Flexible Infrared Photonic Crystal Films With Wide‐Angle Iridescence for Safety Applications

Abstract Materials with retro‐reflective optical properties are essential for various applications, yet solutions for achieving iridescent retro‐reflective colors remain limited. Here, a scalable method is reported to fabricate flexible infrared photonic crystal films that exhibit vivid retro‐reflecting colors over a wide range of angles. Monodisperse polystyrene spheres are shear‐assembled into highly ordered lattices and embedded in UV‐curable resin, forming volume diffraction gratings. The resulting films simultaneously display multidirectional structural colors in forward reflection, retro‐reflection, and retro‐transmission. Upon tilted incident light, retro‐reflective colors that appear against incident light are especially pronounced. The visibility, wavelength, and color shifts over angle of the resulting colors are tunable with the size of the spheres or applied strain, enabling dynamic, direction‐sensitive optical responses with minimal power consumption. This work presents a materials design strategy for multifunctional, scalable structural color systems and opens new possibilities for smart, sustainable safety applications and adaptive visual signaling.