Structural Colors from Modulated Topological Defects in Molecular Smectic Liquid Crystals

Unlike absorption colors, which are produced by light-absorbing dyes and pigments, structural colors arise from the wavelength-dependent interference of light scattered by microstructures produced through the self-organization of soft matter systems, like colloids or chiral liquid crystals. However, applications of structural colors require forming structures with micron-scale periodicities that are well-ordered and defect-free over large areas, which can only be achieved through complex sample processing methods. Here, we demonstrate another approach based on the striated linear topological defects appearing in thin liquid crystal smectic films under hybrid anchoring conditions. We produced structural colors in both reflection and transmission geometries simply by spin-coating the widespread commercial achiral molecular liquid crystal 8CB over glass slides. Optical microscopy, AFM, and optical diffraction techniques show that the colors are due to the modulation period along the defect axis being comparable to the wavelengths of visible light. Our approach dispenses with the tedious synthesis or purification of chiral organic compounds and with achieving ideal periodic microstructures because it relies on the spontaneous formation and uniform modulation of topological defects, allowing for upscaling to the large areas required for applications.