Open Access
Add to BibSonomyAbstract
We demonstrate a liquid crystal (LC) microlens array (MLA) fabricated by LCs possessing negative dielectric anisotropy, in conjunction with a cell with a three-electrode structure. The presented LC MLA is polarization-insensitive and can be operated in both concave and convex modes. The shortest focal length of the LC MLA is –2.54 and 2.22 mm in concave and convex mode, respectively. Disclination lines that are usually observed in conventional hole-patterned LC lens can also be avoided because of the vertical alignment treatment of LCs.
© 2014 Optical Society of America
1. Introduction
Optical anisotropy of liquid crystals (LCs) has been used for various electro-optical devices, such as optical switches, light modulators, and displays. Among these devices, microlens array (MLA) with tunable focal length has potential applications in 2D/3D switchable displays and tunable photonic devices [1–5]. Different methods for fabricating LC lenses have been presented. Examples of these methods are diffractive focus by Fresnel zone pattern [6–9], substrate with surface relief profile [10–12], formation of polymer networks in LCs [13,14], and shaping the electric field with hole-patterned electrode [15–22]. However, most of the demonstrated LC lenses are polarization-dependent because of the intrinsic optical anisotropy of LCs. Due to the use of polarizers, only half of the incident light is involved in focusing properties; thus the LC lenses have low efficiency. To overcome this drawback, numerous approaches, such as LC lens with orthogonally aligned LC layers [23], vertical alignment (VA) nematic LC lens with a curved profile on the substrate [24], LC lens based on axially symmetric photo-alignment method [25], blue phase LC lenses [26], LC Fresnel lens with an orthogonal binary configuration fabricated by photoalignment method [27], and optically isotropic switchable LC MLA [28] have been presented. Nonetheless, the substrate thickness between orthogonal LC layers could lead to deviation in focal lengths with different incident polarization lights between orthogonally aligned LC layers; the fabrication of the curved structure in VA nematic LC lens is complicated; photo-aligned LC lenses are unstable at high temperature; the blue phase LC lens and the optically isotropic switchable LC MLA have narrow tunable ranges. LC lenses with variable focal lengths over a wide range, from negative to positive values, have been presented [29–33]; however, the lenses are polarization-dependent.
In this paper, we demonstrate a polarization-insensitive LC MLA that can be operated in concave and convex modes. The LCs with negative dielectric anisotropy (defined as negative LCs hereafter) and a three-electrode structure cell are used in this experiment. The operating principle, the experimental procedure, and the obtained results are explained in this paper.
2. Operating principle of the LC MLA
The operation scheme of the LC MLA is depicted in Fig. 1. The inner side of the bottom substrate is deposited with a planar indium tin oxide (ITO) electrode. The inner and outer sides of the top substrate are deposited with the hole array-patterned ITO electrode and the planar ITO electrode, respectively. The negative LCs and VA alignment layer are adopted in this experiment. The presented LC MLA can be operated in both concave and convex modes. Figure 1(a) shows that without voltage applied to the cell, the LC molecules are aligned vertically, and the LC MLA cannot diverge or converge incident light. Figure 1(b) illustrates that when voltage is applied between the bottom substrate electrode and the hole array electrode of the top substrate, the LCs at the periphery of the hole are aligned parallel to the substrate surface, but those at the center of the hole are vertically aligned. The effective refractive index for light that is transmitted through the sample exhibits a gradient profile with the axially symmetrical radial form, wherein the LCs in the periphery of the hole have a larger refraction index than those in the center of the hole. The incident light is therefore diverged and the LC MLA is operated in concave mode. Figure 1(c) shows that once external voltage is applied between the bottom substrate electrode and the planar ITO electrode of the top substrate, the hole array electrode is kept at equipotential with the bottom ITO electrode. The refraction index of the LCs in the periphery of the hole is smaller than that of the LCs in the center of the hole. The incident light is converged and the LC MLA is operated in convex mode.
Fig. 1 Operation schemes of the LC MLA: (a) without voltage applied to the cell, (b) concave mode, and (c) convex mode.
