Mode purity and structural analysis of x-ray vortices generated by spiral zone plates

In the visible spectrum vortex beams have found various applications, ranging from optical tweezers to super-resolution imaging. Recently, these beams have been demonstrated using X-rays and electron beams. However, so far, no in-depth discussion has been carried out on the vortex quality, which could become essential for a variety of vortex applications. Here, we investigate the mode conversion efficiency (MCE), vortex structure and stability (in terms of vortex splitting) of the vortex fields generated by spiral zone plates (SZP). We have designed and fabricated SZPs with varying topological charge of both binary and kinoform profile. Kinoforms are known for their 100 % diffraction efficiency in the ideal case. In this work, both types are contrasted with regard to the vortex quality. Utilizing ptychographic coherent diffraction imaging and by comparing to simulations the wavefront of the generated fields is characterized. It was found, that the MCE and vortex structure exhibit the same dependencies on material and ZP properties as the diffraction efficiency (DE) and that the kinoform profile in this sense also improves the vortex quality. With growing SZP charge the MCE decreases. The results link the parameters of optics to the properties of the vortices and help to maximize the performance of ZP based vortex generators for future applications.