Research Progress of Electrically Driven Multi-Stable Cholesteric Liquid Crystals

Abstract

Electrically driven multi-stable cholesteric liquid crystals can be used to adjust the transmittance of incident light. Compared with the traditional liquid crystal optical devices, the multi-stable devices only apply an electric field during switching and do not require a continuous electric field to maintain the various optical states of the device. Therefore, the multi-stable devices have low energy consumption and have become a research focus for researchers. However, the multi-stable devices still have shortcomings before practical application, such as contrast, switching time, and mechanical strength. In this article, the latest research progress on electrically driven multi-stable cholesteric liquid crystals is reviewed, including electrically driven multi-stable modes, performance optimization, and applications. Finally, the challenges and opportunities of electrically driven multi-stable cholesteric liquid crystals are discussed in anticipation of contributing to the development of multi-stable liquid crystal devices.

Keywords: bistable mode; cholesteric phase; electrical driving mode; liquid crystals; multi-stability; multi-stable mode; optical devices; tri-stable mode.

Figure 1

Schematic diagram of bistable mode switching for positive dielectric liquid crystals.

Figure 2

Schematic diagram of bistable mode switching for negative liquid crystals. Adapted from Ref. [29].

Figure 3

(a) Transmission at 0 V as a function of the amplitude of the applied voltage pulse with the frequency of 2 kHz and (b) transmission at 0 V as a function of the amplitude of the applied voltage pulse with the frequency of 60 Hz. Adapted from Ref. [31].

Figure 4

(a) Schematic diagram of bistable mode switching for dual frequency liquid crystals and (b) voltage dependent electro-optic transmittance of device at frequency 1 kHz and 50 kHz. The consequential microphotographs of the planar, focal conic, and homeotropic textures in transmissive mode under crossed polarizer at 200× are shown in the inset. Adapted from Ref. [33].

Figure 5

Schematic diagram of bistable mode switching with homeotropic texture (A and B are the dielectric effect and the electrohydrodynamic effect, respectively).

Figure 6

(a) The transmittance under applied voltage (100 Hz) curve of the devices with various polymer concentrations (where the upward arrow indicates the change in transmittance with increasing voltage. The arrow pointing to the left indicates that the device can stabilize at a certain transmittance after removing the electric field) and (b) the transmittance under applied voltage (20 kHz) curve of the devices with various polymer concentrations. Adapted from Ref. [35].

Figure 7

Textures of three stable states: (a) planar state, (b) focal conic state, (c,d) ULH state with optical axis at 0° and 45° with respect to the polarizer. Adapted from Ref. [39].

Figure 8

(a) Schematic diagram of device, (b) physical diagram of the tri-stable device, and (c) transmission-spectrum of three stable states. Adapted from Ref. [39].

Figure 9

Schematic diagram of a liquid crystal cell with in-plane electrodes. Adapted from Ref. [50].

Figure 10

(a) Transmission spectra of the sample B1 without polymer networks and the sample B3 with polymer networks, (b) photographs of the opaque state and schematic diagrams of the focal conic state for sample B1 without polymer networks, (c) photographs of the opaque state and schematic diagrams of the focal conic state for sample B3 with polymer networks. Adapted from Ref. [66].

Figure 11

(a) Schematic diagram of devices, (b) the change of haze value and specular transmittance with time of the device, and (c) photographs of the fabricated ion-doped CLC cell placed on printed paper. Adapted from Ref. [74].

Figure 12

(a) Vertical switching between the planar and homeotropic states and (b) in-plane switching between the planar and in-plane-field-induced states. Adapted from Ref. [79].

Figure 13

Schematic diagram of the two-step polymerization process. (a) The initial state of the liquid crystals mixture, (b) the non-liquid crystal monomers are polymerized under ultraviolet light, (c) the liquid crystal molecules in the polymer are in focal conic texture, (d) the liquid crystal molecules are in planar texture under high frequency electric field, (e) the liquid crystal monomers are polymerized with the increase of temperature, (f) after the electric field is removed, the liquid crystals with planar texture remain stable under polymer anchoring force, (g) at low frequency electric field, liquid crystal molecules switch from planar texture to focal conic texture and this state can remain stable after the electric field is removed. Adapted from Ref. [83].

Figure 14

(a) Mechanical properties and (b) stability test. Adapted from Ref. [84].

Figure 15

Temperature variation with time in dye-doped bistable devices under different textures. Adapted from Ref. [86].

Figure 16

(a) Schematic diagram of dye-doped tri-stable devices and (b) physical diagram of colored mode devices. Adapted from Ref. [2].