Journal Article X-ray optics coherent diffraction imaging Noise2Noise Research Support, Non-U.S. Gov't X-ray diffraction imaging X-ray reflection phases backward diffraction coherent X-rays covalent electron density dynamical diffraction ensemble diffraction microscopy high-resolution monochromator laser excitation machine learning mixed-scale dense network phase retrieval algorithms ptychography pump–probe experiments sapphire Fabry–Perot resonator synchrotron three-wave diffraction