A high-definition vertically aligned liquid crystal (LC) microdisplay exhibits a excellent contrast ratio, but its fringing field effect splits the bright state unevenly and leads to a very slow response time. By utilizing a circularly polarized light instead of conventional linearly polarized light, we have overcome the long-standing problems of poor sharpness, low brightness, and slow response time. Confirming computer simulations agree with the experimental results well. This approach can be applied to both reflective and transmissive LC microdisplays

Chen, Shu-Hsia

Fan-Chiang, Kuan-Hsu

Wu, Shin-Tson

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2005 High-definition vertically aligned liquid crystal microdisplays using a circularly polarized light Kuan-Hsu Fan-Chiang Shu-Hsia Chen 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 Fan-Chiang, Kuan-Hsu; Chen, Shu-Hsia; and Wu, Shin-Tson, "High-definition vertically aligned liquid crystal microdisplays using a circularly polarized light" (2005). Faculty Bibliography 2000s. 5178. https://stars.library.ucf.edu/facultybib2000/5178 High-definition vertically aligned liquid crystal microdisplaysusing a circularly polarized lightKuan-Hsu Fan-Chianga and Shu-Hsia ChenDepartment of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University,Hsinchu 300, Taiwan, Republic of ChinaShin-Tson WuCollege of Optics and Photonics, University of Central Florida, Orlando, Florida 32816Received 8 February 2005; accepted 20 June 2005; published online 14 July 2005A high-definition vertically aligned liquid crystal LC microdisplay exhibits a excellent contrastratio, but its fringing field effect splits the bright state unevenly and leads to a very slow responsetime. By utilizing a circularly polarized light instead of conventional linearly polarized light, wehave overcome the long-standing problems of poor sharpness, low brightness, and slow responsetime. Confirming computer simulations agree with the experimental results well. This approach canbe applied to both reflective and transmissive LC microdisplays. © 2005 American Institute ofPhysics. DOI: 10.1063/1.1999837Liquid crystal LC microdisplays have been widelyused for data projectors, large-screen rear-projection televi-sions RPTVs, and near-to-eye NTE virtual projections.1Several active matrix technologies2 have been developed, in-cluding high temperature polycrystalline silicon HTPS, lowtemperature polycrystalline silicon LTPS, and liquid-crystal-on-silicon LCOS devices. The ever-increasing de-mands on high resolution and high contrast ratio lead tosmall pixel sizes 10 m and interpixel gaps. As the in-terpixel gap is reduced, the lateral components of the fring-ing fields generated by the voltage difference between theadjacent pixels play a critical role to the optical performance.The influences of the fringing-field-effect in different LCmodes have been investigated previously.3–5 Among them,the vertically aligned VA LCOS is particularly promisingbecause of its inherent high contrast ratio. However, the lightloss of a VA-LCOS due to the fringing-field-effect is rela-tively high 30%  when a linearly polarized light inemployed;3,6 this device is abbreviated as LPVA cell for con-venience. Hence, the sharpness and brightness of the dis-played images are deteriorated tremendously. Moreover, theoptical transition time of the LPVA LCOS switched from thealternate bright and dark states to the all-bright state is ex-tremely slow which results in blurred images.7In this letter, we demonstrate the circularly polarizedlight illuminated vertically aligned CPVA liquid crystal cellwhich not only preserves the light in the presence of thelateral fringing field but also gives a fast optical switchingtime 10 ms to eliminate the blurring of the moving im-ages. We first simulate the LC directors distribution and op-tical performances of the CPVA and the LPVA cells and thenapply the de Vries theory8 to interpret these optical proper-ties. Finally, we present the confirming experimental resultsusing a LCOS device.Although our analyses are concentrated on the reflectiveLCOS devices because of their severe fringing-field-effect,these treatments can be applied equally well to the transmis-sive microdisplays, such as HTPS and LTPS panels. Figures1a and 1b depict the optical configurations used for theCPVA and LPCA device studies, respectively. As shown inFig. 