Design of polychromatic focusing optics for neutron reflectometers: rethinking REFocus optics

Neutron reflectometry is a widely used non-destructive technique for analyzing surface and interfacial structures. In this study, we focused on REFocus optics, which employ an elliptical mirror to enhance measurement efficiency and extend the measurable Q range for a measurement with continuous neutron beams. The REFocus optics leverage a large beam divergence and polychromatic illumination to naturally provide angle and wavelength dispersions under specular reflection conditions. Using a position-sensitive detector simultaneously encodes both the dispersions into Q values, enabling highly efficient measurements compared with conventional methods. Here, we propose a compact REFocus optics design and demonstrate that combining a wide beam divergence with the novel concept of utilizing higher-order reflections covers a wide Q range from 0.08 to nearly 3 nm^-1. Additionally, this approach yields a reduction in acquisition time of up to 100 times compared with conventional optics and offers practical advantages in cost and implementation. These findings provide a promising pathway for improving reflectometry performance at reactor sources.