Janus Photonic Graphics by Stratified Assembly of Two Distinct Colloids

Colloidal photonic crystals have been micropatterned and layered to achieve advanced optical functionalities, yet existing strategies typically rely on delicate, time‐consuming, and multistep processes that limit scalability and reproducibility. Here, we present a single‐step bottom‐up approach for fabricating double‐sided Janus photonic micropatterns through the spontaneous yet spatially separated crystallization of two distinct colloidal species. By tuning the density of an aqueous medium with sucrose to lie between those of poly(methyl methacrylate) (PMMA) and polystyrene (PS) particles, centrifugal sedimentation stratifies the two species in opposite directions. Simultaneously, depletion attraction drives independent crystallization at the top and bottom interfaces, yielding PMMA and PS colloidal crystals with distinct reflection colors. Regioselective heterogeneous nucleation on hydrophobic micropatterns further enables the formation of double‐sided photonic graphics with independently controllable colors and designs. Depending on film thickness, the two structural color patterns can be optically coupled through additive color mixing in thin crystalline layers or decoupled in thick layers for flip‐switchable optical responses. This facile and scalable process unites density‐controlled stratification with depletion‐mediated assembly, providing a versatile platform for the construction of dual‐layered photonic architectures with potential in smart labeling, optical encryption, and dynamic display technologies.