Broadband Circular Polarizing Film Based on Chiral Nematic Liquid Crystals

Abstract

polarizers use birefringent crystals requiring a specific thickness and suffer from bulky configurations, [13] many efforts have focused on cholesteric liquid crystals, such as cholesteric polymer networks with a pitch gradient, [14][15][16] or have focused on newly functional optical materials such as compact 3D gold helix metamaterials [3,17,18] and broadband graphene fiber polarizers. [19] Because these approaches are costly and challenging to scale, an inexpensive and straightforward method to achieve broadband circular polarization is in demand.Cellulose is the most abundant biopolymer in nature and can be produced from several natural resources, including plants, tunicates, or bacteria. [20,21] Treatment of extracted cellulose fibers with sulfuric acid produces cellulose nanocrystals (CNCs) with negatively charged sulfate half-ester surface groups. [22,23] This surface charge, whose electrostatic repulsion allows CNCs to form stable colloidal dispersions in water, gives CNCs unique selfassembling properties. Above a critical concentration, CNCs spontaneously organize into a chiral nematic lyotropic liquid crystal, and the structure is retained in films of CNCs following evaporation of the water. [24,25] This chiral nematic mesophase is characterized by stacked layers of oriented CNC spindles, where the orientations of the spindles within the layers rotate with a characteristic twist. Because of the structure, the films behave as 1D photonic crystals, reflecting circularly polarized light having the same sense as the cholesteric screw orientation. [26] This property of CNCs is attractive for producing iridescent films with many potential applications. [27][28][29][30][31][32][33][34] The wavelength reflected from a chiral nematic structure depends on a modified Bragg's Law, λ = nPcosθ, [35] where n is the average refraction index, P is the helical pitch length, and θ corresponds to the angle between incident light propagation direction and the helix axis. Thus, for a specified refractive index (usually determined by the components), the wavelength of the reflected light varies as a function of θ and P. As a result, both nonuniform pitch lengths and a distribution of helical axis orientations would lead to wide-band reflection.For lyotropic liquid crystals of CNCs, the assembly from an isotropic phase into a dense, anisotropic layered structure is a dynamic process that can be influenced by factors such as the pH, ionic strength, additives, and surface tension. [36][37][38][39] This dynamic self-assembly opens the path to introducing defects into the CNC films that lead to different pitches and/or orientations, giving a variety of reflection bands. In this work, we designed a micelle-assisted assembly approach to modulate the Films capable of broadband circular polarization of light are important for many optical applications. Owing to their intrinsic chiral nematic mesostructure, freestanding films of cellulose nanocrystals can reflect circularly polarized light, but this is typically restricted to a narrow ...

Cellulose nanocrystal-based liquid crystal structures and the unique optical characteristics of cellulose nanocrystal films

“…The CNC-based LC films can also be applied to a composite device because of its optical characteristics. Cao et al (2018) added hundreds of sub-micrometric micelles of anionic surfactant into CNC aqueous suspension to prepare CNC-based LC film. Under the transmission circular dichroism and UV-Vis spectroscopy, the left circularly polarized reflection band of this film is significantly widened.…”

Retrieving the Coassembly Pathway of Composite Cellulose Nanocrystal Photonic Films from their Angular Optical Response

“…[10][11][12][13] The versatility of CNCs and their ability to accommodate these guests allowed for the development of a variety of applications, namely, structural pigments, [14] anti-counterfeit coatings, [15] swelling, [16] or mechanochromic sensors, [17,18] as well as depolarizing [19] or broadband reflectors. [20] Importantly, the presence of nonvolatile additives often significantly alter the optical properties of the produced films. [21] In the presence of TMOS and related sol-gel precursors, a redshift in the reflected wavelength is commonly reported and associated to a larger cholesteric pitch, p, but its effect on the orientation of its helical axis, m, remained relatively unexplored.…”

Effect of Monomer Composition on the Performance of Polymer‐Stabilized Liquid Crystals with Two‐Step Photopolymerization

“…The central reflection wavelength λ is given by λ = ( n e + n o )/2 × P = n × P , and the reflection bandwidth Δ λ is given by Δ λ = ( n e − n o ) × P = Δ n × P , where the n e , n o , n , Δ n , and P are the extraordinary refractive index, the ordinary refractive index, the average refractive index, the birefringence, and the helical pitch, respectively . Generally, the Δ n value of liquid crystal is around 0.1~0.2, and the bandwidth of the CLC with a single pitch is narrow, which cannot meet some application requirements, such as polarizer‐free reflective displays, smart switchable reflective windows, and so on …”