Open Access
Add to BibSonomyAbstract
We report a new approach to preparing a lenticular microlens array (LMA) using polyvinyl chloride (PVC)/dibutyl phthalate (DBP) gels. The PVD/DBP gels coated on a glass substrate form a membrane. With the aid of electrostatic repulsive force, the surface of the membrane can be reconfigured with sinusoidal waves by a DC voltage. The membrane with wavy surface functions as a LMA. By switching over the anode and cathode, the convex shape of each lenticular microlens in the array can be converted to the concave shape. Therefore, the LMA can present a large dynamic range. The response time is relatively fast and the driving voltage is low. With the advantages of compact structure, optical isotropy, and good mechanical stability, our LMA has potential applications in imaging, information processing, biometrics, and displays.
© 2016 Optical Society of America
1. Introduction
Adaptive lenticular microlens arrays (LMAs) have been developed intensively in recent years. These LMAs have found widespread applications in image processing, beam steering, biometrics, and displays. Several approaches, including liquid crystal (LC) [1–9], electrowetting [10], and dielectrophoretic effect [11,12], have been demonstrated. Each approach has its own merits and demerits. For a LC LMA, its focal length can be precisely tuned with a low voltage. Usually the gap of the lens cell is printed with a concave shape [1,3] or the electrode is etched with a Fresnel pattern [5, 9], either the device fabrication procedure or the driving scheme is complicate. When a LC LMA is used for switchable two-dimensional (2D)/three-dimensional (3D) displays, it could integrate with a 90° twisted nematic (TN) LC cell (or called polarization rotator) [7]. By driving the TNLC cell, the LC LMA could provide a switchable focus with fast response time and low driving voltage. Because two LC cells are employed, the optical system is bulky and heavy. To obtain a polarization-insensitive LMA, polymer dispersed liquid crystal (PDLC) [13] or blue phase LC materials [14,15] can be considered. Due to a little phase shift, a PDLC or blue phase LC lens usually presents a very limited dynamic range.
Different from LC LMAs, a LMA based on electrowetting and dielectrophoretic effect is polarization independent because the fluids are optically isotropic. A fluidic LMA usually employs two liquids in order to reduce the gravitational effect. Although its focal length can be tuned directly by a voltage, the device still faces the challenges of bulky structure, complex fabrication, and mechanical stability.
In addition to LC and liquid, polyvinyl chloride (PVC)/dibutyl phthalate (DBP) gels can be used to prepare adaptive lenses too. The gels can either be freestanding or attach to a substrate. Therefore, the structure of these lenses is simple and compact. In previously demonstrated lenses [16–19], the PVC/DBP gels are filled in a circular container, so the aperture of these lenses is circular. By applying a DC voltage, the surface profile of the gels could be deformed. Therefore, the focal length of these lenses could be tuned. Due to the inherent arrangement of the electrode, these lenses usually require a high driving voltage, and their dynamic response is quite slow.
Inspired by the circular PVC/DBP lenses, here we report an approach to preparing a LMA using PVC/DBP gels. The PVC/DBP gels coated on a glass substrate form a membrane. The glass substrate has an interdigitated electrode. By applying a DC voltage to the electrode, the surface of the membrane can be deformed with sinusoidal waves. The membrane with wavy surface has a LMA character. When the anode and cathode of the LMA are switched over, the peaks (troughs) of the waves are converted to the troughs (peaks). Therefore, the focal length of the LMA can be largely tuned. In contrast to previous PVC/DBP lenses, the dynamic response of our LMA is relatively fast, and the driving voltage is low.
1. Device structure
The cross-sectional structure of our MLA is depicted in Fig. 1(a). It consists of a glass substrate, an electrode, and PVC/DBP gels. The gels attached on the substrate form a membrane. The electrode on the glass substrate has an interdigitated pattern, as given in Fig. 1(b). The common electrode has two terminals marked by “A” and “B”. When a positive DC voltage is applied across the terminals (anode A and cathode B), a periodical electric field is generated between adjacent electrode stripes. The stripes connected to cathode B can charge to the PVC/DBP gels by the electric field. Suppose the stripes with plus symbol “+” are connected to the anode, and the adjacent stripes with negative symbol “-” to the cathode, the charges carried by DBP molecules in the PVC gels have a tendency to shift and accumulate on the surface of the gels upon the “+” stripes. Due to the electrostatic repulsive force, the PVC/DBP gels creep up the “+” stripes and form a wavy configuration as shown in Fig. 1(c). Owing to the surface tension, the membrane is smoothly curved. Therefore, the PVC/DBP membrane functions as a LMA. When a light passes through the LMA from the substrate side, it can be converged (diverged) by the convex (concave) surface. The width of adjacent stripes is half of the aperture of the lenticular lens.
