Printed Twisted Thin\nFilms with Near-Infrared Bandgaps\nand Tailored Chiroptical Properties

In this work, twisted helical cellulose nanocrystals\nfilms with\npreprogrammed circular polarization and near-infrared reflectance\nare fabricated via a blade-based 3D printing method. The films are\ncomposed of stacked nanoscale slabs with high birefringence from unidirectionally\norganized cellulose nanocrystals. By changing the printing director,\nwe achieved two types of films: twisted helical stacks and anisotropic\nBragg stacks. These films are highly transparent and clear, and the\nachiral anisotropic Bragg stack shows near-infrared spectral region\nreflectance (1.3–1.4 μm). In contrast, the twisted helical\nfilms show concurrent left- and right-handed circularly polarized\nproperties, as opposed to left-handed natural cellulose nanocrystals\nfilms. We observe dual chiroptical properties with circular dichroism\npeaks due to circular Bragg reflectance in the visible region and\nsuggest that the circularly polarized properties are extended to the\nnear-infrared region. These observations prompted us to explore the\ntransition between anisotropic Bragg stacks and continuous helical\nfilms via simulations. We show that the printed twisted films can\nact as optical metamaterials with dual helicity and fill the gap between\nknown photonic structuresthe conventional continuous chiral\nnematic material with a chiroptical appearance and the achiral Bragg\nstack with a controlled photonic bandgap. These printed twisted stacked\nfilms hold the potential of larger-scale printed ordering of unique\nanisotropic nanostructures for circularly polarized-sensitive photonic,\nthermal, and energy management applications.