Surface Localized Polymer Aligned Liquid Crystal Lens

Kent State University From the SelectedWorks of Philip J. Bos March 25, 213 Surface Localized Polymer Aligned Liquid Crystal Lens Lu Lu, Kent State University - Kent Campus Vassili Sergan Tony Van Heugten Dwight Duston Achintya Bhowmik, et al. Available at: https://works.bepress.com/philip_bos/53/

Surface localized polymer aligned liquid crystal lens Lu Lu, 1 Vassili Sergan, 2 Tony Van Heugten, 3 Dwight Duston, 3 Achintya Bhowmik, 4 and Philip J. Bos 1,* 1 Chemical Physics Interdisciplinary Program and Liquid Crystal Institute Kent State University, Kent, OH 44242, USA 2 Department of physics and Astronomy, California State University, Sacramento, 6 J Street, Sacramento, CA 95819, USA 3 Pixel Optics, Roanoke, VA 2419, USA 4 Intel Corporation, 22 Mission College Blvd, Santa Clara, CA 9554, USA * pbos@kent.edu Abstract: The surface localized polymer alignment (SLPA) method allows complete control of the polar pretilt angle as a function of position in liquid crystal devices. In this work, a liquid crystal (LC) cylindrical lens is fabricated by the SLPA method. The focal length of the LC lens is set by the polymerization conditions, and can be varied by a non-segmented electrode. The LC lens does not require a shaped substrate, or complicated electrode patterns, to achieve a desired parabolic phase profile. Therefore, both fabrication and driving process are relatively simple. 213 Optical Society of America OCIS codes: (16.371) Liquid crystals; (22.363) Lenses; (12.24) Displays. References and links 1. S. Sato, Liquid-crystal lens-cells with variable focal length, Jpn. J. Appl. Phys. 18(9), 1679 1684 (1979). 2. C. W. Chiu, Y. C. Lin, P. C. P. Chao, and A. Y. G. Fuh, Achieving high focusing power for a large-aperture liquid crystal lens with novel hole-and-ring electrodes, Opt. Express 16(23), 19277 19284 (28). 3. H.-C. Lin and Y.-H. Lin, An electrically tunable-focusing liquid crystal lens with a low voltage and simple electrodes, Opt. Express 2(3), 245 252 (212). 4. H. Ren, D. W. Fox, B. Wu, and S.-T. Wu, Liquid crystal lens with large focal length tunability and low operating voltage, Opt. Express 15(18), 11328 11335 (27). 5. G. Q. Li, D. L. Mathine, P. Valley, P. Ayräs, J. N. Haddock, M. S. Giridhar, G. Williby, J. Schwiegerling, G. R. Meredith, B. Kippelen, S. Honkanen, and N. Peyghambarian, Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications, Proc. Natl. Acad. Sci. U.S.A. 13(16), 61 614 (26). 6. L. Lu, L. Shi, P. J. Bos, T. Van Heugten, and D. Duston, Late-newspaper: comparisons between a liquid crystal refractive lens and a diffractive lens for 3D displays, SID Int. Symp. Dig. Tech. 42, 171 174 (211). 7. Y.-P. Huang, C.-W. Chen, and Y.-C. Huang, Superzone fresnel liquid crystal lens for temporal scanning autostereoscopic display, J. Disp. Technol. 8(11), 65 655 (212). 8. P. J. Bos and A. K. Bhowmik, Liquid-crystal technology advances toward future True 3-D flat-panel displays, Inf. Display 27, 6 9 (211). 9. G. Lawton, 3D displays without glasses: coming to a screen near you, Computer 44(1), 17 19 (211). 1. L. Lu, V. Sergan, T. Van Heugten, D. Duston, A. Bhowmik, and P. J. Bos, Distinguished paper: tunable polymer localized liquid crystal lenses for autostereoscopic 3D displays, SID Int. Symp. Dig. Tech. 43, 383 386 (212). 11. N. A. Dodgson, Autostereoscopic 3D displays, Computer 38(8), 31 36 (25). 12. M. P. C. M. Krijn, S. T. de Zwart, D. K. G. de Boer, O. H. Willemsen, and M. Sluijter, 2-D/3-D displays based on switchable lenticulars, J. Soc. Inf. Disp. 16(8), 847 855 (28). 13. Y.-Y. Kao, Y.-P. Huang, K.-X. Yang, P. C.-P. Chao, C.-C. Tsai, and C.-N. Mo, An auto-stereoscopic 3D display using tunable liquid crystal lens array that mimics effects of GRIN lenticular lens array, SID Int. Symp. Dig. Tech. 4, 111 114 (29). 14. Y.-Y. Kao, P. C. P. Chao, and C.-W. Hsueh, A new low-voltage-driven GRIN liquid crystal lens with multiple ring electrodes in unequal widths, Opt. Express 18(18), 1856 18518 (21). 15. M. Ye, B. Wang, and S. Sato, Realization of liquid crystal lens of large aperture and low driving voltages using thin layer of weakly conductive material, Opt. Express 16(6), 432 438 (28). 16. M. C. Tseng, F. Fan, C. Y. Lee, A. Murauski, V. Chigrinov, and H. S. Kwok, Tunable lens by spatially varying liquid crystal pretilt angles, J. Appl. Phys. 19(8), 8319 (211). 17. F. S. Yeung, J. Y. Ho, Y. W. Li, F. C. Xie, O. K. Tsui, P. Sheng, and H. S. Kwok, Variable liquid crystal pretilt angles by nanostructured surfaces, Appl. Phys. Lett. 88(5), 5191 (26). #184495 - $15. USD Received 3 Jan 213; revised 25 Feb 213; accepted 25 Feb 213; published 14 Mar 213 (C) 213 OSA 25 March 213 / Vol. 21, No. 6 / OPTICS EXPRESS 7133