Fig. 1 Structure and operation mechanism of the LMA. (a) cross-sectional structure, (b) the pattern of the electrode, (c) anode to terminal A and cathode to terminal B, and (d) anode to terminal B and cathode to terminal A.
When a negative DC voltage (anode B and cathode A) is applied to the electrode, the charged DBP molecules have to shift toward the new “+” stripes, causing its wavy surface to be flattened. As a result, the wavy membrane can recover to its original shape. If the amplitude of the voltage is sufficiently high, then a new configuration can be established, as shown in Fig. 1(d). In contrast to the convex shape shown in Fig. 1(c), the newly balanced profile presents a concave shape.
Because the shape of the membrane can be converted from convex to concave, the LMA can obtain a large dynamic range. Similar to a solid lens, the focal length (F) of each lenticular microlens in the array can be expressed by
Where r is the radius of curvature of the lenticule and n is the refractive index of the PVC/DBP gels. Since r can be either positive (convex) or negative (concave), its focal length (F) can be either positive or negative accordingly.2. Experiment
To prepare a MLA, an indium-tin-oxide (ITO) glass substrate is chosen. The size of the glass substrate is 2.1 × 2.5 cm. The ITO electrode is etched with an interdigitated pattern. The width of the ITO stripe and the gap of adjacent ITO stripes are 8 μm and 12 μm, respectively.
To prepare PVC/DBP gels, a PVC powder is chosen. The PVC powder (degree of polymerization 1300) is dissolved in tetrahydrofuran (THF; Sigma-Aldrich Co. Ltd) to form a solution. The concentration of the PVC is ~5 wt%. Then the solution is fully dissolved in a large amount of methanol to get the PVC precipitation. The precipitation is then filtered and re-dissolved in THF. To get the purified PVC, the re-precipitation procedure is repeated three times. After that, the purified PVC is dissolved in THF with the concentration of ~0.1 g/ml. Finally the PVC solution is doped with a DBP. The weight of the DBP is ~9 times the weight of the PVC. The PVC and DBP are thoroughly mixed until a uniform mixture is obtained. A small amount of the PVC/DBP solution is dripped on the ITO glass substrate, and the substrate is then rotated using a spin coater. The coating speed is ~1000 rpm/min. When a uniform film is obtained, the substrate is then placed in a dry place at room temperature. After evaporating the THF, a clear PVC/DBP membrane on the substrate is obtained. The thickness of the PVC/DBP membrane is ~25 μm. The refractive index of the PVC/DBP gels is measured to be n ~1.49 [20].
4. Results and discussion
The surface of the PVC/DBP membrane is flat, so it has no lens character. The membrane can be simply evaluated using an optical microscope (OM). The substrate/membrane is placed on the stage of the OM. A USAF resolution target placed under the membrane is used as an object. The observed image is recorded using a CCD camera. Figure 2(a) shows the image of the object. The image is clear and it can resolve up to group 6 and element 3. The corresponding resolution is ~80 lp/mm. This result implies that the membrane is highly transparent with negligible surface distortion. When a DC voltage is applied to the electrode, the image expands only in the direction perpendicular to the ITO stripes. Figure 2(b) shows the image when V = 50 V. The image is blurry and spreads only in horizontal direction. The blurry image implies that the lens takes effect but in defocused state. The image expanding implies that the lens is a lenticular lens. Due to the narrow ITO stripes, the diffraction effect also exists in the observed image.
Fig. 2 Image of the resolution target observed through the PVC/DBP membrane, the ITO stripe of the substrate is placed in vertical position. (a) V = 0, (b) V = 50 V.
To study the light focusing property of the LMA, an experimental setup shown in Fig. 3 is built. A beam from the He-Ne laser (λ~633 nm) is expanded by the beam expander. The substrate/membrane is placed behind the beam expander in vertical position. After passing through the membrane, the output beam is recorded using a CCD beam profiler (BGS-USB-SP503, Newport). A voltage source can provide a DC voltage to the LMA.
