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A simple and low-cost technique is proposed to construct a tunable liquid crystal (LC) microlens with a crater polymer structure, which is prepared using micro-drop technology and 2-step UV polymerization. The dimensions and the geometric profile of the restructured polymer surface significantly depend on the volume of the micro droplet, and the UV irradiation dose. In this work, the focal length of the LC microlens is controlled electrically from infinity to 7.8 cm. Such a microlens has prospective applications in optical communications, image processing, and switchable 2D/3D displays.
© 2013 Optical Society of America
1. Introduction
LC microlenses are significantly attractive for a variety of applications in machine vision, cameras, optical communications, and eyeglasses as they possess the benefits of being low-cost and light weight, as well as having no mechanical moving parts [1–9]. When the applied voltage exceeds the Fréedericksz transition threshold, the LC molecules are reoriented in the direction of the electric field, which alters the refractive index of the LC and affects the phase retardation of the input light. Thus, the focal length of the lens device can be modulated by controlling the voltage. In recent years, several approaches have been developed for fabricating LC microlenses, including spherical-shaped LC lenses [4], LC lenses with a slit electrode [5], adaptive lenses using LC concentration redistribution [6], LC lenses based on polymer network distribution [7], and hole-patterned LC lenses [8]. Typically, these methods are technically complex and expensive.
Herein, we develop a simple and economical approach to fabricating a LC microlens device with a crater polymer structure by using the droplet evaporation method. Although the evaporation of solvent drops from polymers has been extensively employed to develop microstructures on polymer surfaces for a variety of applications in recent years [10–13], our approach is unique and different from the previous works. In previous approach, solvent droplets are deposited onto a polymeric surface; a spherical cavity subsequently forms after the complete evaporation of the solvent due to several physical effects [11], such as the Laplace pressure, surface tension, and the dissolving of the polymer layer. Organic solvent drops are typically applied to not only push the underlying surface, but also dissolve part of the polymer material. Thus, the crater polymer as a template to fabricate microlenses is then realized. However, organic solvent is always harmful to people’s health and the environment. To make the living environment greener, a readily available and low-cost material—namely, water—is utilized to replace the organic solvents in this work.
To make a concave surface via droplet evaporation method, a water droplet is first micro-dropped on the surface of a pre-polymer using a pipette. The pre-polymer is restructured to form a microvessel by the gravity squeeze of droplet and then cured completely by UV illumination during the droplet evaporation. We can use this crater polymer as a template to make tunable LC microlenses. The shape and dimension of the restructured polymer can be easily controlled by the UV exposure condition and droplet volume. Consequently, a highly efficient and electrically controllable LC microlens is produced. In this way, liquid droplets can be deposited onto a polymeric surface by a drop-on-demand inkjet apparatus. Therefore, the proposed method can be potentially extended to make microlens arrays by using inkjet printer, resulting in a relatively simple, low-cost, environmentally friendly, and energy-saving procedure.
2. Experiment
2.1 Fabrication of the LC microlens with a crater polymer mold
Figure 1 shows the schematic diagram for the fabrication of the LC microlens using a crater polymer mold. The upper and lower substrates are formed separately and then assembled to form the LC microlens. The related procedures for each of the series are described in the following sections.
2.1.1 Formation of crater polymer on the upper substrate
First, a UV-curable monomer (NOA61, Norland) was coated on the ITO glass substrate using a steel blade and then exposed to UV light (with an intensity of 1.0 mW/cm2) from an Hg lamp for certain time to form the pre-polymer layer, which was subjected to partial photopolymerization. A pipette dispenser (Nichipet; purchased from Nichiryo Ltd.) was used to sequentially microdrop a controlled volume of deionized (DI) water onto the surface of the partially cured pre-polymer layer; as shown in Fig. 2, the droplet adheres to the underlying surface and extends outward. As the liquid droplet and the pre-polymer are not miscible, the geometric symmetry of the liquid droplet is preserved by cohesion. During the droplet evaporation, a second high-intensity UV exposure (4.0 mW/cm2 for 400 sec) is applied to cure the polymer completely and promotes droplet evaporation. The evaporation occurs within a few seconds, a microscopic crater on the substrate surface is thus formed as the prepolymer is displaced by the gravitational force of the water and then hardened by UV exposure. The water droplet on the surface of pre-polymer reflects UV radiation and the hardening polymer around droplet shrinks with a side effect of a crater formation in the not cured region. The dimension of spherical-like cavity, which can be employed as a template for a LC microlens, is comparable to the deposited droplet size. In this way, the desired crater geometry can be easily controlled by the volume of the liquid droplet and the degree of hardness of pre-polymer material, which is influenced by the 1st-UV curing time. The refractive index of the fully cured NOA 61 polymer film is ~1.559 at wavelength λ = 589nm [14].
