Optically Rewritable Liquid Crystal Display with LED Light Printer

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Optically Rewritable Liquid Crystal Display with LED Light Printer Man-Chun Tseng, Wan-Long Zhang, Cui-Ling Meng, Shu-Tuen Tang, Chung-Yung Lee, Abhishek K. Srivastava, Vladimir G.

1 Optically Rewritable Liquid Crystal Display with LED Light Printer Man-Chun Tseng, Wan-Long Zhang, Cui-Ling Meng, Shu-Tuen Tang, Chung-Yung Lee, Abhishek K. Srivastava, Vladimir G. Chigrinov and Hoi-Sing Kwok State Key Laboratory on Advanced Displays and Optoelectronics Technologies The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong Abstract Optically rewritable liquid crystal display (ORW-LCD) is an electronic paper display invented at the HKUST SKL. The azimuthal alignment direction of the ORW- LCD can be reoriented by an external polarized light source. In this paper, a TFT based polarizationcontrollable light-printer with 420nm LED light source for ORW-LCD is proposed. Experimental results with gray scale images are also presented. The azo dye molecules will be aligned in the direction perpendicular to the exposure light polarization direction. As a result, the LC structure is always stabilized by the alignment film. The ORW-LCD can be rewritten more than 10,000 times and the written information can be kept for a long time without the need of any electrical power. The ORW-LCD can be made on flexible e-paper device, Optical Rewritable Paper, based on the photo-rotation mechanism. possible; Easy implementation of gray scales; Ready to be flexible displays [1]. 1. Introduction ORW Technology: It is known that when linear polarized light is exposed to the azo-dye photoalignment material, the azo-dye molecules will be aligned along the direction perpendicular to the light polarization plane. In principle, the azo-dye molecules can orient at any azimuthal angle, therefore, there are several features of ORW technology that render it a high potential candidate for next generation e-paper: Written information on ORW display can be kept for a long time without the need of any electrical power (environmental friendly); Almost unlimited rewriting times (durable); Since ORW cell is optical addressed rather than electrical addressed, all electronic circuits on the LC display panel are abandoned (simple structure, ultra-high resolution and no aperture ratio issues which created by TFT and data bus ); Low cost but high resolution images are Figure 1. Mechanism of the ORW-LCD ORW Structure: The mechanism of an ORW LC cell is shown in Figure 1. It consists of two substrates, glass or plastic, with different aligning materials [2]. One aligning material is optically passive and it does not react to the exposure light. It provides fixed aligning direction on one substrate. Normal polyimide for TN LCD is used. Whereas the other aligning material is optically active, the azo-dye SD1 is used, and it changes its alignment orientation when being exposed to linear polarized light. The SD1 has been reported to provide excellent alignment quality for liquid crystal molecules with strong anchoring energy [3]. Alignment of liquid crystal layer by this material can be achieved by exceptionally low irradiation energy doses of about 50mJ/cm2. In addition, the required film thickness to align liquid crystal layer is thin and which is about 10nm. Moreover, it has extreme high patterning resolution for more than 75nm which is favorable for many photonic devices [4].The liquid crystal layer is designed to work in wave-guiding mode, hence a relatively thick LC layer is needed. At last, the assembled cell is put in

