An electrically controllable liquid crystal (LC) microlens with polymer crater, which is simply prepared by droplet evaporation, has been previously proposed as a focusing device possessing excellent characteristics in optical performance, especially for the capability of tunable focal lengths. As the alignment layer on the crater surface cannot be effectively rubbed, non-uniformly symmetrical electric fields in the LC lenses usually induce disclination lines during operation. In this paper, a polymer surface stabilization technique is applied to successfully prevent disclination lines and greatly improve the performance of the LC microlens with the polymer crater.
© 2014 Optical Society of America
The research of liquid crystal (LC) microlenses which have recently attracted great interest have promising applications in machine vision, photonics, optical communications, zoom systems and displays [1–3]. In recent years, various approaches have been developed to obtain tunable focus LC microlenses, such as by using a surface-relief profile [4, 5], a shaped electrode [6, 7], the Fresnel zone type [8, 9], or a polymer network technique . Although several approaches have been reported for fabricating microlenses, these methods are technically complicated and expensive. In a previous work, we have proposed a new method to fabricate a LC lens with a polymer microcrater using the micro-drop (MD) technique . In this method, a de-ionized (DI) water droplet is deposited onto the prepolymer surface, and a spherical-shaped microcavity is then formed on the surface as a result of the droplet evaporation. It’s worth mentioning that DI water is here used as the liquid droplet to restructure the polymer surface instead of an organic solvent, which is typically used in conventional methods. Accordingly, the proposed MD method is not only simple and cost effective, but it also provides the freedom to control the lens shape and position, and is relatively environmental friendly.
The fabricated polymer crater was utilized as a microlens mold for liquid crystal injection. When a voltage (V) is applied to the LC microlens, the electric field within the LC layer is 12–16] and have proposed methods to overcome this problem, such as by controlling the LC lenses with two individually applied voltages , adding an extra in-plane electric field to the LC lenses [13, 14], and/or using the polymer stabilization method [15, 16]. Although using an additional electric field efficiently prevents the disclination lines, special fabricating processes or complicated controlling circuits are necessary.
Polymer surface modification is an efficient method for preventing disclination lines. Thus, we have applied this method to address this undesired defect of the proposed LC microlens. Typically, the surface polymer networks necessary for stabilizing the LC directors are achieved by using a UV light exposure on a LC cell doped with a small amount of reactive mesogen (RM). However, we make direct use of the residual UV monomer remaining in the crater material instead of the RM dopant to create the polymer networks on the LC cell surface here. During the UV polymerization, a high voltage is applied to reorient the LC molecules along the electric field direction and a heating process is utilized to make the residual monomer diffuse into the LCs. After polymerization, a thin polymer layer is formed, which fixes the LC directors near the substrate surface of the cell at high inclination angles, which effectively prevents the occurrence of the disclination lines during the operation. A study of the optical properties of a polymer-crater LC microlens made by means of the surface-controlling technique has been conducted. The experimental results indicate that a successful director stabilization is achieved in the LC microlens within the polymer crater. The polymer stabilized (PS) LC microlens is switchable and the measured focusing properties show that the operation voltage and response time are remarkably improved through this method.
The LC microlens with polymer crater has been fabricated by the MD method and 2-step UV curing processes , as shown in Fig. 1. A DI water droplet (with a volume of 0.1 μL) is micro-dropped onto the surface of the prepolymer layer which is then cured with high intensity UV light to form the spherical crater. The droplet pushes the underlying prepolymer surface and produces an outward extension. Since the liquid droplet and the prepolymer will not mix, the liquid cohesion preserves a symmetrical shape to the liquid droplet. Thereby, due to the gravity of the liquid droplet, a spherical crater configuration can be well constructed on the pre-polymer. To evaporate the liquid droplet and achieve the lens-shaped polymer crater on the substrate surface, a subsequent UV exposure must be utilized to completely cure the prepolymer layer. The dimensions and the geometric profile of the restructured polymer surface can be easily controlled by the volume of the micro droplet, and the UV irradiation dose . Figure 2 shows images of the formed polymer crater structure, taken with an optical surface profiler (Zygo, NewView 73003). The observed results indicate that the polymer crater structure can be simply achieved by using the MD method and a 2-step UV exposure process.
To fabricate a homogeneously aligned (HA) LC microlens device in this study, a polymer crater with a diameter of 753μm and a depth of ~33 μm was constructed, and the polyimide SE-3510 (Nissan Chemicals Co.) film serving as a LC horizontal alignment layer, was then coated, baked, and buffed on the bottom ITO substrate. A positive nematic LC (ROP-92775, Dainippon Ink and Chemicals, Inc.; refractive indices no = 1.497 and ne = 1.64 at λ = 589 nm) was subsequently deposited onto the concave cup of the top substrate. Finally, the upper and lower substrates were assembled to form a cell with a cell gap of 60 μm. When a voltage is applied to the fabricated LC microlens, a non-uniform axially symmetric electrical field exists in the LC cell such that the LC molecules near the surface of the upper substrate will be reoriented in reverse directions, as shown in Fig. 3(a). As such, the disclination lines often occur inherently. To avoid the occurrence of disclination, a polymer surface modification technology is here utilized to form polymer networks within a thin boundary layer in the vicinity of the cell substrate that freeze the molecular configuration at a higher angle and pre-determine the preferred direction of the LC molecules on the substrate surface, as shown in Fig. 3(b).