3. Experimental preparation and results
Three ITO-deposited glass substrates are used for the presented LC MLA. The glass substrate is 0.55 mm thick, and the ITO layer deposited on the glass substrate is 100 nm thick. Figure 2(a) shows that the VA polyimide (AL-8088C-0000-21NI, Daily Polymer, Taiwan) is spin-coated on the ITO layer of the top of the bottom substrate and on the ITO layer of the bottom of the middle substrate without mechanical buffering. The bottom and the middle substrates are separated by 75 μm thick Mylar spacer. The ITO layer of the middle substrate is photolithography etched as hole array electrode. The used photomask and etched substrate with hole-array pattern are shown in Figs. 2(b) and 2(c), respectively. The diameter of the hole is 300 μm, and the distance between the adjacent holes is 0.125 mm. The ITO layer of the top substrate is placed face down. After assembling, the cell is filled with nematic LC MLC-6886 (, for λ = 589.3 nm at 20 °C, Merck). The dimension of the presented LC MLA is 20 mm x 20 mm. The LC MLA is subjected to 1 kHz AC voltage with square wave in the experiment.
Fig. 2 (a) Structure of the presented LC MLA. Microscope image of (b) photomask and (c) hole array electrode on the etched substrate.
Figure 3 depicts the interference fringes of the LC MLA at various voltages. A He-Ne laser with a wavelength of 632.8 nm is used as the incident light. The LC MLA is placed between a pair of crossed polarizers. The transmitted image with interference fringes is recorded by a CCD camera [33]. The crossed dark pattern in each microlens indicates the LC molecules are aligned in axially symmetrical form. In conventional hole-patterned LC lenses with LC alignment parallel to the glass substrate, when the voltage is applied to the LC lens, the reverse tilted LCs in the hole area create disclination lines to degrade the optical performance [34]. In the current study, when the voltage is applied to the LC MLA cell, the vertically aligned negative LCs rotate outward in axially symmetrical form, thereby avoiding disclination lines. Figures 3(a)–3(d) show that when the LC MLA is operated in concave mode, the interference fringes gradually increase with voltage, and the maximum fringes appear at 4.3 V. As the applied voltage exceeds 4.3 V, the fringes decrease because the LCs at the center and the periphery of the hole are gradually aligned parallel to the substrate surface. Figures 3(e)–3(h) demonstrate that when the LC MLA is operated in convex mode, the fringes gradually increase with voltage before becoming stable. The fringes decrease once the applied voltage exceeds 140 V. Notably, the spiral pattern in convex mode may be also a disclination due to lens characteristic, which can be solved with polymer stabilized method [35]. In convex mode, the thick middle substrate (giant electrode spacing) of ~0.55 mm provides the LC MLA the high operation voltage, which can be improved by using the ultra-thin middle glass substrate [36]. In concave mode, as shown in Fig. 3(b), the small fringes are observed between the adjacent holes (red circles), owing to the fringing electric field generated at the periphery of the holes. In convex mode, the dark state appears between the adjacent holes because the LCs in this area are aligned vertically. The response time of the LC MLA is also measured. In concave mode, when the voltage are turned on from 0 to 4.3 V and turned off from 4.3 to 0 V, the measured response time are 12 and 8 s, respectively; in convex mode, when the voltage are turned on from 0 to 55 V and turned off from 55 to 0 V, both the measured response time are 7 s. Owing to the complex electric gradient in the cell, the exact torques exerted on LCs are difficult to estimate. However, it is reasonable to predict that the electric torque exerted on LCs in convex mode is higher than that exerted on LCs in concave mode, because of the applied high voltage.
Fig. 3 Interference fringes of the LC MLA. (a)–(d) Operated in concave mode at voltages of 0, 3, 4.3, and 10 V, respectively; (e)–(h) operated in convex mode at voltages of 0, 40, 55, and 140 V, respectively. The red dotted circles represent the holes on the electrode.