2 between polarizers for viewing. The ORW cell structure is very similar to passive LCDs, except that it even doesn t need ITO coated substrates, which makes that the larger panel it is, the more cost effective it is when compare with other e-paper technologies. It is compatible with conventional LCD manufacturing facilities. (c) Grayscale Generation: Several grayscale images generation methods on ORW LCDs have been proposed: At Eurodisplay 2015, we realized 8 bits grayscale images on ORW LCDs by using printed transparency as photo masks [7], which is based on the spatial gray levels method. In this method, gray levels depend on the ratio of DARK to BRIGHT area in each pixel (see Figure 2). change with enough thermal energy, it also requires highly uniform exposure system which will increase the cost of light-printer; H. Wu et al. printed a grayscale image on the ORW LCD by using DMD system [7]. However, they realized gray levels by rotating the polarization plane of incident light and this took much longer time to print gray level images. In this paper, we present the gray levels on ORW LCDs by different liquid crystal twist angles, which can be controlled by the linear polarized light coming from a polarization-controllable light-printer. The lightprinter is making use of a twist nematic (TN) TFT LCD panel as the polarization rotator. 2. The Polarization-Controllable Light Printer Principle: The transmission or gray levels of a twisted nematic LCD that works in the wave guiding mode is giving by the equation 2 I = cos θ (1) where θ is the angle between the analyzer and the output light polarization direction. Therefore, the gray level is obtained by the twist angle in every pixel. A light-printer works by outputting a controlled polarization direction that will be printed onto the azo-dye layer. The azo-dye layer is aligned perpendicular to the polarization direction and this determines each pixel s twist angle of the ORW cell. A TN cell on its own can provide gray level images by controlling the wave guiding efficiency of twist liquid crystal before the analyzer. A desired linear polarized light can be generated by at certain applied voltage. Figure 3 shows the schematics of a modified TN cell polarization rotator. Incident non-polarized light, coming from the left, passes through two basic elements: linear polarizer and TN LC cell. Figure 2. Gray level generated by tuning the dark and bright area ratio of the mask. Image printed on ORW cell by spatial method. However, one drawback is that we have to prepare photo masks every time in advance. It is good only for photocopying many ORW cells; L. Wang et al. exposed the photo-alignment layer with different time durations [6]. But the gray levels are subjected to Figure 3. Optical Schematic of Polarization Rotator.

the ORW LCD independently and simultaneously. Figure 4. Polarization Rotation versus Voltage.

linearly by controlling the applied voltage.

The red line is theoretical calculation result bases on measured retardation of the TN LC cell.

A mono TFT is required because color filter will filter out the blue light for image writing.

7 diagonal, 320 240 resolution and pixel pitch is ~ 0.36mm.

Performance of the final ORW image obtained depends very much on the specification of the printer head. 3.

Figure 6 shows the procedures of using the lightprinter to rewrite the information into an ORW-LCD.

Optical Schematic of the Polarization- Controllable Light-Printer.

3 the ORW LCD independently and simultaneously. Figure 4. Polarization Rotation versus Voltage. Based on TN liquid crystal cell, a single-pixel polarization rotator can rotate the polarization plane from 0 to 90 linearly by controlling the applied voltage. The polarization plane rotation angle as a function of the applied voltage is given in Figure 4. The red line is theoretical calculation result bases on measured retardation of the TN LC cell. The core part of the light-printer head is the mono TN TFT LCD. A mono TFT is required because color filter will filter out the blue light for image writing. The mono TFT we used is a commercially available. It has a size of 5.7 diagonal, resolution and pixel pitch is ~ 0.36mm. The current unit can only support 4 bit (or 16) gray levels. Performance of the final ORW image obtained depends very much on the specification of the printer head. 3. Results By using the polarization-controllable light-printer described above, sample images are produced. Figure 6 shows the procedures of using the lightprinter to rewrite the information into an ORW-LCD. A user friendly computer interface is also designed for controlling the light-printer. Figure 5. Optical Schematic of the Polarization- Controllable Light-Printer. (d) Structure: Figure 5 shows optical schematic for the light-printer based on a polarization rotator. The 420nm LED module (1) is equipped with a convex lens (2) which focuses the light beam. After the Fresnel lens (3) and linear polarizer (4), a uniform linear polarized illumination with low divergence is achieved. The mono-color TN TFT LCD (5) plays the role of polarization plane controller. The image content is transferred to the ORW LCD (6) by this exposure system. Therefore, the polarizationcontrollable light-printer can print desired polarization plane, which gives exact gray level of each pixel on

(e) Figure 6. The Process flow of re-writing information to ORW cells by using the polarization-controllable lightprinter. Figure 6 shows that a picture is chosen through the computer interface.

The ORW cell is put into the light printer in Figure 6(c). After click the start button of the LED light source power controller, the rewrite process begins, which is shown in figure 6(d).