The 3D light intensity distributions at different voltages are recorded as shown in Fig. 4. At V = 0, the output light is relatively weak, as shown in Fig. 4(a). The average intensity of the peaks is ~1100 arbitrary units. The light intensity distribution is relatively uniform. The light stripes are mainly due to the diffraction of the finger-patterned electrode. When a high voltage is applied to the electrode, the intensity of the peaks starts to increase. This result implies the surface of the PVC/DBP membrane is waved, and a LMA character appears. Figure 4(b) shows the intensity distribution when V = 30 V. The LMA causes light to focus. The average intensity of the peaks reaches ~2100 arbitrary units. At V = 50 V, the peaks is almost saturated, as shown in Fig. 4(c). The average intensity of the peaks is ~3800 arbitrary units. The saturated intensity implies that the focal point of the LMA locates at the CMOS chip surface of the CCD camera.
Fig. 4 Light intensity distribution. From (a) (see Visualization 1) to (b) to (c), the driven voltage is V = 0, V = 30 V and V = 50 V, respectively. From (d) to (e) to (f) (see Visualization 2) the time of V = −50 V impacting on the device is 4 s, 7 s and 12 s, respectively. The dynamic video is recorded by switching over the anode and cathode when V = 50 V.
When the voltage is removed, the intensity of the peaks has a tendency to decrease slowly. This result means the PVC/DBP membrane starts to discharge, and the LMA character begins to diminish. To accelerate the speed of the membrane changing from one configuration [Fig. 1(c)] to the other one [Fig. 1(d)], a simple method is to switch over the anode and cathode. When V = −50V is impacting on the membrane for ~4 s, the intensity of the peaks reduces to the original state as shown in Fig. 4(d). Such a result implies that the membrane completely recovers to its original shape. When the membrane is continuously impacted, each stripe shifts to its adjacent stripe as shown in Fig. 4(e). The impacting time is ~7 s. Such a result implies that the configuration shown in Fig. 1(c) is converted to the one shown in Fig. 1(d). The intensity of the peaks increases with time. The peaks reach almost saturation after 12 s, as given in Fig. 4(f). To visually observe the dynamic change of the light intensity distribution, two videos are recorded in Fig. 4. The Visualization 1 shows the change of light intensity when a voltage from 0 to 50 V is gradually applied to the electrode. Visualization 2 shows the change of the light intensity when V = 50 V is changed to V = −50 V. The voltage V = −50 is constantly applied to the membrane until a stable intensity distribution is built. From the dynamic response, the surface of the membrane can be rapidly waved. In contrast to previous PVC/DBP lenses [16, 19], the driving voltage of the LMA is largely reduced. Increase the amplitude of the voltage can quickly increase/decrease the light intensity of the peaks. As a result, the response time of the LMA can be largely reduced.
Using the experimental setup shown in Fig. 3, the focal length of the LMA at different voltages can be measured. To measure the focal length of the lenticular microlens, an imaging glass lens (not shown) is placed between the LMA and the CCD camera [Fig. 3]. We first adjust the position of the imaging lens so that it focuses on the membrane surface. Then we adjust the imaging lens again until the bright focus stripes can be observed. The travel distance of the imaging lens is the focal length. The focal length of one lenticular microlens measured at different voltages is shown in Fig. 5.
At V = 0, the focal length of the lens is at infinity. When the applied voltage exceeds ~28 V, the lens takes effect. As the voltage increases, the focal length is decreased gradually. When the voltage changes from 0 V to 50 V, the focal length of the lenticular microlens can be tuned from infinity to ~88 μm. Similarly, by applying a negative voltage to the electrode, the focal length of the lenticular micorlens presents the same tendency as that of the positive voltage but negative due to its concave shape.
