Diffractive cascades for polychromatic hard X-ray focusing

TL;DR

This paper introduces topology-optimized diffractive cascades for hard X-ray focusing, achieving higher efficiency and broader energy range than traditional zone plates, with robustness to real-world perturbations.

Contribution

It presents a novel design approach using topology optimization to create diffractive cascades with improved efficiency, energy range, and depth of focus for X-ray applications.

Findings

Achieves 40% focusing efficiency with aspect ratio 8, surpassing zone plates.

Enables focusing of beams beyond 20 keV and bandwidths over 1%.

Demonstrates robustness to alignment, fabrication, and heating perturbations.

Abstract

Diffractive focusing of hard X-rays has traditionally required structures with large aspect ratios due to the limited interaction of most materials with X-rays. This has increased the complexity of fabricating diffractive X- ray lenses, restricting their widespread deployment. Here, we utilize topology optimization to design diffractive cascades to focus X-rays. When restricting the structures to a maximum aspect ratio of 8, a diffractive cascade can achieve a focusing efficiency of 40%, far exceeding the 3% efficiency of a zone plate with the same aspect ratio. Diffractive cascades also allow the focusing of beams with energies beyond 20 keV and bandwidths exceeding 1%, loosening the restrictions on other system components. We characterize the robustness of these cascades to alignment, fabrication, and heating perturbations, demonstrating the ability of our designs to operate under real-world conditions. Finally, we exploit the flexibility of our framework to include multiple depths in the objective function. This enables a depth of focus exceeding that of a zone plate or a cascade designed using single-plane optimization. This work demonstrates the utility of topology optimization in the X-ray regime and the possibility of advancing X-ray manipulation across a range of tasks.