The image can be observed under the crossed polarizers (Figure 6(f)). More detail can be shown in the following link: https://youtu.

Conclusion We have successfully developed an electronic light printer system for the ORW applications. The printer head is a mono-color TN TFT LCD.

4 (e) Figure 6. The Process flow of re-writing information to ORW cells by using the polarization-controllable lightprinter. Figure 6 shows that a picture is chosen through the computer interface. The data of the chosen picture will be sent to TN TFT panel driver automatically. Then a normally dark (ECB mode) ORW-LCD is prepared in Figure 6. The ORW cell is put into the light printer in Figure 6(c). After click the start button of the LED light source power controller, the rewrite process begins, which is shown in figure 6(d). After the LED light turning off, the print process is finished. The picture is transferred to the ORW cell and it can be take out from the printer. The image can be observed under the crossed polarizers (Figure 6(f)). More detail can be shown in the following link: (f) substrate, a circular shape PET film ORW-LCD based on the photo-rotation mechanism is successfully made and the result is shown in Figure Conclusion We have successfully developed an electronic light printer system for the ORW applications. The printer head is a mono-color TN TFT LCD. Any digital content with gray scale can be printed on ORW LCDs by importing the content to the printer head. The image quality of the current system is limited by the pixel size and grayscale resolution of the TFT head. For further improvement, a higher resolution monocolor TFT LCD with more gray levels is highly desirable. 5. Impact The optically rewritable (ORW) LCD technology is demonstrated to be workable. Extremely simple structure, high image quality, large area potential, energy saving and low cost features make it a very competitive technology for large displays, signage and advertising boards. Moreover, 3-D ORW LCDs are also ready for real applications [8]. 6. Acknowledgement The support from GRANT ITC-PSKL12EG02 is gratefully acknowledged. Figure 7. Grey levels strips, portrait printed by the light-printer. Figure 8. Flexible ORW-LCD demo. Figure 7 shows the gray level strips. Figure 7 shows a portrait in gray levels which are transferred from the mono-color TFT LCD by the light-printer. Moreover, The ORW-LCD can be made on flexible References [1] A. Muravsky, A. Murauski, X. Li, V. Chigrinov and H. Kwok, Optical rewritable liquid crystal alignment technology, JSID, Vol 15, pp (2007). [2] A. Muravsky, A. Murauski, V. Chigrinov, and H. Kwok, New properties and applications of rewritable azo-dye photoalignment, JSID, Vol 16, pp (2008). [3] Man-Chun Tseng, O. Yaroshchuk, Tetyana Bidna, A. Srivastava, V. Chigrinov and H. S. Kwok, RCS Advances, [4] E. Shteyner, A. Srivastava, V. Chigrinov, and H. S. Kwok, Soft Mattr, 9, , [5] W. Zhang, J. Sun, A. K. Srivastava, V. G. Chigrinov, and H. S. Kwok, 3-D Grayscale Images Generation on Optically Rewritable Electronic Paper. SID Symposium Digest of Technical Papers, 46: 40 (2015). [6] L. Wang, J. Sun, A. Srivastava, and V. Chigrinov, Thermal Stability of Gray-scale Levels on Optical Rewritable Electronic Paper, IDW 11,

5 (2011). [7] H. Wu, W. Hu, H. Hu, X. Liu, G. Zhu, J. Choi, V. Chigrinov, Y. Lu, Arbitrary photo-patterning in liquid crystal alignments using DMD based lithography system, Optics Express, Vol. 20, No. 15 (2012). [8] J. Sun, A. K. Srivastava, W. Zhang, L. Wang, V. G. Chigrinov, H. S. Kwok, Optically rewritable 3D liquid crystal displays, Optics Letters, Vol. 39, Issue 21,pp, (2014).

A large bistable negative lens by integrating a polarization switch with a passively anisotropic focusing element

A large bistable negative lens by integrating a polarization switch with a passively anisotropic focusing element Hung-Shan Chen, 1 Yi-Hsin Lin, 1,* Abhishek Kumar Srivastava, Vladimir Grigorievich Chigrinov,