The electro-optic properties of a 90 degrees twisted-homeotropic liquid crystal (LC) cell with a chiral dopant whose handedness is opposite to the LC twist are studied. In the voltage-off state, the LC directors exhibit a homeotropic alignment. However, in the intermediate voltage state, the bulk LC directors behave like a homogeneous alignment due to the balanced torques between the electric field, surface anchoring, and reversed chiral dopant. Potential applications of this mode for liquid crystal displays, especially for dual-cell-gap transflective liquid crystal displays, are emphasized

Ge, Zhibing

Lee, Ju-Hyun

Wu, Shin-Tson

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2007 Electro-optic properties of a homeotropic liquid crystal cell with 90 degrees surface rubbings and a reverse-handed chiral dopant Ju-Hyun Lee University of Central Florida Zhibing Ge University of Central Florida Shin-Tson Wu University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Lee, Ju-Hyun; Ge, Zhibing; and Wu, Shin-Tson, "Electro-optic properties of a homeotropic liquid crystal cell with 90 degrees surface rubbings and a reverse-handed chiral dopant" (2007). Faculty Bibliography 2000s. 7338. https://stars.library.ucf.edu/facultybib2000/7338 Appl. Phys. Lett. 90, 201108 (2007); https://doi.org/10.1063/1.2739412 90, 201108© 2007 American Institute of Physics.Electro-optic properties of a homeotropicliquid crystal cell with 90° surface rubbingsand a reverse-handed chiral dopantCite as: Appl. Phys. Lett. 90, 201108 (2007); https://doi.org/10.1063/1.2739412Submitted: 07 April 2007 . Accepted: 21 April 2007 . Published Online: 14 May 2007Ju-Hyun Lee, Zhibing Ge, and Shin-Tson WuElectro-optic properties of a homeotropic liquid crystal cell with 90°surface rubbings and a reverse-handed chiral dopantJu-Hyun Lee, Zhibing Ge, and Shin-Tson WuaCollege of Optics and Photonics, University of Central Florida, Orlando, Florida 32816Received 7 April 2007; accepted 21 April 2007; published online 14 May 2007The electro-optic properties of a 90° twisted-homeotropic liquid crystal LC cell with a chiraldopant whose handedness is opposite to the LC twist are studied. In the voltage-off state, the LCdirectors exhibit a homeotropic alignment. However, in the intermediate voltage state, the bulk LCdirectors behave like a homogeneous alignment due to the balanced torques between the electricfield, surface anchoring, and reversed chiral dopant. Potential applications of this mode for liquidcrystal displays, especially for dual-cell-gap transflective liquid crystal displays, are emphasized.© 2007 American Institute of Physics. DOI: 10.1063/1.2739412The 90° twisted-nematic TN liquid crystal LC cell1has been widely used for notebook computers because of itssimple fabrication process, high optical efficiency, and weakcolor dispersion. The weak color dispersion originates fromthe polarization rotation effect of the twisted LC directors.2However, its contrast ratio and viewing angle are limited. Towiden viewing angle, a special discotic phase compensationfilm has been developed.3 For large screen liquid crystal dis-play LCD televisions, high contrast ratio and wide view areimportant. To achieve these goals, multidomain verticalalignment MVA mode note that VA is originally calledhomeotropic alignment4 has been developed.5,6 The MVAmode exhibits an excellent dark state which is insensitive tothe cell gap, wavelength, and temperature. However, due tothe birefringence effect the color shift of a MVA LCD atoblique angles becomes an important issue. Efforts to mini-mize color shift are being actively pursued.The electric field-induced TN structure in a VA cell hasbeen developed to combine the advantages of a VA mode forhigh contrast and a TN mode for weak color dispersions.7–11For simplicity, let us call this mode as chiral homeotropicCH cell. The CH cell is realized by doping a small amountof chiral compound to a negative dielectric anisotropy LC in a homeotropic cell. In the voltage-off state, the LCdirectors are aligned nearly perpendicular to the substratesexcept for a small pretilt angle in