Over 95% Optical Reflectance From Structurally Modulated All‐Biopolymer Multilayers

ABSTRACT Sustainable photonics aims to replace conventional optical materials with environmentally benign alternatives, yet its development is fundamentally constrained by the narrow intrinsic refractive‐index variability of biopolymers. Here we show that a dense–porous periodic architecture yields nearly perfect optical reflection using only polysaccharides. By exploiting the difference in skeletal rigidity between amorphous pectin and crystalline cellulose nanofibers (CNFs), we form alternating dense, high‐refractive‐index pectin layers and porous, low‐refractive‐index CNF layers through a combination of electrodeposition with solvent exchange. This dense–porous periodic architecture yields reflectivities exceeding 97% in the visible range and 95% in the near‐infrared—a range unprecedented for biopolymer‐based optical materials. Moreover, the strategy is simple, scalable, and highly versatile, enabling free spectral design across the visible and near‐infrared regions, broadband metallic‐like reflection, and planar patterning within a single fabrication process. Combined with the low thermal conductivity arising from porous layers and the coexistence of visible transparency and near‐infrared reflection, the resulting films show potential as transparent thermal‐management materials. This work establishes a new materials design principle for biodegradable, polysaccharide‐based photonic systems and advances the field of sustainable photonics.