18. T. Nose, S. Masuda, S. Sato, J. L. Li, L. C. Chien, and P. J. Bos, Effects of low polymer content in a liquidcrystal microlens, Opt. Lett. 22(6), 351 353 (1997). 19. H. W. Ren, Y. H. Fan, and S. T. Wu, Liquid-crystal microlens arrays using patterned polymer networks, Opt. Lett. 29(14), 168 161 (24). 2. V. V. Presnyakov and T. V. Galstian, Electrically tunable polymer stabilized liquid-crystal lens, J. Appl. Phys. 97(1), 1311 (25). 21. V. V. Sergan, T. A. Sergan, and P. J. Bos, Control of the molecular pretilt angle in liquid crystal devices by using a low-density localized polymer network, Chem. Phys. Lett. 486(4-6), 123 125 (21). 22. L. Lu, T. Sergan, V. Sergan, and P. J. Bos, Spatial and orientational control of liquid crystal alignment using a surface localized polymer layer, Appl. Phys. Lett. 11(25), 251912 (212). 23. L. Lu, V. Sergan, and P. J. Bos, Mechanism of electric-field-induced segregation of additives in a liquid-crystal host, Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 86(5), 5176 (212). 1. Introduction Liquid crystal (LC) lenses [1] have a variety of applications, such as focusing and zooming in personal portable devices [2 4], ophthalmic corrections [5], accommodation corrections for 3D displays [6]; and active lenses in switchable autostereoscopic 3D displays [7 1]. In the active lens application, the LC lenticular lens offers an approach to achieve glasses-free 3D displays [8, 11]. By turning the optical power of the LC lens off or on, 2D or 3D images can be shown on the same display [7, 12]. Furthermore, the viewing distance and zones of the autostereoscopic 3D displays can be adjusted by using a tunable LC lenticular lens, with a low value of crosstalk [13]. LC tunable lenses typically control the refractive index of LC with multiple electrode structures [7, 14], or hole patterned electrodes [3, 15]. On the other hand, variable pretilt lenses have been developed from non-uniform alignment layers [16, 17], but the nonuniformity can result in image degradation. Another approach uses polymer networks [18 21] to set the orientation of LC directors; however, this approach is problematic, since it leads to a small amount of light scattering. The light scattering issue can be reduced by localizing the polymer layer in the vicinity of the LC layer boundary [22, 23], with the thickness of the polymer layer less than the wavelength of visible light. This technique is called surface localized polymer alignment (SLPA) method (Fig. 1). UV Electrode Surface Reactive Monomers UV Fig. 1. The surface localized polymer alignment method for controlling LC directors spatially In the SLPA method, under the application of an external electric field, reactive monomers (RMs) drift to the electrode surface, with the facilitation of polar groups on RMs [23]. The SLPA layer allows complete control of the polar pretilt angle as a function of position [22] by selecting the UV illumination region and the strength of the electric field during the polymerization process. It is the objective of this paper to demonstrate a LC cylindrical lens via the SLPA method. This LC lens has a simple fabrication process it does not require special electrode structures or complex photolithographic patterning. Additionally, the optical power of the lens can be tuned by the applied voltage with just one single non-segmented electrode. It also provides the potential to track the viewer position relative to the display in the autostereoscopic application. #184495 - $15. USD Received 3 Jan 213; revised 25 Feb 213; accepted 25 Feb 213; published 14 Mar 213 (C) 213 OSA 25 March 213 / Vol. 21, No. 6 / OPTICS EXPRESS 7134

