Cholesteric liquid crystals (ChLC) is of great interest to researchers due to their special self-assembled helical structure and selective light reflection properties. The cholesteric phase is considered as a special state of the nematic phase and is also often referred to as chiral nematic liquid crystals [1]. Unlike the mainly oriented sequential character of the nematic liquid crystal phase as an elongated molecule, ChLCs exhibit a spontaneous helical structure with the twist axis perpendicular to the local orientation [2,3,4]. The ChLC molecules are arranged in layers, with the long axis of the molecules parallel to the layer planes and each plane rotated at a certain angle with respect to its neighboring planes [5]. The pointing vector of liquid crystal molecules in the layer direction returns to the initial orientation state after 360° of rotation, and this periodic layer spacing is called the pitch (P) [6,7]. The pitch can change with the chemical environment, temperature, electric field, and so on.

Due to the unique helical structure of ChLCs, they exhibit special optical properties, such as selective light reflection [8,9], circular dichroism [10,11], and rotational properties [12,13], which make them widely used in various fields. In recent years, the selective reflection of ChLCs has become a hot research topic of wide interest. ChLCs with reflection wavelength in the visible region can be applied to temperature indication [14], liquid crystal display panel light brightening film [15], anti-counterfeit trademark [16], mirrorless low-threshold laser [17,18], etc. ChLCs with reflection wavelengths in the near-infrared region can be applied to energy-saving and environment-friendly architectural glass or paint [19], etc. ChLCs with reflection wavelengths in the mid-infrared and far-infrared regions have potential applications in military shielding and stealth [20]. Thus, ChLCs with broadband reflective properties have significant appeal and wide market demands due to their outstanding characteristics. However, single-pitch ChLCs tend to have a reflection width less than 200 nm due to their excellent selective reflection of light based on the Bragg relationship [21]. How to realize the broadening of the reflection bandwidth within the desired reflection spectrum is still one of the critical problems to be solved for the application of ChLCs. 

2. Broadening the Reflection Bandwidth

Because of the periodic variation in refractive index caused by the helical structure, ChLCs can selectively reflect the incident light along the helical axis direction. Due to the circular dichroism of the ChLC structure, its selective reflection is not independent of the linearly polarized light, but only the circularly polarized light with the same rotation direction as the ChLC structure. Therefore, the left-handed or right-handed ChLC reflection incident light is limited to 50%. ChLCs with a single-pitch selectively reflect the light of a wavelength between λ_min = Pn_o and λ_max = Pn_e. Here, n_o and n_e are the ordinary and extraordinary refractive indices of the locally uniaxial structure, respectively. The bandwidth of the selective reflection spectrum ∆λ is given by ∆λ = λ_max − λ_min = (n_e − n_o) P = ∆nP. Here, ∆n = n_e − n_o  is the birefringence. According to the above equation, we can seeit can be seen that the bandwidth ∆λ is dependent on the birefringence ∆n and the pitch P at normal incidence. Since the birefringence Δn of ChLC materials is generally less than 0.4 [20], the bandwidth of a ChLC with a single-pitch selective reflection of incident light is narrow (generally less than 200 nm), and it is difficult to adjust the bandwidth by adjusting Δn [22,23]. Therefore, it is necessary to adjust the pitch gradient or the non-uniform pitch distribution in the ChLC material system to achieve the broadening of the reflection bandwidth. As we all know, ChLCs are usually observed in liquid crystal molecules containing chiral units or obtained by adding chiral compounds to nematic liquid crystals. However, the pitch of small molecule ChLCs is usually uniform, single, and narrow due to their low viscosity. Therefore, in order to obtain the broadband reflection of a ChLC, it is usually achieved in a polymer/ChLC system. This may be due to the high viscosity of polymer and the fixed network structure, which limit the diffusion between different length pitch structures, so as to form microstructures with different length pitch distributions. According to the monomer content, the polymer/ChLC system can be divided into small-molecule ChLCs (without monomer), cholesteric liquid crystalline polymers (with 100% monomer), and polymer-stabilized ChLCs (with a small amount of monomer). ChLC polymers are completely polymerized by liquid crystal monomers containing chiral units. Regardless of the mesogenic units in the main chain and side chain, ChLC polymers have high molecular weight and high mechanical properties. Due to the moderate to highly crosslinked network, the ChLC polymer network retains only a small amount of ChLC properties, replicating the helical structure of ChLC in space [24]. Due to high viscosity or the fixed network structure, the orientation and diffusion of ChLC polymers are almost unchanged when stimulated [25]. Polymer-stabilized cholesteric liquid crystals (PSCLC) have a large number of small-molecule ChLCs as continuous phases and the mass fraction of the polymer is usually less than 10%, which can be formed by the polymerization of mesogenic monomers or the monomers without mesophase [26]. Due to the low content of polymer monomers, PSCLC can not only form polymer networks, but also properly retain the ChLC characteristics, which plays a key role in the formation and fixation of microstructure with pitch gradient distribution or uneven pitch distribution. HTherein, we following contents will introduce the approach to broaden the reflection bandwidth from the following aspects: multilayer system, responsive chiral molecules, light-controllable polymerization rate, thermally induced molecular diffusion, two-phase coexistence material system, memory effects of the template, and electromagnetic-induced molecular diffusion.