spite of the existence ofthe chiral dopant. The alignment layers on the top and bot-tom substrates are rubbed in orthogonal directions. As a re-sult, this boundary condition induces a twisted structure uponthe LC directors when the applied voltage exceeds a thresh-old Vth. The chiral dopant is chosen so that its induced twistdirection is the same as that induced by surface rubbing.However, the polarization rotation effect of the chiral ho-meotropic cell is weaker than that of a TN cell because itsboundary layers are aligned perpendicular to the substrates.Therefore, for the CH cell to achieve similar polarizationrotation effect to a TN cell would require a much thicker cellgap which results in a longer response time.In this letter, we demonstrate a modified CH mode toovercome the drawbacks observed in the conventional CHmode. Different from the conventional CH mode, our modeuses a chiral dopant whose helical sense is reversed to that ofthe LC twist induced by surface rubbings. For simplicity,hereafter we call this mode as reversed chiral homeotropicR-CH mode. Due to the reversed sense of the chiral dopant,our R-CH mode shows several attractive features. For in-stances, its voltage-off state between two crossed polarizersis as dark as that of a VA cell and its voltage-on state V3 Vth is like a homogeneous cell. A good dark state leadsto a high contrast ratio and enriched phase retardation leadsto a thinner cell gap which, in turn, reduces the responsetime. Moreover, when the R-CH mode is used in a dual-cell-gap transflective LCD,12 its voltage-dependent transmittanceVT and voltage-dependent reflectance VR curves overlapvery well.In order to examine the electro-optic characteristics ofthe R-CH mode, we performed numerical simulations andcompared results with those of TN, VA, and CH modes. TheLC material used in the simulations is ZLI-2293 for TN andMLC-6608 for VA, CH, and R-CH modes. Table I summa-rizes the parameters used in the simulations. The negativesign in d / p cell gap/chiral pitch length means that the twistdirection of the LC director exerted by the chiral dopant isopposite to that induced by the surface rubbings. Duringsimulations, we used MATLAB codes based on the finite ele-ment method to calculate the one dimensional static LC di-rectors profile and extended Jones matrix method13 to calcu-late the voltage-dependent transmittance and reflectance ofour transflective LCD.Figures 1a and 1b show the LC twist angle the tiltangle is similar to those of a CH cell11 and not shown hereunder various operation voltages for the conventional CHmode and the R-CH mode, respectively. The threshold volt-age of the cells is about 2.1 Vrms. In the CH mode Fig. 1athe LC twist angle increases monotonously along the cell gapdirection. However, in the R-CH mode, besides the boundaryregions the twist angle stays at 40°–50° in the bulk regionwhen the applied voltage is between 2.2 and 6.0 Vrms whichis 3 Vth. Therefore, in this voltage range the R-CH cellbehaves like a homogeneous cell. This effect originates fromthe balanced torques from the electric field, twisting powerof the chiral dopant, and the surface anchoring energy of theorthogonal boundary alignment layers. In the high voltageregime, the electric field is so strong that the chiral effectbecomes less significant. Under such a circumstance, the LCdirectors are governed mainly by the electric field and sur-aElectronic mail: swu@mail.ucf.eduAPPLIED PHYSICS LETTERS 90, 201108 20070003-6951/2007/9020/201108/3/$23.00 © 2007 American Institute of Physics90, 201108-1face anchoring effects. As a result, the twist angle increasesmonotonously along the cell gap direction, as shown in Fig.1b.In the voltage-off state, the R-CH cell behaves like a VAcell so that its dark state is excellent. In the 1 VthV3 Vth region, as shown in Fig. 1b, the bulk region be-haves like a homogeneous cell. By contrast, from Fig. 1athe conventional CH cell has a continuous twist across thecell gap. Therefore, the R-CH cell exhibits larger phase re-tardation than the CH cell for a given dn value. This effectis clearly illustrated in Fig. 2.Figure 2 