Since the PVC/DBP gel is soft, its surface profile could not be measured directly. Here we can indirectly measure the surface configuration of the membrane using optical glue NOA81 (Noland Adhesive). The glue is coated on the surface of the membrane in a dark room. Then the surface of the membrane is waved using a DC voltage. After that, the NOA81 is cured by a UV light (~10 mW/cm2). When the NOA 81 is solidified, it is separated from the membrane. The cured NOA81 and the membrane share the same topography. At V = 0, partial area of the NOA81 is cured first. Then a voltage of V = 50 V is applied to the electrode. When the surface of the membrane is waved, the NOA81 on the waved membrane is cured. The surface of NOA81 can be analyzed using a scanning electron microscope (SEM). Figure 6(a) shows the surface of NOA 81 cured at V = 0. The flat surface implies that the membrane is flat too. Figures 6(b) shows 2D configuration of the NOA81 cured at V = 50 V. A grating-like pattern implies that the surface of the membrane has a wavy configuration. To clearly observe this structure, an enlarged cross-sectional configuration with three periods is taken, as given in Fig. 6(c). Indeed the bottom surface of the cured NOA81 presents a concave shape. This result implies that each lenticular microlens presents a convex shape. From the diameter of the lens aperture D = 40 μm, the maximum displacement of the concave shape h~5 μm, the radius of the spherical curvature is calculated to be r~42.5 μm. According to Eq. (1) and n~1.49, the focal length of the lenticular microlens is calculated to be F~83 μm. Such a result is approximate to the focal length measured at V = 50 V in Fig. 5.
Fig. 6 Surface profile of cured NOA 81 taken by SEM. (a) partial area of NOA81 cured at V = 0, (b) the left NOA81 cured at V = 50 V, and (c) cross-sectional configuration of the cured NOA81 with three periods shown in (b).
From our experiments, a PVC/DBP membrane integrating with comb-shaped electrodes can be used to prepare a LMA. In the voltage-off state, the surface of the membrane is flat. By applying a DC voltage, the surface of the membrane can be waved. The membrane with a wavy configuration functions as a LMA. The focal length of the MLA can be easily tuned. Increase the amplitude of the voltage can reduce the focal length and increase the dynamic response. From the given videos, the response time is relatively fast. By switching over the anode and cathode, the convex shapes can be converted to the concave shapes. Because the membrane firmly adheres to the substrate, the LMA could not be distorted by the gravitational effect when it is placed vertically. Therefore, the LMA has good mechanical stability without additional parts to hold. Depending on applications, various MLAs can be fabricated by just changing the pattern of the electrode. Therefore, this approach can extend to prepare an arbitrary LMA. Owing to the compact structure and good optical performance, it is possible for our LMA to replace a LC MLA for switchable 2D/3D displays.
5. Conclusion
We have demonstrated a LMA using PVC/DBP gels. The PVD/DBP gels form a membrane on a glass substrate. The surface of the membrane can be deformed with a wavy configuration by a DC voltage. The waved membrane owns a LMA character. The focal length of each lenticular microlens in the array can be tuned from infinity to ~83 μm when the applied voltage is changed from 0 to 50 V. When the anode and cathode are switched over, the peaks (troughs) of the waves can be converted to the troughs (peaks). Therefore, the focal length of each lenticular microlens can be largely tuned. The dynamic response is relatively fast, and the driving voltage is low. Owing to the advantages of easy fabrication, compact structure, optical isotropy, and good mechanical stability, our LMA has potential applications in imaging, beam steering, biometrics, and displays.
Acknowledgments
The authors thank Prof. Myong-Hoon Lee for the valuable discussions. This work is supported by the National Research Foundation of Korea under Grant 2014064156.