Fig. 2 A schematic diagram of the crater formation. (a) A model of the Micro-Drop technology process. (b) The mechanism in fabricating the polymer crater.
2.1.2 Fabricating LC microlens with the crater polymer
To fabricate a vertically-aligned (VA) LC lens device, the polyimide 5662A (Nissan Chemicals Co.) films, serving as LC vertical alignment layers, were first coated and baked on the two ITO substrates. To make the reorientation of LC molecules induced by the applied voltages in the same direction, the surface of polyimide film on the bottom substrate was rubbed using a rubbing machine. Subsequently, a negative nematic LC (MJ5667; refractive indices no = 1.4842 and ne = 1.5852 at λ = 589 nm; Merck Ltd.) was dispensed on the polymer crater. Finally, the upper and lower substrates were assembled into a cell, with a cell gap of 60 μm.
2.2. Measurement of the lens properties
To characterize the focusing properties of the LC lens, the image quality, interference patterns, and focal length under different applied voltages were measured. The scheme of the measurement system was set up to probe the voltage-dependent focal length of the LC lens, as shown in Fig. 3. Since the proposed VA-LC microlens, which is a concave lens, will diverge the ray. A convex lens was arranged behind the LC sample in this work to investigate the voltage-dependent focal length of this negative lens. In addition, the interference patterns induced by LC lenses were observed under crossed polarizers. The rubbing direction of the LC microlens was 45° with respect to the polarization of both polarizers. The ordinary and extraordinary light waves are incident on the LC cell; the ordinary wave experiences a spatially uniform phase shift while the extraordinary wave experiences a phase shift of the nearly spherical profile. Therefore, as the two waves pass through an analyzer which the directions of polarization are ± 45° with respect to the light wave, the interference between them occurs. The interference fringe patterns captured by a CCD provide the information of the phase shift experienced by the extraordinary wave. In this work, an AC voltage of 1 kHz frequency is applied to the ITO electrodes.
4. Results and discussion
4.1 Polymer crater structure
First, to observe the shape of polymer crater structures, 2D images were taken with a microscopy and optical surface profiler (Zygo, NewView 73003) (see Figs. 4(a) and 4(b), respectively). The observed results indicate that the droplet evaporation satisfactorily restructured the polymer surface. The concave structure can be simply fabricated using the micro-drop and 2-step UV exposure processes.
Fig. 4 2D images of the polymer crater observed by (a) optical microscope and (b) optical surface profiler.
To investigate the impacts of the liquid droplet volume and the 1st-UV exposure time on the micro-crater structure, the crater morphology was measured with a surface profiler (Taylor-Hobson 1250A). Figure 5(a) shows the depths and diameters of the craters made with different pre-exposure (1st-UV) times while the drop volume was fixed at V ~2μL. Figure 5(b) shows the depths and the diameters of the craters made with different drop volumes while the pre-exposure time was held constant at t = 16 minutes. As seen in Fig. 5(a), when the drop volume is constant and the pre-exposure time is increased from t = 16 minutes to 20 minutes, the diameters and depths of the craters vary from 1982 μm to 2275 μm and from 282 μm to 182 μm, respectively. As the pre-exposure time increases, the pre-polymer layer stiffens so that the liquid droplet cannot penetrate as deeply into the pre-polymer layer, thereby indirectly increasing the diameter and decreasing the depth of the polymer crater.
Fig. 5 The measured crater diameter and depth versus (a) the pre-exposure time, and (b) the drop volume.
Figure 5(b) shows that, when the pre-exposure time is held constant and the liquid volume is increased from 2 μL to 6 μL, both the diameters and depths of the spherical craters increase. As the drop volume increases, the additional gravitational force enlarges the depression and the contact area on the surface of the pre-polymer layer. Thus, both the diameter and depth of the crater increase. These experimental results reveal that the crater morphology can be significantly controlled by adjusting the first UV exposure time and the drop volume.
4.2 Focus properties of the LC microlens
The color of the LC microlens under different applied voltages was observed to be different by polarized optical microscope (POM), as shown in Fig. 6. This color difference indicates that the reorientation of the LC molecule is tuned by varying the applied voltage. Initially, the LC molecules were vertically aligned within the microlens, at Vrms = 0. When the applied voltage exceeded the threshold value, the color of the microlens changed due to the reorientation of the LC molecules in the cell. The phase retardation of the lens increased with the applied voltages, and the bulk of the LC directors were almost homogeneously oriented to the substrates under higher voltages so that the color gradually became brighter.
Fig. 6 The LC microlenses observed by a cross-polarized microscope under different applied voltages. The arrows P and A represent an orientation of crossed polarizer and analyzer, as well as R is the rubbing direction.
