Self-Assembled Nanorods and Microspheres for Functional Photonics: Retroreflector Meets Microlens Array

Patterned micro/nanomaterials display efficient light management capabilities owing to their control of light propagation within multiscale periodic structures. Here a hierarchical photonic structure composed of polystyrene microspheres and cholesteric assembly of cellulose nanocrystals is described, acting as a polarization-sensitive retroreflective coating and microlens array. Micropatterned photonic films are prepared by casting an aqueous cellulose nanocrystal suspension onto a monolayer of polystyrene microspheres substrate through evaporation-assisted transfer imprinting lithography, integrating a bulk cholesteric matrix and patterned surface. By directing light at the as-assembled polystyrene surface, an enhanced structural color develops from the circularly polarized light retroreflection. Whereas when light travelling across the photonic film, the transparent layer of polystyrene microspheres forms into plano-convex microlens to converge the transmitted light into the focus plane and reduce centimeter-scale illuminated image into a high-fidelity miniaturized replica. This simple method, combining self-assembly with imprinting lithography, is expected to pave the way for designing custom-tailored optics with novel functions.