X-Ray Propagation in Tapered Waveguides: Simulation and Optimization
  We use the parabolic wave equation to study the propagation of x-rays in tapered waveguides by numercial simulation and optimization. The goal of the study is to elucidate how beam concentration can be best achieved in x-ray optical nanostructures. Such optimized waveguides can e.g. be used to investigate single biomolecules. Here, we compare tapering geometries, which can be parametrized by linear and third-order (Bezier-type) functions and can be fabricated using standard e-beam litography units. These geometries can be described in two and four-dimensional parameter spaces, respectively. In both geometries, we observe a rugged structure of the optimization problem's ``gain landscape''. Thus, the optimization of x-ray nanostructures in general will be a highly nontrivial optimization problem. 
Sebastian Panknin, Alexander K. Hartmann, and Tim Salditt