To evaluate the optical properties of the tunable LC microlens, we observed the interference fringes between the ordinary and extraordinary rays using a He-Ne laser beam (λ = 632.8 nm) passed through the lens placed between two crossed polarizers. Figure 7 shows the recorded interference fringes at several different applied voltages. When the applied voltage is altered, the appearance of the interference fringes is modified due to the reorientation of the LC directors as caused by the applied electric field. Note that the retardation difference of two adjacent constructive or destructive interference rings indicates a phase change of 2π. The variation in phase retardation induced by the applied voltage represents how the electrical field alters the lens properties. As the voltage increased, an increase in the number of interference fringes occurred, indicating that the curvature of the phase profile of the LC microlens was gradually boosted with increased voltage. In addition, the interference patterns are composed of almost circular fringes, indicating that the LC lens is nearly axially symmetrical. The fringes also directly reveal the refractive index distribution of the lens-like profile, which depends only on the tilt angle of the LC directors.
Fig. 7 The interference fringes of the LC microlens at different voltages of 0, 15, 20, 30, 40 and 80 Vrms.
Based on the measured interference fringes at different voltages, the focal length of the lens cell can be calculated using the equation [8], where r is the radius of the lens aperture, λ is the wavelength of incident laser beam, and N is the number of observed rings in the interference patterns. As a comparison, the focal length of the lens cell was also obtained by measuring the distance between the lens and a focused light spot. Figure 8 demonstrates the calculated and measured voltage-dependent focal length of the LC microlens. At applied voltages that fall below the threshold value, the LC directors within the inner area of the LC lens have not reoriented and, thus, fail to produce the distribution of gradient refractive indices necessary for a lens. Therefore, the focal length of the LC is nearly infinity. However, as the voltage increases, the focal length declines dramatically until it approaches the shortest distance at Vrms = 50 V. At voltages above 50 V, the focal length remains nearly constant. Although the calculated focal length here is a bit different from the measured one, we believe that the inconsistency of focal lengths is caused by the uncertain counted number of interference rings due to the central-asymmetrical distributions of LC directors in the polymer crater. This undesired result is caused by the proposed polymer crater fabricated by hand-dropping a water droplet on the pre-polymer surface, not by a drop-on-demand inkjet apparatus. To overcome this issue, an ink-jet printer should be employed to create a prefect micro-cavity with the central symmetrical crater structure.
Fig. 8 The focal length of the LC microlens with respect to the applied voltage, as directly measured, and as calculated from the number of rings in the interference pattern.
Furthermore, the imaging properties of the LC microlens at the microscopic scale were evaluated using a parallel POM, with the transmission axis of the polarizer set to be parallel to the rubbing direction of the LC cell. When the LC microlens was placed at a distance of 0.2 mm from a focal object—namely, a photomask with several digits and patterns—the image was found to be somewhat deformed. Figure 9 compares the original image of the photomask with three lens-rendered images for different applied voltages. The lens-like refractive index distribution is formed by the applied electric field in the LC layer and then modulates the wavefront of an incident plane light wave. As a result, the image of the proposed lens can be simply modified by the applied voltage.
Fig. 9 (a) A focal object seen only with an optical microscope, and the imaging behavior of the LC lens at (b) V = 0, (c) V = 20 and (d) V = 30 Vrms.
The droplet evaporation restructures the crater-shaped polymer surface well, which in this work is successfully realized using the MD method and a 2-step UV-curing processes. As the mechanism to obtain a high-quality crater surface topology is very complicated, the shape of the microcraters is governed by several physical parameters, such as the drop’s surface tension, the droplet volume, the capillary pressure at the bottom of the drop, and hardness of the pre-polymer. Therefore, the optimum control of the parameters in the fabrication process is critical. We believe this paper provides a unique way for fabricating the LC microlens with a crater structure. Furthermore, the MD technique offers several advantages, including the ability to control the size, shape, and position as well as the low consumption of the lens material and the simplicity of the fabrication process. Accordingly, if an ink-jet printer can be applied to deposit the droplets on the pre-polymer surface, greater freedom and consistency in controlling the shape and position of the polymer crater, reduced consumption of the lens material, and simplified fabrication processes can be potentially achieved.
5. Conclusions
We presented a simple and novel fabrication method for building LC-based microlenses with the polymer microcrater. Both droplet volume and 1st-UV exposure are very crucial to the final polymer crater structure and optical performance of the LC lenses. Compared with other reported LC lenses of various structures and fabrication methods, the proposed method is more suitable for making LC lenses or lens arrays of a very small size, from several tens to several hundreds of micrometers in diameter. As such, the presented method provides extremely simple and low-cost technology for making tunable LC microlens devices, possibly with an ink-jet printer.
Acknowledgment
We would like to acknowledge the funding of the research by the National Science Council of Taiwan (NSC101-2221-E-239-025-MY2) and Suyin Optronics Corp., Taiwan. In addition, we are sincerely appreciated to Mr. Tom Sung of Suyin Optronics Corp. and Ms. Vicky Chang of Jabil Green Point Corp. for their technical assistances.
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