The preparation of the LC microlenses made by the polymer surface modification method is demonstrated in Fig. 4. A sufficiently high voltage and heat treatment were applied to the LC cell in order to attain the desired LC configuration in the networks during photo-polymerization. Herein, it’s worth pointing out that doping the LC material with an extra UV curable monomer is not required, because the residual monomer remaining in the prepolymer layer is directly utilized to fabricate the polymer network. Therefore, the crater material mustn’t be completely cured before assembling the LC cell, in order to avail of the residual monomer.
For preventing the occurrence of disclination lines in this work, the voltage was applied to the LC microlens with a slowly increasing rate from 0 Vrms to 170 Vrms. Subsequently, the LC lens was heated to 60°C for 5 minutes to make the residual monomer dissolve into the LC layer. While maintaining the 170 Vrms potential and the 60°C temperature, an ultraviolet light (15 mW/cm2) was applied for 20 minutes, thus generating the polymer networks near the cell surface which provide a stable anchoring of the LC molecules. Once the polymer stabilized (PS) LC microlens was completed, the polymer structures sustain the LC directors near the crater surface, and effectively prevent disclination lines. This curing process not only produces polymer networks on the surface but also completely cures the prepolymer layer. The refractive index of the fully cured NOA 61 polymer film is ~1.559 at wavelength λ = 589nm.
To study the focusing properties of the LC microlens made with the polymer surface modification in this work, an AC voltage of 1 kHz frequency was applied to the ITO electrodes, and the focal length, the voltage-dependent image quality, and interference patterns were measured. The measurement system shown schematically in Fig. 3 of the previous work  was set up to probe the voltage-dependent focal length of the LC lens.
3. Results and discussion
A crossed-polarized optical microscope (POM) was used to observe the refractive index change of the LC microlens and the presence of any disclination lines which might occur, with respect to the applied voltage. The polarization axis was 45° with respect to the rubbing direction of the LC microlenses. Photographs of the LC microlens without and with the polymer stabilization were taken and are shown in Figs. 5(a) and 5(b), respectively. The concentric rings of color indicate the change in the refractive index. Initially, the LC molecules were horizontally aligned within the microlens, at Vrms = 0. As the applied voltage exceeded the threshold value, the color of the microlens changed due to the reorientation of the LC molecules within the cell. The phase retardation of the lens decreased with an applied voltage, and the bulk of the LC directors were almost vertically oriented to the substrates under higher voltages so that the color gradually became darker. As also seen in Fig. 5(a), disclination lines appear in the LC microlens made without the polymer stabilization. We found that the disclination lines persisted until the applied voltage surpassed 160 Vrms, and then disappeared gradually. On the contrary, the LC microlens made with the polymer stabilization shows a more uniform refractive index distribution and no disclination lines occur in the cell when directly applying an arbitrary voltage, as shown in Fig. 5(b). We can argue that, as a result of the application of our polymerization technique, the LC director configuration was completely stabilized by the polymer network layer near the surfaces of the PSLC microlens so that defect lines do not appear under different applied voltages.
To evaluate the optical properties of the LC microlenses made using the polymer surface modification process, we investigated the phase profile of the stabilized LC microlens by observing the interference fringes between the ordinary and extraordinary rays of a He-Ne collimated and expanded laser beam (λ = 632.8 nm) passed through crossed polarizers. The rubbing direction of the LC cells was 45° with respect to the polarization axes, and the interference fringes of the LC microlens were apprehended with a coupled charge device (CCD). The recorded interference fringes at the applied voltages of 0, 5, 7, 15, 20, and 40 Vrms are shown in Fig. 6. As the applied voltage was altered, the appearance of the interference fringes was modified due to the reorientation of the LC directors 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. Generally, the numbers of rings appearing in the interference patterns of LC lenses provide information on total phase retardation from the center to the edge of the effective apertures. The variation in phase retardation induced by the applied voltage indicates how the electrical field tunes the lens properties. When the voltage increased, a decrease in the number of interference fringes occurred, indicating that the gradient phase profile of the LC lens flattens with increased voltage. In addition, as observed in the photographs, the interference patterns comprise nearly circular fringes, indicating that the LC lenses are very nearly axially symmetrical. This circular symmetry, the lack of distortion, and the absence of disclination lines were preserved during the voltage change. Thus, using polymer stabilization effectively prevents disclination lines in the LC lenses.
