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Retroreflection and Wettability Controlled Smart Indicator Based on Responsive Bilayer Photonic Crystals for Traffic Warning

Due to the increasing human mortality caused by natural disasters and traffic accidents, the development of smart traffic warning systems is urgent. The first smart indicator for traffic warning based on bilayer photonic crystals with both selective retroreflection and response reflection optical properties is developed. The wettability‐controlled reflection evaluates the risk of landslides and warns drivers to slow down, while the retroreflection simultaneously displays iridescent vivid structural colors to observers at different viewing directions relative to the illumination direction and informs drivers the approaching of the accident blackspot. The mechanism involves diffractions in the periodic nanostructures and uses the wettability of vertical surfaces to realize the spectral diversity of reflectance. Moreover, colorful flickering structural colors based on the rotational periodic diffraction and angle dependence of the retroreflection are designed for speed limits warning. It demonstrates an energy‐efficient, smart, and tunable multi‐functionalized bilayer photonic crystal that is especially suitable for nighttime traffic warning, advertisement, and periodic warning display applications.