1a, the incident light is linearly polarized by a polar-izing beam splitter PBS along the x axis. The light willthen traverse through the LC layer, whose rubbing directiondeviates from the x-axis by 45°. The reflective pixel elec-trodes fabricated in the device will reflect the light back tothe LC layer. After passing the LC layer the second time, theoutgoing light will be analyzed by the PBS and directed tothe detector. For the CPVA device, a broadband circular po-larizer is used instead of a PBS, as shown in Fig. 1b. There-fore, the incident light is circularly polarized and modulatedby the LC cell. After traversing through the LC layer twice,the light will be analyzed by the same circular polarizer anddirected to the detector. For the following simulations andexperimental results, the LCOS panel is operated at the al-ternate bright and dark states with the on-state voltage Von=5 V and the off-state voltage Voff=0 V. Here the turned-onvoltage corresponds to the maximum reflectance. The pixelsize is 11.5 m, the interpixel gap is 0.5 m, and the cellgap d=2.6 m. The LC pretilt angle 0=88° and the LCmaterial used for simulations is Merck MLC-6608; its ex-traordinary refractive index ne=1.5578, ordinary refractiveindex no=1.4748, and dielectric anisotropy =−4.2.In order to study the relation between the director profileand the optical properties, the two-dimensional 2D LC di-rector distribution was calculated first by the commercialsoftware 2dimMOS from autronic-MELCHERS GmbH us-ing the finite-element method. The 2D reflectance profileswere then calculated by the Jones Matrix method.9,10 Figure2 presents the simulated results of the LC director distribu-aElectronic mail: simonf.eo89g@nctu.edu.twFIG. 1. Schematic drawing of the systems used for a the LPVA deviceand b the CPVA device. PBS=polarizing beam splitter; P=polarizationaxis, R is the LC alignment direction at the front surface.APPLIED PHYSICS LETTERS 87, 031110 20050003-6951/2005/873/031110/3/$22.50 © 2005 American Institute of Physics87, 031110-1tion and the corresponding reflectance profiles, RCPx topand RLPx middle, at the dark-bright-dark state bottomfor the CPVA and the LPVA systems and Fig. 3 shows thecalculated azimuthal angle, , of the LC directors along thez direction at the specified x positions, x=x1, xa, xb, and x2as denoted in Fig. 2. From Fig. 2, the fringing fields pen-etrate into the voltage-off pixels from both right and leftedges of the voltage-on pixel. Without the electric field, thedirector n of this vertically aligned cell is parallel in a planewith an azimuthal angle of 45° from the x axis. With the fieldon, the LC directors on the substrate surfaces are pinned dueto the strong anchoring boundary condition. The largest polarangle appears near the middle plane because the negativetype 0 LC molecules tend to orient themselves per-pendicular to the electric field. Therefore, the director profilecan be regarded as a screw-like structure with its helical axisalong z axis and have opposite rotational sense in the upperand lower parts.With regard to the optical properties, it is shown in Fig.2 that there are two LC domains separated at xb, where thedark line occurs on the bright pixel of the LPVA system. Atxb, nearly all the directors are aligned in the y direction. Onthe left and right sides, the directors have their maximumtwist angles smaller and larger than 45°, respectively. In theLPVA cell, a reflectance peak is displayed in each domain,i.e., RLPxa and RLPx2, while RLPxb displaying a mini-mum reflectance. The RLPx is lying between RLPxa whichis RLPx2, and RLPxb in the rest area of the voltage-onpixel. The accumulated light loss is above 30%. On the con-trary, RCPx is not influenced by the director distortions inthe x–y plane leading to a high sharpness and high bright-ness image in the CPVA cell.As mentioned above, the LC director profile can be re-garded as a screw-like structure with spatially varying pitch,Pz. Some studies have been made in such structures withdifferent models of Pz.11,12 Here we interpret the opticalproperties qualitatively by de Vries theory8 for simplicity.Assuming a wave propagates along the helical axis of anideal helical structure with a fixed pitch P0. In general, thereare two eigenwaves propagating in the same direction. Theireigenvalues li , i=1,2, satisfy the dispersion relation whichcan be derived from solving the Maxwell’s equations supple-mented by the constitutive equation:13,14 −k02+ li2+q022−4q02lk2−k14=0, where k02= 2 /2ne2+no2 /2, k12= 2 /2ne2−no2 /2, q0=2 / P0, and  is the wavelength of the inci-dent light. When P0 and  are known, one can obtain theellipticity of the eigenwaves as:	 =2liq0±k14 + 4P02li2 − k12, 1where li=k02+q02±4k02q02+k14. The eigenwaves, whose po-larization ellipse axes are either parallel or perpendicular ton, have their phase difference accumulated to l1− l2z. FromEq. 