A fast response nematic liquid crystal (LC) modulator using dual fringe field switching (DFFS) mode is demonstrated. Both top and bottom substrates have pixel and common electrodes to generate complementary fringing fields. The cell consists of a vertically aligned LC layer whose dielectric anisotropy is positive. In a voltage-on state, self-imposed thin LC walls near the center of the slits and electrodes are formed and the surrounding LC molecules exert a strong restoring force. As a result, submillisecond gray-to-gray response time is achieved. Two DFFS cells are used as examples to illustrate the design principles for display and photonic applications. (C) 2008 American Institute of Physics

Choi, Wing-Kit

Ge, Zhibing

Jiao, Meizi

Wu, Shin-Tson

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2008 Submillisecond response nematic liquid crystal modulators using dual fringe field switching in a vertically aligned cell Meizi Jiao University of Central Florida Zhibing Ge University of Central Florida Shin-Tson Wu University of Central Florida Wing-Kit Choi 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 Jiao, Meizi; Ge, Zhibing; Wu, Shin-Tson; and Choi, Wing-Kit, "Submillisecond response nematic liquid crystal modulators using dual fringe field switching in a vertically aligned cell" (2008). Faculty Bibliography 2000s. 493. https://stars.library.ucf.edu/facultybib2000/493 Appl. Phys. Lett. 92, 111101 (2008); https://doi.org/10.1063/1.2896650 92, 111101© 2008 American Institute of Physics.Submillisecond response nematic liquidcrystal modulators using dual fringe fieldswitching in a vertically aligned cellCite as: Appl. Phys. Lett. 92, 111101 (2008); https://doi.org/10.1063/1.2896650Submitted: 14 February 2008 . Accepted: 21 February 2008 . Published Online: 17 March 2008Meizi Jiao, Zhibing Ge, Shin-Tson Wu, and Wing-Kit ChoiARTICLES YOU MAY BE INTERESTED INFast response wide viewing angle liquid crystal cell with double-side fringe-field switchingApplied Physics Letters 83, 5154 (2003); https://doi.org/10.1063/1.1635982Electro-optic characteristics and switching principle of a nematic liquid crystal cell controlledby fringe-field switchingApplied Physics Letters 73, 2881 (1998); https://doi.org/10.1063/1.122617Fast-switching initially-transparent liquid crystal light shutter with crossed patternedelectrodesAIP Advances 5, 047118 (2015); https://doi.org/10.1063/1.4918277Submillisecond response nematic liquid crystal modulators using dualfringe field switching in a vertically aligned cellMeizi Jiao,1 Zhibing Ge,1 Shin-Tson Wu,1,a and Wing-Kit Choi21College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA2Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, TaiwanReceived 14 February 2008; accepted 21 February 2008; published online 17 March 2008A fast response nematic liquid crystal LC modulator using dual fringe field switching DFFSmode is demonstrated. Both top and bottom substrates have pixel and common electrodes togenerate complementary fringing fields. The cell consists of a vertically aligned LC layer whosedielectric anisotropy is positive. In a voltage-on state, self-imposed thin LC walls near the center ofthe slits and electrodes are formed and the surrounding LC molecules exert a strong restoring force.As a result, submillisecond gray-to-gray response time is achieved. Two DFFS cells are used asexamples to illustrate the design principles for display and photonic applications. © 2008 AmericanInstitute of Physics. DOI: 10.1063/1.2896650Fast response time is highly desirable for both displayand photonic devices employing a nematic liquid crystalLC. For LC displays LCDs, the targeted response time is3 ms in order to reduce motion blurs and color break upfor color sequential displays using a single LCD panel.1 Forphase-only spatial light modulators SLMs such as adaptiveoptics,2 the desired response time is less than 1 ms in orderto compensate for the fast-moving atmospheric turbulences.Response time of a LC device is governed by the cell gap,viscoelastic coefficient, anchoring energy, and operatingvoltage.3 To shorten response time, several approaches havebeen developed, e.g., low viscosity materials, thin cell gap,4,5overdrive and undershoot voltage,6,7 and elevated tempera-ture operation.8In