References and links
1. T. Dekker, S. T. de Zwart, O. H. Willemsen, M. G. H. Hiddink, and W. L. IJzerman, “2D/3D switchable displays,” Proc. SPIE 6135, 61350K (2006). [CrossRef]
2. J. Flack, J. Harrold, and J. Woodgate, “A prototype 3D mobile phone equipped with a next generation autostereocopic display,” Proc. SPIE 6490, 64900M (2007). [CrossRef]
3. M. P. C. M. Krijn, S. T. de Zwart, D. K. G. de Boer, O. H. Willemsen, and M. Sluijter, “2D/3D displays based on switchable lenticulars,” J. Soc. Inf. Disp. 16(8), 847–855 (2008). [CrossRef]
4. Y. Liu, H. Ren, S. Xu, Y. Chen, L. Rao, T. Ishinabe, and S.-T. Wu, “Adaptive focus integral image system design based on fast-response liquid crystal microlens,” J. Disp. Technol. 7(12), 674–678 (2011). [CrossRef]
5. Y. P. Huang, C. W. Chen, and Y. C. Huang, “Superzone Fresnel liquid crystal lens for temporal scanning auto-stereoscopic display,” J. Disp. Technol. 8(11), 650–655 (2012). [CrossRef]
6. J.-H. Na, S.-C. Park, S.-U. Kim, Y. Choi, and S.-D. Lee, “Physical mechanism for flat-to-lenticular lens conversion in homogeneous liquid crystal cell with periodically undulated electrode,” Opt. Express 20(2), 864–869 (2012). [CrossRef] [PubMed]
7. H. Ren, S. Xu, Y. Liu, and S. T. Wu, “Switchable focus using a polymeric lenticular microlens array and a polarization rotator,” Opt. Express 21(7), 7916–7925 (2013). [CrossRef] [PubMed]
8. Y.-J. Lee, J.-H. Baek, Y. Kim, J.-U. Heo, Y.-K. Moon, J. S. Gwag, C.-J. Yu, and J.-H. Kim, “Polarizer-free liquid crystal display with electrically switchable microlens array,” Opt. Express 21(1), 129–134 (2013). [CrossRef] [PubMed]
9. T.-H. Jen, X. Shen, G. Yao, Y.-P. Huang, H.-P. D. Shieh, and B. Javidi, “Dynamic integral imaging display with electrically moving array lenslet technique using liquid crystal lens,” Opt. Express 23(14), 18415–18421 (2015). [CrossRef] [PubMed]
10. R. D. Niederriter, A. M. Watson, R. N. Zahreddine, C. J. Cogswell, R. H. Cormack, V. M. Bright, and J. T. Gopinath, “Electrowetting lenses for compensating phase and curvature distortion in arrayed laser systems,” Appl. Opt. 52(14), 3172–3177 (2013). [CrossRef] [PubMed]
11. C.-C. Cheng, C. A. Chang, and J. A. Yeh, “Variable focus dielectric liquid droplet lens,” Opt. Express 14(9), 4101–4106 (2006). [CrossRef] [PubMed]
12. C. V. Brown, G. G. Wells, M. I. Newton, and G. McHale, “Voltage-programmable liquid optical interface,” Nat. Photonics 3(7), 403–405 (2009). [CrossRef]
13. H. Ren, Y.-H. Fan, Y.-H. Lin, and S.-T. Wu, “Tunable-focus microlens arrays using nanosized polymer-dispersed liquid crystal droplets,” Opt. Commun. 247(1–3), 101–106 (2005). [CrossRef]
14. Y. Li and S. T. Wu, “Polarization independent adaptive microlens with a blue-phase liquid crystal,” Opt. Express 19(9), 8045–8050 (2011). [CrossRef] [PubMed]
15. J. Yan, L. Rao, M. Jiao, Y. Li, H. C. Cheng, and S. T. Wu, “Polymer-stabilized optically isotropic liquid crystals for next-generation display and photonics applications,” J. Mater. Chem. 21(22), 7870–7877 (2011). [CrossRef]
16. B. T. Hirai, T. Ogiwara, K. Fujii, T. Ueki, K. Kinoshita, and M. Takasaki, “Electrically active artificial pupil showing amoeba-like pseudophodial deformation,” Adv. Mater. 21(28), 2886–2888 (2009). [CrossRef]
17. H. Xia, M. Takasaki, and T. Hirai, “Actuation mechanism of plasticized PVC by electric field,” Sens. Actuators A Phys. 157(2), 307–312 (2010). [CrossRef]
18. M. Ali, T. Ueki, D. Tsurumi, and T. Hirai, “Influence of plasticizer content on the transition of electromechanical behavior of PVC gel actuator,” Langmuir 27(12), 7902–7908 (2011). [CrossRef] [PubMed]
19. S.-Y. Kim, M. Yeo, E.-J. Shin, W.-H. Park, J.-S. Jang, B.-U. Nam, and J. W. Bae, “Fabrication and evaluation of variable focus and large deformation plano-convex microlens based on non-ionic poly(vinyl chloride)/dibutyl adipate gels,” Smart Mater. Struct. 24(11), 115006 (2015). [CrossRef]
20. J. C. R. Reis, I. M. S. Lampreia, A. F. S. Santos, M. L. C. J. Moita, and G. Douhéret, “Refractive index of liquid mixtures: theory and experiment,” ChemPhysChem 11(17), 3722–3733 (2010). [CrossRef] [PubMed]