The focal length of the LC MLA is obtained by calculating the measured fringes of the LC MLA using the following equation [37]:
where r is the radius of the microlens, N is the number of fringes, and is the wavelength of the incident light. Figure 4 shows the focal lengths of the LC MLA at various voltages. As shown in Fig. 4(a), in concave mode, when a low voltage is applied to the cell, the LCs at the center of the hole are aligned vertically, but the LCs at the periphery of the hole are aligned at low tilt angle, which is increased with voltage. The tilt angle is defined as the angle that deviates from the substrate surface normal. Therefore, the focal length of the LC MLA increases with applied voltage. When the applied voltage reaches 4.3 V, a short focal length of −2.54 mm is obtained, because the LCs at the periphery of the hole are aligned parallel to the substrate surface, but the LCs at the center of the hole are aligned vertically. As the voltage exceeds 4.3 V, the focal length gradually increases because the LCs at the periphery of the hole are aligned parallel to the substrate surface, but tilt angle of the LCs at the center of the hole are gradually increased with voltage. In convex mode, a short focal length of 2.22 mm is obtained at 55 V, above which, the focal length maintains at 2.22 mm, as shown in Fig. 4(b). If the applied voltage exceeds 140 V, the focal length increases gradually because the LCs in the holes are aligned parallel to the substrate surface. The critical electric field of the LC MLA is 0.024 and 0.27 in concave mode and convex mode, respectively. The range of the tunable focal length can increase further if the birefringence of LCs increases.Fig. 4 Measured focal lengths of the LC MLA as various voltages in (a) concave mode and (b) convex mode.
The image performance of the LC MLA is examined with an optical microscope (OM). Figure 5(a) demonstrates that a target, “”, is placed at the stage, and the LC MLA cell is placed 1 cm above the target. A white light source is incident from the bottom of the OM. The 5X objective is placed 1.5 cm above the LC MLA initially. A digital camera is installed atop the OM to observe the images. Figure 5(b) shows the image of the target when the LC MLA is operated without voltage. When the LC MLA is operated in concave mode at 4.3 V, the 5X objective is adjusted to observe a clear upright virtual array image, “”, as depicted in Fig. 5(d). At this objective position, the array image blurs or disappears by changing the applied voltage, as shown in Figs. 5(c) and 5(e). Similarly, the reversed real array image “” is obtained when the LC MLA is operated in convex mode at 55 V, as shown in Fig. 5(g). The array image is then blurs or disappears by changing the applied voltage, as depicted in Figs. 5(f) and 5(h). The results confirm that the LC MLA possesses the capabilities of electrically switchable concave and convex modes.
Fig. 5 (a) Schematic of the OM; (b) the target image without voltage applied to the LC MLA. Target images with LC MLA operated in concave mode at (c) 0 V, (d) 4.3 V, and (e) 10 V. Target images with LC MLA operated in convex mode at (f) 0 V, (g) 55 V, and (h) 140 V.
Figure 6(a) shows the focusing images of the LC MLA operated in convex mode at 55 V. The polarization of the incident light is controlled by a polarizer. The transmittance axis of the polarizer is initially parallel to the Y-axis (0°) and then rotated counterclockwise. As shown in Figs. 6(a) and 6(b), the focusing image and intensity of the presented LC MLA are polarization-insensitive. In concave mode, the LCs are reoriented in axially symmetrical form; therefore, the polarization-insensitive feature of the LC MLA is also expected to exist. The polarization-insensitive feature of the LC MLA still exists when the incident light is circularly polarized. The axially symmetrically aligned LCs provides the LC MLA the polarization-insensitive property. However, owing to the intrinsic birefringence of LCs, various focal lengths and aberrations are expected to be observed, because the incident light can still be decomposed as two orthogonal polarizations originated from the planar aligned LCs in the LC MLA. The complex issues regarding the image quality still remain to be improved.
Fig. 6 (a) Focusing images and (b) focusing intensities of the LC MLA at different polarizations of the incident light.
4. Conclusions
We have demonstrated a polarization-insensitive LC MLA with switchable focal lengths over a wide range, from negative to positive values. With applied voltage, the LC molecules are aligned along the axially symmetrical radial direction; thus, the LC MLA is insensitive to the polarization of the incident light, and the commonly observed disclination lines in conventional hole-patterned LC lens are avoided. The LC MLA provides diverging and converging behavior, depending on the electrical addressing scheme. By adopting LCs with high birefringence and thin middle substrate, the range of the tunable focal lengths can be increased and the operating voltages can be reduced. Research on optimizations of the electro-optical properties and the applications of the LC MLA are under way.
Acknowledgments
This work was supported by the National Science Council of the Republic of China, Taiwan (Contract Nos. NSC 101-2112-M-018-002-MY3 and NSC 103-2811-M-018-001).