1, if P0P0ne−no, then 	→ ±1. In this regime,the eigenwaves are almost circularly polarized, which meansthe oscillation direction of a linear wave will be rotated afterpassing the LC layer. The optical rotatory power the amountof rotation per unit length is given as =k14 / 8q0k02−q02.Also from Eq. 1, if P0ne−no, then 	→0 or . In thisregime, the eigenwaves are almost linearly polarized at thedirection parallel or perpendicular to n.In the CPVA system, the light is preserved in the regionof x1xx2 as presented by the reflectance profile Rcpx inFig. 2. This can be understood easily by the above argumentswith an approximated model, which is a nontwisted LC layersandwiched between two twisted layers. In Fig. 3, thetwisted regimes near the substrates 0zdb and d−dtzd have their effective pitch P0= 2 /dbdb2 /d−dtdt, where z denotes the twist angle ofthe LC director at z and is defined as the difference of azi-muthal angles between that LC director and the surface LCdirector. By using Eq. 1, the eigenwaves in these regimesare almost circularly polarized for the visible light. The inci-dent circularly polarized light remains its polarization stateafter passing the top-twisted regime. Note that a circularlypolarized light can be regarded as the superposition of anytwo orthogonal linearly polarized waves with the same am-plitude and  /2 phase difference. The nontwisted uniformregime dbzd−dt, as shown in Fig. 3, with linearly po-larized eigenwaves provides a phase difference, say = /2,such that the resulted light is linearly polarized in the direc-tion at 45° from n. The reflected light that traverses throughthe bottom-twisted regime twice forward and backwardwill have the same linear polarization. Then the uniform re-gime changes it to be circularly polarized by gathering an-other  /2 phase change. The polarization state of the outgo-ing circular wave will again not be influenced aftertraversing through the top-twisted regime. Finally, almost allof the light passes through the broadband circular polarizer.In other words, the CPVA system preserves the input lightefficiently.On the contrary, for the LPVA system the input light islinearly polarized in the x direction. The top-twisted regimeFIG. 3. Calculated azimuthal angle, , and twist angle, =−45°, of theLC directors along the z direction at x=x1, xa, xb, and x2 as denoted inFig. 2.FIG. 2. Simulated LC director distribution and the corresponding reflectanceprofiles, RLPx and RCPx, at the alternate dark and bright state for theCPVA and the LPVA systems, respectively. The on-state voltage Von=5 Vand the off-state voltage Voff=0.031110-2 Fan-Chiang, Chen, and Wu Appl. Phys. Lett. 87, 031110 2005will slightly rotate the plane of polarization. The nontwistedregime will generally change the light to an elliptically po-larized light. The characteristic of the polarization ellipsedepends on the angle between the local director n and theentering linear wave. At x=xb, most directors are alignedparallel in the y direction. Since the incident light of thenontwisted region is almost linearly polarized in the x direc-tion, one of the linear eignewaves has almost zero amplitudewhile the other remains oscillating in the x direction afterhaving passed through this region. The reflected light thattraverses through the bottom twisted regime twice will main-tain the same polarization. i.e., along the x direction. Thelight will then traverse through the uniform and the top-twisted regions the second time with its polarization almostunchanged as already discussed above. Eventually, the out-going light will be blocked by the analyzer resulting in aminimum reflectance, RLPxb. At x=xa as shown in Fig. 3,most directors without twist deformation are aligned in the45° direction with respect to the x axis. Therefore, the twolinear eigenwaves have almost the same amplitudes in thisregime. By gathering a  phase change after passing throughthe LC layer twice, the outgoing light will be linearly polar-ized at the direction nearly perpendicular to that of the inci-dent light, which results in a maximum reflectance RLPxa.The reflectance for the rest area can be deduced from thesame model and the results are in between RLPxa andRLPxb.Figures 4a and 4b depict the experimental resultswhen the panel is operated at