Robust and Flexible Cellulose Photonic Films With Ambidextrous Circularly Polarized Reflection

ABSTRACT Sustainable cellulose‐based photonic materials that simultaneously combine structural coloration, circular polarization manipulation, and mechanical robustness remain challenging to achieve. Here, we report robust and flexible cellulose‐based bilayer photonic films with ambidextrous circularly polarized reflections, enabled by integrating a left‐handed chiral nematic cellulose nanocrystal (CNC) layer with a stretched regenerated cellulose (SRC) substrate. The CNC layer provides tunable circularly polarized structural coloration via helicoidal self‐assembly, whereas the SRC substrate exhibits pronounced birefringence and functions as a half‐wave retardation layer, thereby facilitating polarization conversion while concurrently enhancing mechanical robustness. As a result, the film exhibits intrinsic left‐handed circularly polarized (LCP) reflection on the front side and right‐handed circularly polarized (RCP) reflection on the reverse side, while the tensile strength increases from 6.6 to 72.4 MPa, accompanied by an increase in elongation at break from 0.8% to 10%. By coupling structural coloration, polarization selectivity, and water‐responsive color modulation, the films further enable multidimensional optical encryption. The coating‐based fabrication offers a scalable route toward large‐area sustainable photonic materials for optical encryption, anti‐counterfeiting, and advanced optoelectronic applications.