References and links
1. T. Nose, S. Masuda, S. Sato, J. Li, L. C. Chien, and P. J. Bos, “Effects of low polymer content in a liquid-crystal microlens,” Opt. Lett. 22(6), 351–353 (1997). [CrossRef] [PubMed]
2. Y. Choi, H. R. Kim, K. H. Lee, Y.-M. Lee, and J. H. Kim, “A liquid crystalline polymer microlens array with tunable focal intensity by the polarization control of a liquid crystal layer,” Appl. Phys. Lett. 91(22), 221113 (2007). [CrossRef]
3. M. Ferstl and A. Frisch, “Static and dynamic Fresnel zone lenses for optical interconnections,” J. Mod. Opt. 43(7), 1451–1462 (1996). [CrossRef]
4. G. Williams, N. J. Powell, A. Purvis, and M. G. Clark, “Electrically controllable liquid crystal Fresnel lens,” Proc. SPIE 1168, 352–357 (1989). [CrossRef]
5. J. S. Patel and K. Rastani, “Electrically controlled polarization-independent liquid-crystal Fresnel lens arrays,” Opt. Lett. 16(7), 532–534 (1991). [CrossRef] [PubMed]
6. T. H. Lin, Y. Huang, A. Y. G. Fuh, and S. T. Wu, “Polarization controllable Fresnel lens using dye-doped liquid crystals,” Opt. Express 14(6), 2359–2364 (2006). [CrossRef] [PubMed]
7. X. Q. Wang, A. K. Srivastava, V. G. Chigrinov, and H. S. Kwok, “Switchable Fresnel lens based on micropatterned alignment,” Opt. Lett. 38(11), 1775–1777 (2013). [CrossRef] [PubMed]
8. S. J. Hwang, T. A. Chen, K. R. Lin, and S. C. Jeng, “Ultraviolet-light-treated polyimide alignment layers for polarization-independent liquid crystal Fresnel lenses,” Appl. Phys. B 107(1), 151–155 (2012). [CrossRef]
9. Y. M. Lou, Q. K. Liu, H. Wang, Y. C. Shi, and S. L. He, “Rapid fabrication of an electrically switchable liquid crystal Fresnel zone lens,” Appl. Opt. 49(26), 4995–5000 (2010). [CrossRef] [PubMed]
10. H. S. Ji, J. H. Kim, and S. Kumar, “Electrically controllable microlens array fabricated by anisotropic phase separation from liquid-crystal and polymer composite materials,” Opt. Lett. 28(13), 1147–1149 (2003). [CrossRef] [PubMed]
11. Y. Choi, J. H. Park, J. H. Kim, and S. D. Lee, “Fabrication of a focal length variable microlens array based on a nematic liquid crystal,” Opt. Mater. 21(1–3), 643–646 (2002). [CrossRef]
12. H. T. Dai, Y. J. Liu, X. W. Sun, and D. Luo, “A negative-positive tunable liquid-crystal microlens array by printing,” Opt. Express 17(6), 4317–4323 (2009). [CrossRef] [PubMed]
13. H. Ren, S. Xu, and S. T. Wu, “Polymer-stabilized liquid crystal microlens array with large dynamic range and fast response time,” Opt. Lett. 38(16), 3144–3147 (2013). [PubMed]
14. M. Xu, Z. Zhou, H. Ren, S. H. Lee, and Q. Wang, “A microlens array based on polymer network liquid crystal,” J. Appl. Phys. 113(5), 053105 (2013). [CrossRef]
15. M. Ye and S. Sato, “Optical properties of a liquid crystal lens of any size,” Jpn. J. Appl. Phys. 41(5B), L571–L573 (2002). [CrossRef]
16. M. Ye, B. Wang, T. Takahashi, and S. Sato, “Properties of variable-focus liquid crystal lens and its application in focusing system,” Opt. Rev. 14(4), 173–175 (2007). [CrossRef]
17. M. Ye, B. Wang, M. Kawamura, and S. Sato, “Image formation using liquid crystal lens,” Jpn. J. Appl. Phys. 46(10A), 6776–6777 (2007). [CrossRef]
18. T. Nose, S. Masuda, and S. Sato, “A liquid crystal microlens with hole-patterned electrodes on both substrates,” Jpn. J. Appl. Phys. 31(5B), 1643–1646 (1992). [CrossRef]
