Non-Spherical Reflow-Formed Microstructures for Retroreflective Structural Coloration

Total-internal-reflection-induced interference in high-refractive-index convex microstructures provides a route to vivid structural coloration without periodic nanostructures. Structural colors from simple convex geometries, such as hemispherical and cylindrical microstructures, have been widely investigated. However, because these structures generally possess simple and symmetric cross-sectional profiles, their local curvature and optical path are relatively uniform, limiting the generation of complex color responses within a single microstructure. Here, we extend this concept by designing and fabricating non-spherical reflow-formed microstructures in which multiple local radii of curvature and cross-sectional geometries coexist within a single element. The curved structures are formed by thermal reflow of photolithographically patterned photoresist precursors. Unlike droplet-based fabrication of hemispherical caps, this reflow-based approach allows the final curved geometry to be controlled through the in-plane shape and volume distribution of the initial lithographic pattern. As representative examples, we fabricate semi-ellipsoidal and semi-toroidal microstructures. In the semi-ellipsoidal geometry, the local cross-sectional profile and radius of curvature encountered by the total-internal-reflection path vary with the incident position, enabling position-dependent multicolor structural coloration within a single microstructure. In the semi-toroidal geometry, the asymmetric cross-sectional profile further modifies the optical path depending on the incident position, producing distinct color responses from opposite sides of the structure. These results suggest that non-spherical reflow-formed microstructures can serve as a versatile platform for TIR-based structural coloration, enabling single-element control over both color multiplicity and directional optical response.