Phys. Rev. Lett. 118, 024801 (2017)

Conventional diffraction theory breaks down at sufficiently high X-ray intensities, such as that achieved with the latest generation of X-ray lasers (XFELs), and in this regime it is possible for an X-ray diffraction pattern to be self-focused beyond the diffraction limit. That's the conclusion of a theoretical study conducted by Joachim Stöhr from the SLAC National Accelerator Laboratory in the USA. Stöhr considered the case of a source formed from a thin film within a circular aperture. When illuminated at high intensities such that spontaneous scattering from the film is replaced by stimulated scattering, the resulting diffraction pattern is no longer the expected Airy pattern. Instead, the stimulated pattern corresponds to the square of the Airy pattern, where the central peak has a reduced width and the outer Airy rings effectively disappear. The new pattern results from the interaction of cloned photon pairs created by stimulated emission.