19. M. Ye, B. Wang, and S. Sato, “Liquid crystal lens with focus movable in focal plane,” Opt. Commun. 259(2), 710–722 (2006). [CrossRef]
20. 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 (2008). [CrossRef] [PubMed]
21. C. Y. Huang, Y. J. Huang, and Y. H. Tseng, “Dual-operation-mode liquid crystal lens,” Opt. Express 17(23), 20860–20865 (2009). [CrossRef] [PubMed]
22. C. J. Hsu and C. R. Sheu, “Using photopolymerization to achieve tunable liquid crystal lenses with coaxial bifocals,” Opt. Express 20(4), 4738–4746 (2012). [CrossRef] [PubMed]
23. M. Ye, B. Wang, and S. Sato, “Polarization-independent liquid crystal lens with four liquid crystal layers,” IEEE Photon. Technol. Lett. 18(3), 505–507 (2006). [CrossRef]
24. Y. Choi, Y. T. Kim, S. D. Lee, and J. H. Kim, “Polarization independent static microlens array in the homeotropic liquid crystal configuration,” Mol. Cryst. Liq. Cryst. 433(1), 191–197 (2005). [CrossRef]
25. A. Y. G. Fuh, S. W. Ko, S. H. Huang, Y. Y. Chen, and T. H. Lin, “Polarization-independent liquid crystal lens based on axially symmetric photoalignment,” Opt. Express 19(3), 2294–2300 (2011). [CrossRef] [PubMed]
26. Y. H. Lin, H. S. Chen, H. C. Lin, Y. S. Tsou, H. K. Hsu, and W. Y. Li, “Polarizer-free and fast response microlens arrays using polymer-stabilized blue phase liquid crystals,” Appl. Phys. Lett. 96(11), 113505 (2010). [CrossRef]
27. D. W. Kim, C. J. Yu, H. R. Kim, S. J. Kim, and S. D. Lee, “Polarization-insensitive liquid crystal Fresnel lens of dynamic focusing in an orthogonal binary configuration,” Appl. Phys. Lett. 88(20), 203505 (2006). [CrossRef]
28. Y. J. Lee, C. J. Yu, J. H. Lee, J. H. Baek, Y. Kim, and J. H. Kim, “Optically isotropic switchable microlens arrays based on liquid crystal,” Appl. Opt. 53(17), 3633–3636 (2014). [CrossRef] [PubMed]
29. Y. H. Fan, H. Ren, X. Liang, H. Wang, and S. T. Wu, “Liquid crystal microlens arrays with switchable positive and negative focal lengths,” J. Disp. Technol. 1(1), 151–156 (2005). [CrossRef]
30. H. Ren, J. R. Wu, Y. H. Fan, Y. H. Lin, and S. T. Wu, “Hermaphroditic liquid-crystal microlens,” Opt. Lett. 30(4), 376–378 (2005). [CrossRef] [PubMed]
31. B. Wang, M. Ye, and S. Sato, “Liquid crystal lens with focal length variable from negative to positive values,” IEEE Photon. Technol. Lett. 18(1), 79–81 (2006). [CrossRef]
32. H. T. Dai, Y. J. Liu, X. W. Sun, and D. Luo, “A negative-positive tunable liquid-crystal microlens array by printing,” Opt. Express 17(6), 4317–4323 (2009). [CrossRef] [PubMed]
33. O. Pishnyak, S. Sato, and O. D. Lavrentovich, “Electrically tunable lens based on a dual-frequency nematic liquid crystal,” Appl. Opt. 45(19), 4576–4582 (2006). [CrossRef] [PubMed]
34. M. Ye, B. Wang, and S. Sato, “Driving of liquid crystal lens without disclination occurring by applying an in-plane electric field,” Jpn. J. Appl. Phys. 42(8), 5086–5089 (2003). [CrossRef]
35. Y. W. Kim, J. Jeong, S. H. Lee, J. H. Kim, and C. J. Yu, “Improvement in switching speed of nematic liquid crystal microlens array with polarization independence,” Appl. Phys. Express 3(9), 094102 (2010). [CrossRef]
36. X. Zhao, C. Liu, D. Zhang, and Y. Luo, “Tunable liquid crystal microlens array using hole patterned electrode structure with ultrathin glass slab,” Appl. Opt. 51(15), 3024–3030 (2012). [CrossRef] [PubMed]
37. H. W. 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 (2007). [CrossRef] [PubMed]