addition to fast response time, high contrast ratio,wide viewing angle, low operating voltage, and high trans-mittance are all important for direct-view LCDs. To achievewide view and high contrast ratio, film-compensated multi-domain vertical alignment MVA has been widelyemployed.9 To realize the useful electro-optic effect of aMVA LCD, a negative dielectric anisotropy 0 LC hasto be used because the applied electric field is in the longi-tudinal direction. Several attempts using transversal fringingfield to drive a VA cell filled with a positive LC have alsobeen investigated10,11 to generate rubbing-free andprotrusion-free MVA mode.12,13 However, all of these ap-proaches show a relatively slow response time.In this letter, we demonstrate a dual fringe field switch-ing DFFS mode to achieve a fast response time. The LCemployed has a positive  and the LC layer is around10 m in order to accumulate sufficient phase change. Theelectrodes on both substrates are in complementary positionsin order to enhance the light efficiency and transmission uni-formity. Two cells are used as examples to illustrate theDFFS mode for wide view display and submillisecond re-sponse SLM applications.Figure 1 shows the device structure, electric field direc-tions, and LC director reorientations of the DFFS mode in avoltage-on state. To achieve a good dark state for wide-viewpurpose, the LC cell is configured between two crossedpolarizers and a set of compensation films. For phase-onlySLM applications, the compensation films and polarizers arenot required.In Fig. 1, the electrode structure on each substrate is thesame as a conventional fringe field switching LCD which hasa planar common electrode and slit pixel electrodes.14,15 Inthe DFFS mode, the electrode width W and electrode gapG are around 2–5 m, and the LC cell gap d is around10 m, depending on the LC material employed. The topand bottom pixel electrodes are intentionally shifted by half apixel electrode width in order to form complementary do-mains on two boundary layers for enhancing the optical ef-ficiency and transmission uniformity.When there is no voltage applied, all the LC directorsare aligned vertically to the substrate. Therefore, the lightpassing through the LC layer experiences no phase retarda-tion and is blocked by the crossed analyzer, resulting in anexcellent dark state. On the other hand, when the appliedvoltage exceeds a threshold Vth, the LC directors are tiltedby the fringing fields and the phase retardation effect takesplace. In such a thick cell, the electric field near the substratesurface is much stronger than that in the middle of the cell.So only the boundary layers are reoriented while the middlebulk layers remain untouched. Moreover, the LC moleculesaElectronic mail: swu@mail.ucf.edu.FIG. 1. Color online Device structure of DFFS mode in a voltage-on state.The LC employed has a positive .APPLIED PHYSICS LETTERS 92, 111101 20080003-6951/2008/9211/111101/3/$23.00 © 2008 American Institute of Physics92, 111101-1at the center of each electrode and electrode gap are not tiltedbecause in these regions, the electric fields are too weak toreorient the LC directors. These unswitched LC directorsform disclination lines which function as boundary walls toimpose a strong restoring force, as shown in Fig. 1. There-fore, fast rise and decay times can be achieved.From Fig. 1, the LC directors form two domains neareach substrate surface because of the strong fringe fields. Ifwe use the zigzag electrodes, a four-domain structure will beformed which in turn contributes to wide viewing angle afterfilm compensation. Moreover, the LC reorientation shown inFig. 