2.1. Multilayer System

This method of multilayer system is mainly used to broaden the reflectance bandwidth of the composite system by superimposing ChLC samples with different pitches. Firstly, multiple polymer/liquid crystal films with different pitches should be prepared. Usually, copolymerization of different amounts of chiral monomers or doping of different chiral compounds into PSCLC are used to form multiple ChLC polymers or PSCLC films that can reflect incident light of different wavelengths, respectively. Then, ChLC film layers with different pitches are stacked together in a certain order. Finally, the stacked films are bonded or polymerized together to obtain a reflection band wider than any film layer. Usually, the obtained reflection band is between the maximum and minimum reflection wavelengths in each ChLC film. Kralik obtained a ChLC film reflected wavelength that covers the visible light region, by superimposing three layers of ChLCs reflecting red light, green light, and blue light, respectively [27]. Choi manufactured a continuous wide reflective band by superimposing three right-handed polymer cholesteric liquid crystal (PCLC) layers with different pitches and a sandwich structure of nematic liquid crystals (NLCs) made up of these PCLC films. Samples of PCLC with different reflection bands were spin-coated on the substrate, in turn, with the PVA film acting as an alignment layer for the upper PCLC layer and a barrier layer preventing the lower PCLC layer from being dissolved by the new PCLC film stack. Through this stacking process, broadband multilayer stacked PCLC films were obtained. A broadband reflective film covering the visible light range was obtained by stacking RGB PCLC films. While each PCLC layer provides a smooth reflection band, the multi-layer stacked layers exhibit some degradation in the reflection band. This phenomenon is due to defects in abrupt pitch shifts in PCLC layers and inserted isotropic polymer layers [28]. Recently, a new approach to the preparation of broadband reflective ChLC films based on inkjet printing and non-stick technology was proposed in ourne research  [29]. We fFilled the separate C, M, Y channels with chiral molecules, polymerizable monomers and liquid crystals, respectively, and controlled the inkjet volume by software (Acro-rip). In order to prepare a broadband reflective film by stacking layers, we separately printed tthe precursor liquid crystal layer were printed separately on each substrate with different chiral additions. After UV polymerization of the first liquid crystal layer, the second liquid crystal layer was transferred to the first liquid crystal layer for polymerization. Then, a third, fourth, or even more layers were stacked in the same way and the thickness of each PSCLC film was controlled by adding polyimide films. The advantages of the single-pitch multilayer stacking method are that the preparation process is very simple and the wavelength and the reflection range of each film are controllable. However, the stacked multilayer system is not a direct combination of monolayer characteristics, and there is certain diffusion between the film interfaces, which, to a certain extent, affects the plane orientation of ChLC molecules in the interface, resulting in low reflectivity and transmittance of the final obtained films. In addition, the thickness of the film obtained by the above method is relatively thick, and does not have the function of dynamic adjustment, which reduces the practicability.