shows the VT curves of three CH cells with d=5, 6, and 7 m and a R-CH cell with d=5 m. In the lowvoltage region, both cells exhibit vertical alignment so thattheir dark state is excellent at normal incidence. As the volt-age exceeds a threshold voltage Vth2.1 Vrms, the trans-mittance starts to increase. For the CH cells, it takes 7 mto reach the 100% transmittance as compared to 5 m forthe R-CH cell. This is because the R-CH cell exhibits morebirefringence effect than the CH cell. Due to the thinner cellgap requirement for the R-CH cell, its response time is fasterthan the corresponding CH cell.This enriched birefringence effect of the R-CH cellmakes it suitable for transflective liquid crystal display TR-LCD applications using dual cell gaps.14 In a TR-LCD, eachpixel is divided into two subpixels: one for transmission andanother for reflection. In the transmissive mode, the back-light passes through the LC layer once while in the reflectivemode the ambient light traverses the LC layer twice. To bal-ance the phase retardation, the cell gap for the reflectivepixels is made to be one-half of that of the transmissivepixels.Figure 3 shows the normalized transmittance and reflec-tance of a TR-LCD using dual cell gaps under crossed cir-cular polarizers. The surface reflections of all the opticalcomponents and absorption of polarizers are neglected. In aTABLE I. LC and cell parameter used for computer simulations.LC mode TN VA, CH, R-CHLC materials ZLI-2293 MLC-6608Elastic constant K11 ,K22 ,K33 pN 12.5, 7.2, 17.9 16.7, 7.0, 18.1Dielectric constants para ,perp 14.1, 4.1 3.6, 7.8Refractive indices =550 nm no ,ne 1.6312, 1.4880 1.5578, 1.4748Cell gap transmissive part d 3.3 m5.0 m for VA and R-CH5.0, 6.0, and 7.0 m for CHd / p p: helical pitch +0.25 0 for VA, +0.25 for CH, and −0.25 for R-CHRubbing direction bottom/top 0° +x axis /90° +y0°/90° for CH and R-CH0°/180° −x for VAOptic axis of polarizer bottom/top 0°/90°0°/90° for CH and R-CH45°/135° for VAPretilt angle  2° 88°Anchoring Strong anchoring in all the cells studiedFIG. 1. Color online Director profiles twist angle under various opera-tion voltages in a the conventional chiral homeotropic mode and b thereverse chiral homeotropic mode.FIG. 2. Color online Transmittance curves of the conventional CH modewith different cell gaps compared to the transmittance curve of the R-CHmode. The LC cells are sandwiched between two crossed linear polarizers.201108-2 Lee, Ge, and Wu Appl. Phys. Lett. 90, 201108 2007TR-LCD, a broadband circular polarizer is commonly usedin order to obtain a high contrast ratio for the reflectivemode. Three LC modes CH, R-CH, and VA are comparedhere. Their VT and VR curves are labeled as a and b,c and d, and e, respectively. For the VA cell shown inFig. 3e, the VT and VR curves overlap exactly because ofthe double cell gaps employed. From Figs. 3a and 3b, theVT and VR curves of the CH cell-based TR-LCD do notreach 100%. This is attributed to the twisted LC directorsand circular polarization employed. The polarization rotationeffect of a TN cell works well for a linearly polarized lightbut not well for a circularly polarized light.15 As a result, thetwisted structure of the LC layer under crossed circular po-larizers decreases the obtainable maximum transmittance andreflectance. On the other hand, the R-CH cell behaves like ahomogeneous cell in the voltage-on state. Thus, its VT andVR curves overlap quite well below V6 Vrms. Above6 Vrms, the transmittance decreases faster than the reflec-tance, similar to the CH mode. This result can be explainedfrom the LC director profile which goes back to the twistedstructure, as shown in Fig. 1b. In other words, in the R-CHcell, the birefringence effect is dominant in the low voltageregime while the twist effect reigns in the high voltage re-gime.It is known that birefringence effect has larger color dis-persion than the polarization rotation effect.15 Thus, the VAcell would exhibit the largest color dispersion, then R-CHcell, CH cell, and finally TN cell. To verify this intuition, wecompare the color dispersion of these four LC configurationsin transmissive