1 is similar to the bend structure of a pi cell16 in thelateral direction. This would minimize the backflow effectand lead to a fast response time.The structures and performances of the proposed DFFSmode are optimized using a commercial three-dimensionalLC simulator TECHWIZ developed by Sanayi and the opticalcalculation is based on the extended 22 Jones matrixmethods.17 To obtain fast response time, high transmittance,and wide viewing angle for display applications, we use cell1 as an example. The device parameters for cell 1 are listedas follows: cell gap d=14 m, electrode width W=3 m,and electrode gap G=3 m. The LC material used in simu-lations is basically the same as Merck E7 except for a lowerviscosity: K11=11.7 pN, K22=8.8 pN, K33=19.5 pN, n=0.223, =14.4, and 1=150 mPa s. For comparison, therotational viscosity of E7 is 1250 mPa s at room tempera-ture 22 °C. Some LC mixtures with similar birefringencebut with a lower viscosity 180 mPa s are commerciallyavailable. A lower viscosity leads to a faster response time.The voltage-dependent transmittance VT of cell 1 isshown as curve 1 in Fig. 2, whose threshold voltage is Vth2.5 Vrms, on-state voltage Von13 Vrms, and maximumtransmittance Tmax91.8% normalized to the maximumtransmittance of two parallel linear polarizers. Meanwhile,we use cell 2 as an example for photonic applications. Thecell gap is d=11 m, W=2 m, and G=2 m. With de-creased electrode width and gap, the tilted LC layer shrinkscloser to the substrate, which means that fewer LC moleculescontribute to the phase retardation effect. Therefore, a higherbirefringence LC material is required to obtain a sufficientphase change. Curve 3 in Fig. 2 represents the VT curve forcell 2 using the E7-like material. Clearly, its phase retarda-tion is insufficient to reach 1 at 15 Vrms. Increasing cell gapdoes not lower the operating voltage much because it onlythickens the unswitched middle layer which makes no con-tribution to the overall phase retardation. A more effectiveway is to employ a high n LC mixture. The parameters forthe high birefringence LC material UCF-2 type18,19 usedin calculation are listed as follows: K11=17.3 pN,K22=10.4 pN, K33=38.3 pN, n=0.416, =18.27, and1=200 mPa s. The VT curve for cell 2 filled with UCF-2is shown as curve 2 in Fig. 2, where Vth3 Vrms andTmax93% at Von13 Vrms.The gray-to-gray GTG response times of cell 1 and cell2 were calculated and results are summarized in Tables I andII, respectively. During calculations, we divided the VTcurve uniformly into eight gray levels 1–8. Each of theGTG response time was calculated. Here, the response timeis defined as 10%–90% transmittance change. The cell gapsof the proposed two cells are optimized to achieve a fastresponse time. If the cells are too thin, the middle LC layerswill rotate slightly with a slower speed which, in turn, leadsto a slower GTG response time. On the other hand, if the gapis too thick, some problems such as narrow viewing angleand strong cross-talks between the neighboring pixels wouldoccur. The optimized cell gaps, which depend on the LCmaterial employed, are 14 and 11 m for cell 1 and cell 2,respectively. The average GTG rise time for cell 1 is 1.88 msand average GTG decay time is 1.68 ms, which are fastenough for color sequential displays. To further shorten re-sponse time, a lower viscosity LC or overdrive and under-shoot voltage method can be employed. By combining thesetwo effects, the response time is expected to be reduced byadditional approximately three to five times. For cell 2, theaverage GTG rise time is 0.91 ms and decay time is 0.68 msat room temperature. In comparison, a VA cell withd=4.64 m MLC-6608 has an average GTG rise time ofFIG. 2. Color online Simulated VT curves for proposed cells. Curve 1 isfor cell 1 with E7-like LC: d=14 m, W=3 m, and G=3 m; Curve 2 isfor cell 2 with high n LC material: d=11 m, W=2 m, and G=2 m;Curve 3, a comparison curve, is for a cell with E7-like LC: d=14 m,W=3 m, and G=3 m.TABLE I. Calculated GTG scale response time in unit of ms of cell 1.1 2 3 4 5 6 7 81 5.42 3.40 2.86 2.53 1.96 1.68 0.952 1.05 2.62 2.30 1.85 1.58 1.42 0.803 1.05 1.96 2.24 2.10 1.97 1.58 0.774 1.09 2.12 2.20 2.36 2.00 1.72 0.925 1.13 1.93 2.03 2.53 2.48 1.79 0.916 1.34 1.93 2.02 2.03 3.26 2.44 0.957 1.25 1.96 2.11 2.21 2.56 3.00 1.258 1.46 2.17 2.36 2.74 3.12 3.44 4.13111101-2 Jiao et al. Appl. Phys. Lett. 92, 111101 200838.8 ms and decay time of 42.1 ms.20 The response timeof our DFFS mode is 20 times faster.Cell 1 is used as an example for display applications. Toobtain a wide viewing angle, we use an A+ plate and a C−plate on each side of the cell. This configuration is quitecommon for wide view display devices.9 The optic axis ofthe A+ plate is parallel to the transmission axis of the polar-izer on the same side. The parameters of the compensationfilms used in our simulation are listed as follows: For