To verify that a twisting of the LC directors in the cell has not occurred, the rubbing direction of the LC cell was aligned with one of the crossed polarizers and we observed if it stayed black while applying a voltage. This test definitely confirmed that the LC molecules of the proposed device are horizontally-aligned. As a result, the refractive index distribution of the lens-like profile depends exclusively on the tilt angle of the LC directors. The tuned refraction modulated the wavefront of the incident light, thereby modifying the focusing properties of the LC microlens. Thus, we validate that the applied electric field has reoriented the LC directors and altered the refractive index distribution. According to the measured interference fringes at different voltages, the focal length of the lens cell can be calculated by the equation ,Figure 7 demonstrates the calculated and measured voltage-dependent focal length of the LC microlens. The measured focal length of the LC microlens was found to be approximately −8.8 cm at the null voltage, and the focal length increases directly with an increase in the applied voltage. The focal length became nearly constant, peaking (approximately −30 cm), at voltages beyond 50 Vrms. While the measured voltage-dependent focal length here is noticeably consistent with the calculated one, a small deviation between the two focal lengths comes from the uncertainty in the counted number of interference rings due to the central-asymmetrical distributions of LC directors in the polymer crater. This unexpected result is caused from the polymer crater being fabricated by hand, and not with a drop-on-demand inkjet apparatus.
The imaging properties of the LC microlens at the microscopic scale were also evaluated using a parallel-polarized optical microscope (POM), with the transmission axis of the polarizer set parallel to the rubbing direction of the LC microlens. The LC microlens was placed on a photomask with several digits and patterns, which acted as a focal object, where the distance between the LC microlens and the object was 0.2 mm. Figure 8 shows the lens-rendered image of the microlens without surface modification and three images of the polymer stabilized LC microlens for various applied voltages. Comparing Fig. 8(a) with 8(b), we find that the occurrence of a disclination line in the unstabilized LC microlens makes the image blurry, as shown by the dashed red circle in Fig. 8(a). We conclude that the disclination line significantly lowers the image quality. When the applied voltage on the polymer-stabilized LC microlens exceeded the threshold value (V = 0 Vrms), the central part of the image was magnified due to the variations of the refractive index in the cell, and the image gradually became almost fixed at a voltage of 45 Vrms. Since a polymer network is used in the system, the mismatch of refractive indices between the LC molecules and the polymer networks leads to a slight optical scattering. To overcome this issue in the future, a more appropriate polymer/LC material combination should be chosen with a view towards fabricating a PSLC lens with an optimum morphology of the polymer network.
In addition, a remarkable improvement in LC microlenses’ response times was achieved by the polymer surface modification process. As disclination lines occurred in the LC microlenses without polymer stabilization, the response time for the reorientation of the LC molecules was severely slowed under the applied voltages. For example, the response time of the LC microlens without polymer surface treatment was found to be ~7.3 s at an applied voltage of 70 Vrms. On the contrary, the PSLC microlenses had a much faster response time (~0.9 s) which is a drastic reduction to one-eighth of that without the polymer stabilization. This attractive result is produced by the anchoring effect of the polymer network, which has stabilized the LCs near the surfaces of the PSLC microlens in a consistent direction conducive towards avoiding the occurrence of disclination lines when applying a direct voltage. Besides, due to the large cell gap used here, the driving voltage and response time of the proposed LC microlens are higher than those of the typical LC microlens. The non-attractive lens properties are mainly caused by the proposed polymer crater being fabricated by hand-dropping a water droplet on the pre-polymer surface, and not by a drop-on-demand inkjet apparatus, since the precise volume of liquid in the droplet used to depress the pre-polymer surface is really difficult to control manually. Herein, to obtain a more perfect crater structure, the thickness of the polymer layer should be increased. According to Eq. (1), as the thickness of the polymer layer is increased, the applied electric field on the LC layer should decrease and then a higher operation voltage is required to drive the proposed LC microlens. At the same time, a longer response time of the microlens is needed. If the droplets can be deposited onto the pre-polymer surface by an ink-jet printer, a greater accuracy and consistency in controlling the tiny volume of the droplet can be attained. Thus, a symmetrical shape of the polymer crater, and a reduced polymer layer thickness can be potentially achieved. Accordingly, the thickness of the polymer and LC layers can be significantly reduced, and then the driving voltage and response time are expected to be notably improved. To create a perfect LC microlens for practical applications, an ink-jet micro-dropper should be employed.
In this work, a simple, novel, and low-cost fabrication method, using liquid micro-droplets and UV exposure to build PSLC microlenses with polymer craters was proposed. To achieve an optimal performance of a LC microlens nested within a polymer crater and to successfully avoid the occurrence of disclination lines, a polymer surface modification has been employed which takes direct advantage of the residual UV monomer remaining in the crater material, instead of doping the LCs with a RM. According to the experimental results, the fabricated PSLC microlenses demonstrated superior performance during operation, and the polymer surface stabilization notably improved the response time, making it eight times faster than those made without the surface stabilization treatment. The polymer stabilization process has been reliably repeated in our work, and is expected to be repeatable in subsequent treatments.
The authors would like to acknowledge the funding of the research by the National Science Council of Taiwan (NSC101-2221-E-239-025-MY2). In addition, we sincerely appreciate Ms. Vicky Chang of Jabil Green Point Corp. for her technical assistance.
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