2. Lens fabrication A positive cylindrical lens is considered. The lens has high optical path difference (OPD) in the center and low OPD on the edge. In this work, we use 22 µm planar cells filled with BL6 nematic mixture (Δn =.286, Δε = 17.3) at room temperature. The mixture contains 1.2 wt% of the reactive monomer RM-257 (both by EM Industries) and.12 wt % of Irgacure 651 photoinitiator (by Aldrich). Polyimide PI-2555 (by HD MicroSystems) is coated inside of the planar cells and anti-parallel rubbed to provide the initial alignment of LC mixtures. RMs are confined to the surface and polymerized with a particular applied voltage that sets the pretilt uniquely. A pre-polymerization voltage (1V, 6Hz AC) is applied on the cell for 1 minutes before the following process, which helps the segregation of RMs to the surface [23]. A movable narrow slit (.8 mm) allows only a thin line of UV curing light to pass, which only induces one narrow strip of the cell to be cured at a time. The slit is moved to a location, a particular voltage is applied to the entire cell to create a desired tilt in the liquid crystal, and the UV light is turned on to cure only that narrow strip for 1 seconds. Then, the UV light is turned off. The narrow slit is moved to the next location (2 µm per step). A different voltage is applied to the lens to create a different tilt angle, and then the UV light is turned on to cure that second strip. This process is repeated until the entire cell is cured, allowing a gradient of tilt angles to be created. The voltage applied to the entire cell (cure voltage) at different UV illumination locations is called cure voltage pattern (Fig. 2). A higher cure voltage gives a bigger pretilt angle. According to the correlation between cure voltage and resultant pretilt angle, an approximately parabolic cure voltage pattern is required to generate a positive lens. This cure voltage pattern needs to be further optimized to achieve a desired parabolic phase profile. 5 Cure Voltage (*2V) 4 3 2 1-2.5-2. -1.5-1. -.5.5 1. 1.5 2. 2.5 Cure Location (mm) Fig. 2. The cure voltage pattern for making a 5mm wide 2 diopter SLPA LC Lens The high power LED collimator source with a 22-mm clear aperture (by Mightex, model: LCS-365-2, peak wavelength ~365nm) is used for the polymerization process. Intensity of the UV light source is set at a low level (5 mw/cm 2 ) to prevent the formation of non-uniform textures [22]. An LED power controller is used to control the UV light on/off sequentially. A step motor and a step motor controller are used to precisely control the position of the slit as designed in the cure voltage pattern profile. 3. Lens characterization We have fabricated a cylindrical lens (with a width of 5mm) by using the method described above. In our experiment, the Michelson interferometer method and the interferometric analysis software IntelliWave are used to check the OPD (in number of waves) across the LC lens. Wavelength of the laser beam is 543nm. Figure 3(a) shows a photograph of the cell, with an observed spatially varying OPD profile. Figure 3(b) shows the parabolic OPD profile (without applying an electric field) across the UV light cured area in the SLPA LC cell. #184495 - $15. USD Received 3 Jan 213; revised 25 Feb 213; accepted 25 Feb 213; published 14 Mar 213 (C) 213 OSA 25 March 213 / Vol. 21, No. 6 / OPTICS EXPRESS 7135

(b) OPD across the cured area Fig. 3. OPD measurement across the cured area in the LC cell To characterize the LC lens quality, it is placed in front of a camera focused at infinity aimed at an image (a modified USAF 1851 chart).5 meter away. The quality assessment is made by comparing the LC lens with a high quality Newport glass lens (f =.5m). However, the glass lens is a spherical lens which can focus on the horizontal direction and vertical direction at the same time. Figure 4(a) shows the image without a correction lens in front. With the Newport glass lens, it brings back the focus on both vertical lines and horizontal lines (Fig. 4(d)). Depending on the axis direction of the LC lens, the LC lens can bring the camera in focus either on the vertical lines or on the horizontal lines (Fig. 4(b) and 4(c)). Figure 4(e) and 4(f) are the same as Fig. 4(b) and 4(c), but with the masks added on the figure to make the edge definition (of the vertical or horizontal lines) more obvious. The modulation transfer function (MTF) value gives a quantitative comparison between the LC cylindrical lens and the Newport glass lens. MTF = (I max I min )/(I max + I min ), where I max and I min are maximal and minimal intensity. It falls from 1 to with the increasing of spatial frequency. By analyzing the slanted-edge picture, we can calculate the MTF value of different lenses. This slanted-edge methodology is described by standard ISO 12233 and used in the QuickMTF software. The same setup is used as it is in Fig. 4, but with a tilted USAF 1951 chart as a subject. Accordingly, we get 3 images as Fig. 4(b)-4(d), but tilted. Slanted edges in those figures are analyzed by the QuickMTF software. The first step is edge detection in the tilted image. When a right edge is detected, the QuickMTF software is able to calculate the MTF of that edge. For each tilted image, 5 edges are selected and accordingly 5 MTF are calculated. Then, an averaged MTF value is calculated to represent the quality of the lens. #184495 - $15. USD Received 3 Jan 213; revised 25 Feb 213; accepted 25 Feb 213; published 14 Mar 213 (C) 213 OSA 25 March 213 / Vol. 21, No. 6 / OPTICS EXPRESS 7136

