Hierarchical Colorful Structures by Three-Dimensional Printing of Inverse Opals.

Structural coloration is a recurring solution in biological systems to control visible light. In nature, basic structural coloration results from light interacting with a repetitive nanopattern, but more complex interactions and striking results are achieved by organisms incorporating additional hierarchical structures. Artificial reproduction of single-level structural color has been achieved using repetitive nanostructures, with flat sheets of inverse opals being very popular because of their simple and reliable fabrication process. Here, we control photonic structures at several length scales using a combination of direct laser writing and nanosphere assembly, producing freeform hierarchical constructions of inverse opals with high-intensity structural coloration. We report the first 3D prints of stacked, overhanging and slanted microstructures of inverse opals. Among other characteristics, these hierarchical photonic structures exhibit geometrically tunable colors, focal-plane-dependent patterns, and arbitrary alignment of microstructure facet with self-assembled lattice. Based on those results, novel concepts of multilevel information encoding systems are presented.