2.2. Responsive Chiral Molecules

ChLCs with helical structures can be formed by adding chiral molecules to the nematic phase liquid crystals. Helical twisting power (HTP) represents the ability of chiral molecules to induce the formation of helically arranged structures in nematic liquid crystals, which is directly related to the pitch of the helical superstructure of ChLCs (HTP = (PX_c)⁻¹). Here, P and X_c are pitch and chiral molecule concentration, respectively. Therefore, the HTP of the chiral molecule plays a decisive role in the pitch of ChLCs. The responsive chiral molecule allows a change in HTP of the chiral molecule under different external fields (thermal, optical), which changes the pitch of the ChLCs and allows for the adjustment of the reflection wavelength [30]. Duan synthesized a responsive chiral compound of which the HTP decreases with increasing temperature and formed a PSCLC film with a pitch gradient distribution by using two polymerizable monomers [31]. One is 1,4-di- [4-(6-acryloyloxy) hexyloxy benzoyloxy]-2-methyl benzene (C6M), a free-radical polymerizable monomer with high polymerization activity at low temperatures, and the other is ethylene glycol diglycidyl ether (EDGE), a cationic polymerizable monomer with high polymerization activity at high temperatures. Based on the large HTP temperature dependence of a chiral dopant, polymer networks with different pitches were formed by polymerizing C6M to fix a small pitch at low temperature and EDGE to fix a large pitch at high temperature in sequence. The research by Zhang confirmed that an appropriate concentration of free-radical monomers and a sufficient concentration of cationic monomers are essential for the formation of PSCLC films with broadband reflectivity [32]. Furthermore, it was shown that the increased functionality and rigidity of the cationic monomers had a positive effect on the broadening of the reflection bandwidth. Due to the rapid development of photoresponsive chiral compound materials, the modification of the HTP of chiral compounds by light, combined with the PSCLC to fix the distribution of the pitch gradient, has become a new approach to broaden the reflection bandwidth of ChLC films. The degree of molecular isomerization along the thickness direction is distributed in a gradient, leading to the distribution of the pitch gradient. The presence of the polymer network causes the fixation of the helical structure. The network density formed from top to bottom gradually decreases, leading to a significant difference in the effect of azo–chiral compound replies and further realizing the broadening of the reflection bandwidth. The reflection wavelength of the composite can be expanded to 1000–2400 nm. Lu synthesized a new cyclic chiral azobenzene compound, Azo-o-Bi, which shows high HTP and optochemically reversible cis-trans isomerization in organic solvents and liquid crystals. Due to the strong absorption of UV light by the chiral Azo-o-Bi molecule, the cis-isomers with high HTP accumulated on the side close to the UV irradiation. The gradient distribution of the pitch in the direction of UV propagation induced a broadband reflection ChLC film [34]. Yang constructed broadband reflectivity and super-reflectivity by exploiting the strong UV-intensity dependence of the photo-isomerization of chiral molecular motors [35]. Under different light intensities of UV irradiation, the HTP of chiral molecules can be significantly reduced, and the stronger the light intensity, the greater the change in HTP values. By irradiation with different light intensities of UV light, the pitch and handedness of the ChLCs change accordingly. The gradient distribution of the light intensity in the direction of the cell induces a pitch gradient in the ChLCs, resulting in a broadband reflection with a bandwidth of 1080–2050 nm.

2.3. Light-Controllable Polymerization Rate

Polymerization rate is one of the important mechanisms to obtain pitch gradient or a non-uniform distribution of pitch in ChLC materials. As the structure of the molecular arrangement of the LC phase can be fixed by the photopolymerization reaction of liquid crystalline polymers, the polymerization rate of monomers can be adjusted by controlling the UV light intensity in the direction of liquid crystal cell thickness, thus, realizing the regulation of ChLC pitch. The stronger the light irradiation, the faster the monomer polymerization and the more monomer consumption. The non-chiral monomer diffuses in the direction of more consumption, which drives the chiral molecules to diffuse in the opposite direction to form a pitch gradient. However, when the rate of monomer polymerization is too fast, the monomer is no longer dominated by diffusion but preferentially fixed, which is not conducive to the formation of pitch gradient. In addition, other factors, such as polymerization monomer type, UV absorbing dye concentration, photoinitiator, sample thickness, polymerization temperature, UV light intensity, and other experimental conditions, have important effects on the polymerization rate, and adjusting these parameters can effectively regulate the reflection wavelength and bandwidth [36,37]. This method of light-controllable polymerization rate was first proposed by Dutch scientist Broer [38]. The material system of bifunctional chiral liquid crystal polymerizable monomer, monofunctional liquid crystal polymerizable monomer, ultraviolet light absorbing dye and photoinitiator was used for the first time. UV intensity gradient occurs in a certain direction due to the existence of the dye. The top of the cell polymerizes at a faster rate than the bottom. The polymerizable monomer diffuses to the top and chiral molecule diffuses to the bottom accordingly, thus, forming a pitch gradient. The reflectance spectrum can be effectively broadened by using multiple gradients of UV-induced polymerization in ChLCs and controlling the rate of photopolymerization. The intensity gradient of the UV light varies depending on the distance between the UV lamp and the cell, which affects the polymerization rate and leads to the formation of spiral structures with different pitches [39]. In addition, it was found that certain liquid crystal materials possess UV light absorption properties inherently. It is possible to create UV intensity gradients in the direction of the liquid crystal cell thickness without the use of dye, thus, triggering differences in polymerization rates. Mitov found that asymmetric irradiation of liquid crystal cassettes with weak UV light in small molecule liquid crystals, bifunctional liquid crystal polymerizable monomers, and photoinitiator material systems can induce pitch gradients in polymer networks. The concentration of the polymer network is higher near the UV source and lower away from it, resulting in a significant broadening of the selective reflectance from 80 nm to 220 nm, while the polymer network produced under symmetric irradiation has no significant gradient distribution and the reflection bandwidth is narrower than that of asymmetric irradiation [40]. Based on the mechanism of different polymerization rates to induce pitch gradient, Hu synthesized an isosorbide derivative chiral thiol molecule with double-terminal thiyl functional group (RIS) with high HTP, and prepared a PSCLC film with broadband reflection properties by thiol-acrylate click chemistry for the first time. The click reaction polymerizes faster than the free radical polymerization of the acrylic monomer [41]. The chiral compounds RIS containing two sulfhydryl groups and C6M can click chemistry under UV light irradiation, while the remaining C6M that did not undergo a click reaction can polymerize by itself. At the side closer to UV light, the light intensity is stronger and the polymerization rate is faster, so C6M and RIS diffuse to this site with a shorter pitch. Therefore, a pitch gradient is formed in the direction of UV irradiation to achieve broadband reflection. Compared with the acrylate-based ChLC film, this system has a wider reflection bandwidth under the same preparation conditions.

