When a light ray is allowed to pass through a birefringent material, it splits into two rays that are plane polarized in mutually orthogonal planes or circular-polarized in opposite directions. Controllable birefringence is highly desired to advance its use in optics and biomedicine applications.
Study: Programmable Birefringent Patterns from Modulating the Localized Orientation of Cellulose Nanocrystals. Image Credit: Yurchanka Siarhei/Shutterstock.com
An article published in the journal ACS Applied Materials and Interfaces demonstrated an approach to tuning the alignment of cellulose nanocrystals with a controllable orientation at localized precision. In this approach, the one-dimensional (1D) cellulose nanocrystals were allowed to align along the periphery of the template and the controlled three-phase contact line by evaporation.
This alignment tuned the localized orientation of cellulose nanocrystals, resulting in the formation of cellulose nanocrystal films with the ability to display polarized light-depended birefringent extinction patterns.
The cellulose nanocrystal films helped achieve versatile pattern designs when employed with different shaped templates and template arrays with varied layouts. Due to the localized modulation of cellulose nanocrystals, the films exhibited dynamically transformable designs of polarized angle-dependent birefringent patterns.
The construction of an N-nary encoding system was also demonstrated based on sunlight-transparent cellulose nanocrystal films, which were utilized for abundant information storage. Under polarized light, these systems exhibited visible extinction patterns that enrich the area of bio-based photonics.
Birefringent Materials and Birefringence Phenomenon
Propagation of an incident light through the optical anisotropic materials results in light splitting into two orthogonal vibrating rays (ordinary and extraordinary rays) with different refractive indices and transmitting velocities. The recombination of these ordinary and extraordinary rays in a phase difference termed retardation cause constructive or destructive interference.
Due to their essential function in modulating the polarization of light, birefringent materials are explored in academic and industrial research and applied to the laser industry, optical communication, polarimetry, and scientific instrumentation.
Bioinspired engineering materials with birefringence properties are applied in sensing, optics, and anticounterfeiting fields. However, the currently available birefringent materials are prepared by uniaxial deformation of anisotropic structures. Fabricating structures with controllable birefringence at localized preciseness is challenging due to ineffective control of the orientation of the localized structure.
Aqueous cellulose suspensions have been reported to be birefringent when the crystals are aligned magnetically, electrically, or mechanically. Moreover, needle-like cellulose nanocrystals are intrinsically anisotropic and are considered promising birefringent materials.
Although various approaches, including microfluid channels-based geometry confinement, electric and magnetic alignment, mechanical stretching, and co-assembly with inorganic particles or polyols, were applied to improve their chiral optical properties. Only a few reports mentioned cellulose-based birefringent structures.
Programmable Birefringent Patterns using Cellulose Nanocrystals
In the present work, the birefringent materials were constructed with programmable extinction patterns by aligning the intrinsically anisotropic cellulose nanocrystals through confined assembly on different templated and a three-phase controlled contact line.
The resulting cellulose nanocrystal films were of micro- to a millimeter in size with precisely aligned cellulose nanocrystals having locally tunable orientation. Compared to other birefringent materials, the obtained cellulose nanocrystal films exhibited versatile birefringent extinction designs, interference color, and dynamically transformed patterns depending on the polarized angle.
The template-confined self-assembly of modified cellulose nanocrystal films showed their assembly in triangle, circle, square, clover, and hexagonal shapes, demonstrating the Maltese cross extinction pattern. Rotating these films did not alter the position and intensity of the Maltese cross, indicating the centrosymmetric orientation of modified cellulose nanocrystals.
By leveraging the programmed extinction patterns under polarizing light and transparency of cellulose nanocrystal films under sunlight, the N-nary encoding system was constructed, revealing the promising applications of cellulose nanocrystal films in information and display.
To summarize, a birefringent film of microscale size with programmable extinction patterns and interference colors was fabricated based on the alignment of modified cellulose nanocrystals by evaporation confined on a template. The modified cellulose nanocrystals were strictly aligned along the template’s periphery and the three-phase contact line, resulting in the films with anisotropic nanocrystals having locally modulated orientation.
Depending on the shape of the individual template, tailored Maltese cross extinction patterns with curved sides and tilted angles were obtained. Additionally, complex and detailed extinction layouts containing birefringent patterns were achieved by aligning the modified cellulose nanocrystals over the template arrays. Moreover, the patterns could transform into different designs by simple rotation.
The advantages of programmed extinction patterns under polarized light and transparency of cellulose nanocrystal films under sunlight helped develop an N-nary encoding system for information encryption. Thus, the template-based approach for producing cellulose nanocrystal-based optical materials with adjustable birefringence properties contributed to the field of bio-based photonics.
Wang, H., Shao, R., Meng, X., He, Y., Shi, Z., Guo, Z., Ye, C. (2022) Programmable Birefringent Patterns from Modulating the Localized Orientation of Cellulose Nanocrystals. ACS Applied Materials and Interfaces. https://pubs.acs.org/doi/10.1021/acsami.2c12205