Ian McNulty, Experimental Facilities Division, Argonne National Laboratory
X-ray vortices

Phase singularities are a ubiquitous feature of waves of all forms and represent a fundamental aspect of wave topology. In a singular optical field, a point phase discontinuity exists when the complex amplitude vanishes and the phase is undefined. A surface of constant phase about the singularity describes a helix containing m turns for each wavelength traveled along the propagation axis. This helical wavefront is known as an optical vortex with charge m. Phase singularities have been extensively studied with visible light, and it is now recognized that vortices are a common feature of coherent light propagation. Optical vortices are also noteworthy because they carry orbital angular momentum.

Motivated by rapid progress in coherent x-ray phase retrieval methods and the potential effects of singularities on them, we recently demonstrated the production and detection of x-ray vortices of various charge states at the Advanced Photon Source. The vortices, created with a optical structure that imparted a helical phase ramp to the incident coherent x-ray wavefield, were observed interferometrically.

Optical vortices are finding application in areas as diverse as manipulation of trapped microscopic particles and observation of faint astronomical objects. We may find that x-ray vortices will become similarly useful as well as being worthy of further study.