A+plate, ne=1.5590, no=1.5866, and d=25.65 m; for C−plate, ne=1.50, no=1.65, and d=8.02 m.21 Figure 3 showsthe isocontrast contour plots of film-compensated cell 1. The100:1 isocontrast contour line extends beyond the 70° view-ing cone. This is adequate for most display applications, in-cluding televisions, monitors, and mobile devices. However,it is quite difficult to achieve a good dark state for cell 2because its dn is too large to be compensated precisely.With the employed large dn, the accumulated phase retar-dation is quite large even under a very small pretilt angleresulting in a non-negligible light leakage. Thus, cell 2 ismore suitable for laser beam steering and adaptive opticswhere the incident laser beam is nearly normal to the LCcell. The incident laser polarization should be set parallel tothe LC reorientation axis horizontal direction in Fig. 1 inorder to obtain phase-only modulation. For such applica-tions, the required phase change is 2. This can be achievedby using reflective mode, where the incident light traversesthe LC layer twice, or by increasing the LC birefringence.In conclusion, a submillisecond GTG response DFFS LCmode is demonstrated. This mode has potential applicationsfor displays and phase-only applications. For display appli-cations, the operating voltage needs to be decreased in orderto lower power consumption. To lower the on-state voltage,we need to optimize the electrode structures, cell gap, andLC parameters. For phase-only modulations, we need to in-crease the phase change to 2. This can be done by using areflective device, increasing the LC birefringence, or furtheroptimizing the device configuration.1J. H. Lee, X. Zhu, and S. T. Wu, J. Disp. Technol. 3, 2 2007.2D. Gu, B. Winker, B. Wen, D. Taber, A. Brackley, A. Wirth, M. Albanese,and F. Landers, Proc. SPIE 5553, 68 2004.3S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays Wiley, NewYork, 2001.4S. Gauza, X. Zhu, S. T. Wu, W. Piecek, and R. Dabrowski, J. Disp.Technol. 3, 250 2007.5M. Jiao, Z. Ge, Q. Song, and S. T. Wu, Appl. Phys. Lett. 92, 0611022008.6S. T. Wu and C. S. Wu, J. Appl. Phys. 65, 527 1989.7S. T. Wu, Appl. Phys. Lett. 57, 986 1990.8S. T. Wu, A. M. Lackner, and U. Efron, Appl. Opt. 26, 3441 1987.9Q. Hong, T. X. Wu, X. Zhu, R. Lu, and S. T. Wu, Appl. Phys. Lett. 86,121107 2005.10R. A. Soref, J. Appl. Phys. 45, 5466 1974.11W. K. Choi and S. T. Wu, U.S. Patent No. 7,298,445 B1 20 November2007.12S. H. Lee, H. Y. Kim, I. C. Park, B. G. Rho, J. S. Park, H. S. Park, and C.H. Lee, Appl. Phys. Lett. 71, 2851 1997.13S. H. Hong, Y. H. Jeong, H. Y. Kim, H. M. Cho, W. G. Lee, and S. H. Lee,J. Appl. Phys. 87, 8259 2000.14C. Z. Xiang, X. W. Sun, and X. Y. Jin Appl. Phys. Lett. 83, 5154 2003.15H. Y. Kim, G. R. Jeon, D. S. Seo, M. H. Lee, and S. H. Lee, Jpn. J. Appl.Phys., Part 1 41, 2944 2001.16P. J. Bos and K. R. Beran, Mol. Cryst. Liq. Cryst. 113, 329 1984.17A. Lien, Appl. Phys. Lett. 57, 2767 1990.18H. Wang, T. X. Wu, S. Gauza, J. R. Wu, and S. T. Wu, Liq. Cryst. 33, 912006.19S. Gauza, H. Wang, C. H. Wen, S. T. Wu, A. Seed, and R. Dabrowski, Jpn.J. Appl. Phys., Part 1 42, 3463 2003.20H. Wang, T. X. Wu, X. Zhu, and S. T. Wu, J. Appl. Phys. 95, 5502 2004.21X. Zhu, Z. Ge, and S. T. Wu, J. Disp. Technol. 2, 2 2006.TABLE II. Calculated GTG scale response time in unit of ms of cell 2.1 2 3 4 5 6 7 81 1.70 1.33 1.24 1.20 1.13 1.00 0.602 0.37 1.05 1.04 1.02 0.95 0.83 0.483 0.37 0.72 1.11 0.98 0.90 0.90 0.434 0.38 0.70 0.69 1.22 0.91 0.75 0.405 0.40 0.69 0.70 0.68 1.22 0.80 0.396 0.42 0.68 0.71 0.70 0.66 1.06 0.447 0.45 0.70 0.77 0.74 0.72 0.68 0.538 0.53 0.80 0.86 0.90 0.92 1.04 0.96FIG. 3. Color online Isocontrast contour plot of cell 1. The LC applied isE7-like, d=14 m, W=3 m, and G=3 m. The parameters of the com-pensation films are listed as follows: for A+ plate ne=1.5590, no=1.5866,and d=25.65 m; for C− plate ne=1.50, no=1.65, and d=8.02 m.111101-3 Jiao et al. Appl. Phys. Lett. 92, 111101 2008

Submillisecond response nematic liquid crystal modulators using dual fringe field switching in a vertically aligned cell

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Submillisecond response nematic liquid crystal modulators using dual fringe field switching in a vertically aligned cell

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