modes without using any compensationfilms. Under normal incidence of a standard white light, wecalculated the color values, x and y, of the transmitted lightas a function of operation voltage ranging from 0 to 10 Vand plotted the results on the Commission Internationale del’Eclairage CIE color coordinate.As expected, the TN mode shows the weakest colorvariation which is inherited by the polarization rotation ef-fect. In contrast, the VA mode which uses pure birefringenceeffect shows the largest color variation because of its purebirefringence effect. The conventional CH mode shows aslightly larger color variation than the TN mode. In a CHmode, the voltage-on state has a surface region where the LCdirectors are not perfectly switched due to the surface an-choring force. These boundary layers contribute to the bire-fringence effect more than the polarization rotation effect.While in a TN cell, the surface regions contribute towaveguiding effect more than birefringence effect. The colorvariation of the R-CH mode lies between the CH mode andthe VA mode. Therefore, the R-CH mode has weaker colordispersion than the commonly employed VA cell in a TR-LCD.Wide view angle is another important requirement forTR-LCDs. The viewing angle of the single-domain TN, VA,CH, or R-CH cells is inadequate and thus four domains haveto be considered.16 The VA cell is known to have a muchhigher contrast ratio than TN. Among the VA, CH, and R-CHcells compared, the VA cell requires the thinnest cell gap.Thus, its response time is the fastest, provided that the sameLC material is employed. Nevertheless, its color dispersionis the strongest. On the other hand, the CH cell exhibits theweakest color dispersion, but its cell gap is the thickest.Therefore, the R-CH cell presents a good compromise be-tween response time and color dispersion.In conclusion, we have demonstrated a twisted-homeotropic cell with reverse-handed chiral dopant. Itsvoltage-off state is like a VA cell and voltage-on state like ahomogeneous cell. Its advantage over the TN, VA, or con-ventional chiral homeotropic cells is higher contrast ratio,weaker color dispersion, and smaller cell gap, respectively.This operation mode is particularly attractive for transflectiveLCDs employing dual cell gaps.The authors are indebted to the financial support fromChi Mei Optoelectronics.1M. Schadt and W. Helfrich, Appl. Phys. Lett. 18, 127 1971.2D. K. Yang and S. T. Wu, Fundamentals of Liquid Crystal Devices Wiley,New York, 2006.3H. Mori, J. Disp. Technol. 1, 179 2005.4M. F. Schiekel and K. Fahrenschon, Appl. Phys. Lett. 19, 391 1971.5Y. Koike and K. Okamoto, Fujitsu Sci. Tech. J. 35, 221 1999.6H. Kim, J. Song, S. Park, J. Lyu, J. Souk, and K. Lee, J. Soc. Inf. Disp. 1,3 2000.7F. Ogawa, C. Tani, and F. Saito, Electron. Lett. 12, 70 1976.8D. de Rossi and J. Robert, J. Appl. Phys. 49, 1139 1978.9S. W. Suh, S. T. Shin, and S. D. Lee, Appl. Phys. Lett. 68, 2819 1996.10J. S. Patel and G. B. Cohen, Appl. Phys. Lett. 68, 3564 1996.11S. T. Wu, C. S. Wu, and K. W. Lin, J. Appl. Phys. 82, 4795 1997.12M. Okamoto, H. Hiraki, and S. Mitsui, U.S. Patent No. 6,281,952 August28, 2001.13A. Lien, Liq. Cryst. 22, 171 1997.14X. Zhu, Z. Ge, T. X. Wu, and S. T. Wu, J. Disp. Technol. 1, 15 2005.15S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays Wiley, NewYork, 2001.16R. Lu, X. Zhu, S. T. Wu, Q. Hong, and T. X. Wu, J. Disp. Technol. 1, 32005.FIG. 3. Color online Normalized transmittance and reflectance curves offour LC modes as a function of operation voltage: a CH: VT curve filledcircle, b CH: VR curve filled triangle, c R-CH: VT curve opencircles, d R-CH: VR curve open triangles, and e VA: overlapped VTand VR curves solid line.201108-3 Lee, Ge, and Wu Appl. Phys. Lett. 90, 201108 2007

Electro-optic properties of a homeotropic liquid crystal cell with 90 degrees surface rubbings and a reverse-handed chiral dopant

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Electro-optic properties of a homeotropic liquid crystal cell with 90 degrees surface rubbings and a reverse-handed chiral dopant

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