(b) LC Lens V (c) LC Lens H ( )with masks (e) image (b) (f) image (c) with masks (a) Without a lens (d) Glass lens Fig. 4. Comparing the LC lens with the glass lens: (a) focus in infinity; (b) the LC lens focuses on the vertical lines; (c) the LC lens focuses on the horizontal lines; (d) the spherical glass lens focuses on both horizontal and vertical lines; (e) and (f) are the same as (b) and (c), but with masks added for clarity. MTF (Contrast, %) 1 9 Spherical Glass lens 8 7 Cylinderical LC Lens Horizontal Cylinderical LC Lens Vertical 6 5 4 3 2 1.1.2.3.4.5 Cycles per pixel Fig. 5. MTF value vs. cycles per pixel of the LC cylindrical lens and glass spherical lens In Fig. 5, x-axis is the cycle per pixel, which represents the spatial frequency. Y-axis is the MTF value. With the Newport spherical glass lens, the contrast falls slowly with the increase of the frequency. As shown, the LC lens has very similar optical performance as the Newport spherical glass lens. The LC lens has a slight difference when it focuses on the horizontal #184495 - $15. USD (C) 213 OSA Received 3 Jan 213; revised 25 Feb 213; accepted 25 Feb 213; published 14 Mar 213 25 March 213 / Vol. 21, No. 6 / OPTICS EXPRESS 7137

direction or vertical direction. This situation is not related to the horizontal vs. vertical orientation of the lens, but to the LC alignment distortion. Because of the in-plane monomers migration during the UV curing process between each step, some regions of the surface localized polymer layer are not perfectly homogeneous. Accordingly, the LC alignment distortion is resulted. This alignment distortion can be solved by optimizing the lens fabrication process (such as cure intensity, cure voltage and cure time per step) and implementing different LC/monomers formulations to achieve a more homogeneous surface localized polymer layer. The fabricated LC lens shows good tunability with the application of an external voltage. Under different external applied voltages, OPD across the SLPA LC cell (symbols in Fig. 6(a)) fits the ideal lens profile (red lines in Fig. 6(a)) very well. Correspondingly, the focal length of the fabricated LC lens is changed from.5 meter (V), to 2 meter (2.5V), to 4.8 meter (4V) with a non-segmented ITO electrode (Fig. 6(b)). However, between.5 meter and 2 meter focus, the central region of the lens requires a higher voltage than the outer region in order to approach the ideal lens profile. Consequently, the lens focus cannot be tuned from.5 meter to 2 meters gradually with one single non-segmented electrode. However, with 2 or 3 segmented electrodes, the lens could have different applied voltages in the central and outer regions, and could be tuned through the focal lengths between.5 meter and 2 meters, not only the discrete focal length in the graph. OPD (μm) 6 5 4 3 2 1 V 1 V 2.5 V 4 V 9 V Fit curve 1 2 3 4 5 Position (mm) (a) Focal length (m) 5 4 3 2 1 1 2 3 4 5 Applied Voltage (V) (b) Fig. 6. (a) OPD of the cured area fits well with the ideal lens curve under different applied voltages; (b) lens focus is tuned with different applied voltages. 4. Summary In this paper, we demonstrate a simple lens fabrication approach by the surface localized polymer alignment (SLPA) method. Optical power of a LC lens and the lens width can be accurately controlled by the polymerization process and can be easily adjusted for different display and ophthalmic applications. The fabrication process does not require any complex electrode patterns or pre-shape cell surfaces. Both fabrication and driving process of the lens are relatively simple. Furthermore, we show the potential of adjusting of the LC lens focus with a non-segmented electrode. Acknowledgment We thank Dr. Oleg Kurtsev for the discussion on MTF calculation, and X. Cai for English corrections. #184495 - $15. USD Received 3 Jan 213; revised 25 Feb 213; accepted 25 Feb 213; published 14 Mar 213 (C) 213 OSA 25 March 213 / Vol. 21, No. 6 / OPTICS EXPRESS 7138