the alternating bright and darkstates for the LPVA and the CPVA devices, respectively. Theimages were captured by CCD through an optical polarizingmicroscope. It is clearly shown that the image of the LPVAdevice is degraded by the fringing-field-effect severely. Thedark lines in the bright pixels originate from the distortion ofthe LC director profile illustrated in Figs. 2 and 3. By con-trast, Fig. 4b shows that the dark lines are totally elimi-nated in the CPVA device. Therefore, the sharpness and thebrightness of the displayed images are enhanced signifi-cantly.The CPVA optical system not only improves the staticperformance but also dramatically reduces the dynamic re-sponse time of the LCOS panel. Figures 4c and 4d showthe photos of the LC panel switched from the alternate brightand dark states to the all-bright state for the LPVA and theCPVA devices, respectively. The elapsed time after switchingis 198 ms in Fig. 4c and 33 ms in Fig. 4d. The distortedLC directors take an extremely long time to relax back to theoriginal state. The dark lines in the LPVA device last for atleast several hundred milliseconds after switching whichcauses a serious image blurring effect. Since the distorted LCdirectors are mainly rotating in the x-y plane during the re-laxation process, the influence on the optical response of theCPVA device is very small. Figure 5 shows the measuredreflectance with respect to the elapsed time after switchingfrom the alternate bright and dark states to the all-bright statefor the CPVA device. The result indicates that the transitiontime is less than 10 ms.In conclusion, we have demonstrated a circularly polar-ized light illuminated vertically aligned LCOS microdisplay.The outstanding problems of poor sharpness and low bright-ness in the conventional device illuminated by a linearly po-larized light are overcome. Furthermore, the dynamic transi-tion time of the CPVA device switched from the alternatebright and dark states to the all-bright state is less than 10ms, which is adequate to eliminate the blurring of movingimages in the LPVA device. The CPVA system preserves thesame high contrast ratio 1000:1 as the LPVA. The opti-cal properties are interpreted qualitatively by the de Vriestheory. Potential applications of this device for high contrastand high optical efficiency reflective LCOS and transmissivep-Si TFT-LCD projection displays are foreseeable.This work was partially supported by the National Sci-ence Council, R.O.C., under Contract No. NSC 93-2112-M-009-022. The authors are indebted to Dr. Xinyu Zhu UCFfor useful discussions and Himax Technologies, Inc., for of-fering the experimental samples.1E. H. Stupp and M. S. Brennesholtz, Projection Displays Wiley, NewYork, 1998.2E. Lueder, Liquid Crystal Displays Wiley, New York, 2001.3K. H. Yang and M. Lu, Displays 20, 211 1999.4H. De Smet, J. Van den Steen and A. Van Calster, SID Tech. Dig. 32, 9682001.5K. H. Fan Chiang, S. T. Wu, and S. H. Chen, Jpn. J. Appl. Phys., Part 141, 4577 2002.6Y. Ji, J. Grandhi and M. E. Stefanov, SID Tech. Dig, 30, 750 1999.7S. Zhang, M. Lu, and K. H. Yang, SID Tech. Dig, 31, 898 2000.8H. de Vries, Acta Crystallogr. 4, 219 1951.9R. C. Jones, J. Opt. Soc. Am. 31, 488 1941.10S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays Wiley, NewYork, 2001.11Z. Lu, L. Li, H. Vithana, Y. Jiang and S. M. Faris, Mol. Cryst. Liq. Cryst.Sci. Technol., Sect. A 301, 237 1997.12S. Kutter and M. Warner, Eur. Phys. J. E 12, 515 2003.13P. G. de Gennes and J. Prost, The Physics of Liquid Crystals Clarendon,Oxford, 1993.14P. C. Yeh and C. Gu, Optics of Liquid Crystal Displays Wiley, New York,1999.FIG. 4. Images captured by CCD through the polarizing microscope of theLC panel operated at the alternate bright and dark state for a the LPVA andb the CPVA devices. Images of the LC panel switched from the alternatebright and dark state to the all-bright state for c the LPVA device at 198 msafter switching and d the CPVA device at 33 ms after switching.FIG. 5. Measured reflectance with respect to the elapsed time after switch-ing from the alternate bright and dark state to the all-bright state for theCPVA device.031110-3 Fan-Chiang, Chen, and Wu Appl. Phys. Lett. 87, 031110 2005

High-definition vertically aligned liquid crystal microdisplays using a circularly polarized light

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High-definition vertically aligned liquid crystal microdisplays using a circularly polarized light

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