2.4. Thermally Induced Molecular Diffusion

The method of thermally induced molecular diffusion to produce a non-uniform distribution of pitch was first proposed by Mitov [3,42,43,44,45]. Liquid crystal oligomers of cyclosiloxane side chains with different ratios of chiral and non-chiral side chains were stacked together in an up-down way, and thermal diffusion occurred between the two layers after certain heat treatment; then, the films were rapidly cooled to the temperature below the glass transition to fix the gradient distribution of the pitch. Thus, single-layer ChLC polymer films reflecting the whole visible region were obtained [46]. This method is also applicable to PSCLC systems [44,45,47]. In theisr study, the powders with different pitch are randomly mixed in a certain ratio rather than in an up-and-down way. Yang proposed the “powder mixing method”, using glassy cyclosiloxane side chain liquid crystal polymers or polymerizable monomer/chiral compounds with crystalline phase and cholesteric phase transition. The powders with different pitches were mixed in a certain ratio and heated to liquid crystal phase to achieve the diffusion of material molecules [48]. It is a simple preparation procedure with high controllability of reflection spectrum and bandwidth by adjusting the powder components. On the basis of the above work, Yang mixed chiral dopants of different pitches with photopolymerizable monomers having a crystalline (Cr)- cholesteric phase transition in the solid state and heated them to the ChLC temperature range [49]. The thermal diffusion is accompanied by UV-irradiated polymerization in the parallel orientation state to form monolayer ChLC polymer films with an inhomogeneous pitch distribution structure.

2.5. Two-Phase Coexistence Material System of Cholesteric and Twist Grain Boundary (TGB)

Limited by the stability of the helical structure and the average refractive index of the liquid crystal material, it is difficult to form stable broadband reflection for ChLCs with wavelengths over 2000 nm, which severely limits its application in infrared light shielding covering the near-infrared, mid-infrared, and far-infrared wavelengths. To further broaden the reflection bandwidth, based on our preliminary work, wit can be constructed multi-layered microstructures with both cholesteric phase and TGB phase in the ChLC polymerizable material system and achieved a polymeric liquid crystal film with an ultra-wide reflection band [50]. This material system can form an SmA-like short-range ordering (SSO) structure during the transition from cholesteric phase to SmA phase, also called TGB phase, which has a larger pitch than the cholesteric phase and selective light reflection properties. Based on light-controllable polymerization rate and thermally induced molecular diffusion, the liquid crystal cassette was subjected to temperature gradient and light intensity gradient. The liquid crystals were transformed from cholesteric phase helical structure to SSO structure in the cell thickness direction, and polymer films with both cholesteric phase and TGB phase could be fixed after UV polymerization. Microstructure observation by scanning electron microscopy showed that the pitch of the film had an ultra-wide distribution, and the reflection spectrum of the film could cover 750–2500 nm. Similarly, Zhang prepared ultra-wide infrared reflective films by controlling the curing temperature and utilizing the difference in photopolymerization rates of different acrylate monomers and mesocrystalline phase structure transformation [51]. Due to the higher polymerization rate of the diacrylate monomer than the monoacrylate monomer, a gradient in pitch distribution is created in the thick direction of the liquid crystal cell with both cholesteric phase helical structure and SSO structure.