 His latest interest is the development and exploration of non-linear x-ray phenomena.Such phenomena could be conveniently ignored during the first one hundred years of x-ray science because even for the most advanced synchrotron radiation sources, x-ray interactions with matter proceeded one-photon-at-a-time.Similar to the advent of conventional lasers in the 1960s, the emergence of x-ray lasers now requires the extension of the conventional description of x-ray interactions with matter through the concepts of quantum optics.