High order harmonic generation (HHG) is a source of bright, ultrafast, fully spatially coherent, extreme ultraviolet (EUV) light with applications in ultrafast molecular and materials spectroscopy, element selective ultrafast magnetic dynamics, nano-thermal heat transport and high-resolution imaging. Harmonics have been generated up to a few keV, but the flux has been very low past 100 eV. Thus, applications of HHG have focused on the EUV region. By enhancing the brightness of harmonics at higher energies, we can expand the applications of HHG to the soft x-ray region of the spectrum. The u22water windowu22 is a particularly important region of the spectrum for high resolution biological imaging. In this region, between 284 and 540 eV, water is an order of magnitude more transparent than carbon, providing contrast between various biological materials. This thesis presents two methods to improve the brightness of harmonics in the water window. In the first, harmonics were generated from doubly ionized argon, which extended the cutoff photon energy to 540 eV, 200 eV higher than previously demonstrated from argon. The second method used the recently developed mid-infrared phase-matching technique to fully phase match the harmonic process at soft x-ray photon energies up to 540 eV which increased the brightness of harmonics in the water window by three orders of magnitude. This source was then characterized with the first spatial coherence measurement of any compact light source in this spectral range. In the future, this source can be used for high resolution, element specific, coherent imaging in the water window and ultrafast transient absorption spectroscopy in molecules and materials.
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