas cell driven by a femtosecond multi-mJ mid-infrared optical parametric chirped-pulse\namplification (OPCPA) system at a 1 kHz repetition rate. The cutoff energy was extended to\nâË?¼450 eV with a 2.1 Ã?¼m driver wavelength and a photon flux of 1.5 106 photons/s/1%\nbandwidth was obtained at 350 eV. A comparable photon flux of 1.0 106 photons/s/1%\nbandwidth was observed at the nitrogen K-edge of 410 eV. This is the first demonstration of\nwater-window harmonic generation up to the nitrogen K-edge from a kHz OPCPA system.\nFinally, this system is suitable for time-resolved soft x-ray near-edge absorption spectroscopy.\nFurther scaling of the driving pulseÃ?¼s energy and repetition rate is feasible due to the availability\nof high-power picosecond Yb-doped pump laser technologies, thereby enabling ultrafast,\ntabletop water-window x-ray imaging.
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