Cheng, Ko-Ting

Prof. Ko-Ting Cheng’s research in the field of Display Technology includes the studies of the physical properties of liquid crystals and their relative materials, liquid crystal photoalignment, the applications and developments of liquid crystal electro-optical devices, and others.

Studies of the physical properties of liquid crystals and their relative materials

The researches of liquid crystal doped with azobenzenes / monomers / polymers / nanoparticles / micron-particles / gels have been widely paid much attention to by not only the academics and industries in these recent decade. It is exactly that the basic researches are the studies of their physical properties. With the funding supports for our researches from MOST and industries, as well as the cooperation with other groups, we have recently clarified the electro-optical properties of the described materials into various liquid crystals. Also, we have extended the proposed mechanisms to development of novel liquid crystal electro-optical devices. The azobenzene materials, polymers and nano-particles will be described in detail in the following part.

(1) Chiral azobenzene dopants: The electro-optical properties of the materials can be optically tuned directly. With a few doping of the dopant into liquid crystals, the properties of optically controllable reflection wavelength can be obtained, as shown in Fig. 1. It shows that the reflection band can be red-shifted by shinning with UV light, and can be blue-shifted by illumination with green light. Such a technique can be applied to fabricate multi-stable liquid crystal devices.

Figure 1 Variations in the transmission spectra of chiral azobenzene-doped LCs during illumination with (a) UV light (8.3 mW/cm², red shifting) and (b) green light (5.3 mW/cm², blue shifting) at various durations.

(2) Blue phase liquid crystal: Blue phase liquid crystal, existing between isotropic state and cholesteric liquid crystals, can be generated by slow cooling (<0.5 ^oC/min) from isotropic state. Optically, blue phase liquid crystal shows optical isotropic so that its optical properties are extremely different from those of nematic liquid crystals. We have recently produced a binary liquid crystal states, including blue phase liquid crystal and isotropic state at a temperature in a liquid crystal cell, as shown in Fig. 2. It can be adopted to fabricate a blue phase liquid crystal lens, grating, and so on.

Figure 2 Textures of blue phase liquid crystal sample observed under a cross-polarizer POM at30 °C. The right-hand region (A_UV) is illuminated with UV light at30 °C, whereas the left-hand region (A_dark) is not illuminated. The black dashed lines indicate the edge of the used photomask.

(3) Polymers: In this section, I’d like to introduce the properties of one kind of polymer, poly (n-vinyl carbazole) (PVK) in liquid crystal. We have examined the effects of thermal and optics onto the material, PVK, in detail. The schematic diagram of the proposed mechanism, particular thermally induced phase separation, is depicted as Fig. 3. In brief, the coated PVK layer onto the substrates will be dissolved into liquid crystal when the temperature is higher than the clearing temperature of the used liquid crystal. At this moment, the liquid crystal cell shows transparent. Subsequently, the heated liquid crystal cell is cooled to room temperature. During cooling, the thermally induced phase separation occurs, and the rough PVK films reform. Finally, a scattering LC cell (with gray scales) can be obtained. Hence, the reported particular TIPS involves a combination of dissolution of PVK into liquid crystal s and thermally induced phase separation.

Figure 3 (a) initially transparent state; (b) with thermal treatment (setting temperature is higher than the clearing temperature of the used liquid crystal) and (c) Multi-domain scattering mode light shutter after phase separation of PVK and liquid crystals.

Applications and developments of liquid crystal electro-optical devices

Recently, the researches of liquid crystal electro-optical devices in our group include scattering mode liquid crystal shutters, the development of liquid crystal display techniques, liquid crystal light modulators, liquid crystal lenses, liquid crystal gratings, multi-viewing-angle liquid crystal devices, liquid crystal flexible electronics, and others. The following will show you the relative researches about our liquid crystal electro-optical devices in recent years.

(1) Scattering mode liquid crystal shutters: The researches in this part are based on the effects of thermal and optics onto the employed polymer, PVK. We have demonstrated several modes of scattering mode liquid crystal light shutters. Some of the results have been published in SCI journals, they are Opt. Express 20, 16777 (2012), Opt. Express 20, 26252 (2012), Opt. Express 21, 18492 (2013), Sci. Adv. Mater. 6, 36 (2014), Opt. Express 22 4404 (2014).

(a)                                                            (b)

(c)                                                            (d)

Figure 4 Developments of electrically controllable and all optically controllable liquid crystal display devices based on the mechanism shown in Fig. 3. (a) Variations in stable transmission in relation to the temperature during heating (black dots) and cooling (red squares) of the LC sample fabricated from two non-rubbed PVK-coated glass substrates. The LC sample at25 °C; Inset (a) before (transparent) and, inset (b) after (scattering) thermal treatment via the particular TIPS; (b) Measured transmission of the fabricated scattering mode LC light shutter as a function of an applied AC (1 KHz) voltage. Insets show photographs of the LC light shutter at25 °Cwith the applied AC voltages of inset (a) 0 and inset (b) 18 V; (c) Blue squares and red circles show the scattering performances as a function of the polarization state of the probe beam based on the optically treated DDLCs cell. The insets show the scattering patterns, when the polarized laser beam with (a) [(c)] 0° and (b) [(d)] 90° polarization (normally incident), penetrates through the DDLC cell; (d) Gray-scale images of the LC cell obtained using a digital camera after UV irradiation (7.4mW/cm²) of the scattering mode LC cell (prepared by illumination with a DPSS laser at an intensity of 98 mW/cm² for 60 s) for (a) 15, (b) 30, (c) 60, and (d) 90 s.

(2) All optically controllable liquid crystal light shutters: The studies of demonstration of all optically controllable liquid crystal shutters in this part are based on the effects of UV and visible light onto the chiral azobenzene dopants. Some of the results have been published in SCI journals, including Opt. Express 21, 21840-21846 (2013), Appl. Phys. Lett. 103, 101105 (2013).

Figure 5 An optically activated and multistable shutter. Photographs of the device taken on a backlight before (a) and after (b) illumination with UV light (l ~365 nm) through a photo-mask.

Figure 6 Images of fabricated optically patternable, erasable, and rewritable cholesteric LC display device photographed with a digital camera. (a) Initially transparent LC cell, (b) “NCKU” optically addressed via green light, (c) patterns erased LC cell, and (d) “LC” optically re-addressed.

(3) Photoalignment of liquid crystal and flexible liquid crystal electronics: Photoalignment based on the adsorption effect of methyl red molecules onto substrates is the main mechanism for this research. The desired patterns can be patterned onto the cell by illuminating with green light. The published SCI paper is Appl. Optics 50, 213-217 (2011).

Figure 7 Image of the fabricated photo-rewritable and flexible liquid crystal display device after being bent to a radius of curvature of65 mm.

(4) Blue phase liquid crystals and their applications: The first study about blue phase liquid crystal is to produce a binary structure, including blue and isotropic phases, in one cell. The application is electrically controllable liquid crystal gratings. Additionally, it can be extended to produce other applications, such as liquid crystal lens, etc. The published SCI paper is J. Appl. Phys. 111, 013114-1-5(2012).

(a)                                                                             (b)

(c)

Figure 8 (a) binary structures observed under polarized optical microscope; (b) Measured first order diffraction efficiency as a function of applied AC (1 KHz) voltage, and (c) diffraction patterns of the grating when various AC voltages are applied.