Extreme Ultraviolet Vortices at Free Electron Lasers
Characterization of extreme ultraviolet vortices
Optical vortices can be described as radiation which carries an orbital angular momentum. In contrast to circularily polarized light from synchrotron and free electron laser sources, it is not the electric and magnetic field vector but the phase of the electromagnetic field that rotates around a singularity in a helical fashion. The vortex can be characterized with an integer-numbered topological charge, which describes how often the phase rotates by 360°. Possible applications of these unique beams are the investigation of quadrupole transitions, angle-dependent emission of photoelectrons from supermolecular orbital states, defining torus-shaped beams, or scattering at magnetic vortices.
To demonstrate optical vortices at free electron lasers, we fabricated spiral zone plates, which yield a diffraction pattern with such a phase singularity. The material of choice for the extremely intense EUV radiation of the FERMI free electron laser is silicon. We thus etched spiral zone plates into ultraflat thin silicon membranes, and characterized the radiation using a Hartmann wavefront sensor downstream of the zone plate focal plane.
Beam profiling with developed polymer imprints
Beam profiling of free electron laser radiation in the focus of a Fresnel zone plate is a true challenge. An established method to characterize extremely intense beams in the EUV and soft X-ray regime is to shoot imprint craters into a material with well known damage threshold, and to characterize the shape of the imprint crater. However, this method is limited to a dynamic range of approximately 100, with the consequence that many imprints at varying X-ray fluence have to be recorded to get the full beam profile from maximum intensity to faint beam tails. We adapted the method and treated imprint craters in poly(methyl methacrylate) (PMMA) with organic solvents, in analogy to development processes in lithography. In this way, additional material, which is exposed to radiation but not ablated, can be removed. We are able to show that the developed imprints are extremely sensitive and enhance the dynamic range for beam profiling by an order of magnitude to 1000.