Carboxylated Cellulose Nanocrystals for Depolarization Films

Depolarizer is an important optical element for high-precision fiber optic gyroscopes in space and long-haul and high-bit-rate fiber transmission optical communication. Carboxylated cellulose nanocrystals prepared by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation prefer to form free-standing films with nematic-like mesostructures whose potential for optical application remains underexploited. Herein, we show that carboxylated cellulose nanocrystals by hydrochloride acid hydrolysis and TEMPO-mediated oxidation self-assemble into nematic-like mesophase that is capable of depolarizing both linearly and circularly polarized light in the visible region. By tuning the carboxylate content at retained fiber aspect ratio, the optimal carboxylated cellulose nanocrystal films to depolarize the visible linearly polarized light with the minimal degree of depolarization of ca. 4% are realized. We present the experimental evidence that birefringent microdomains with substantial variation in the size and orientation acting as numerous waveplates are responsible for the depolarization performance. Finally, we demonstrate how carboxylated cellulose nanocrystal films capable of depolarizing broadband visible circularly polarized light can be leveraged for encrypted patterning.