We report on the liquid crystal (LC) alignment induced by sparse polymer ribbons fabricated by the two-photon polymerization-based direct laser writing method. Each ribbon is fabricated by a single scan of the laser through the photoresist and possesses surface relief gratings on both sides. The relief gratings are caused by the optical interference between the incident and reflected laser beams. With the aid of these relief gratings, LC molecules can be well aligned along the selected direction of the ribbons. LC cells with the -shaped and checkerboard-type microstructures are constructed based on the sparse out-of-plane polymeric ribbons. Our results show that with such polymer ribbons a compartmentalized LC alignment in the arbitrary microstructures can be realized.
© 2016 Optical Society of America
Surface alignment layers for liquid crystal (LC) alignment are essential to all LC electro-optical devices. Mechanically rubbed polymer films are conventionally used in LC displays (LCDs), especially in large-area flat panel displays. Nevertheless, the rubbing process exhibits specific disadvantages, such as introduction of dust particles and electrical charging. As a result, contact-free methods for the alignment of LC molecules, in particular various photo-alignment techniques [1 –3], have recently attracted a significant amount of interest and some have already been successfully applied in the LCD industry .
Appealing additional functions achieved by patterned alignment represent the core of certain advanced LCD operation modes [5 –9] and of several non-display LC applications [10,11]. Micron-scale surface configurations that induce a patterned LC alignment can be of a chemical or of a physical nature, i.e., they can be based either on pre-designed chemical [12 –15] or topographical surface modifications [16 –22]. Different techniques are utilized to realize specific orientation of the LC molecules in miniaturized zones by the help of the above described structures; however, the associated fabrication processes are usually quite complicated, and the patterns generated are mostly relatively simple and uniform. In contrast, two-photon polymerization-based direct laser writing (TPP-DLW) is a very convenient process to generate sophisticated nano- and micro-surface patterned structures in an efficient and pre-designed contactless manner [23,24]. Lee et al. demonstrated that a micrograting-type pattern fabricated by the TPP-DLW method can induce LC molecules to align parallel to the grating channels . The anchoring strength of the alignment was manipulated by varying the depth and/or the period of the grating pattern .
Obviously, the methods mentioned above are all based on in-plane alignment structures, i.e., the alignment effect is generated by the flat substrate surfaces that confine the LC film. Nevertheless, investigations of micrometer sized objects placed into the LC layer suggest that alignment could be induced also by out-of-plane structures located in-between the substrates [27 –29]. In our recent study , a 1D periodic scaffold structure of polymer ribbons was fabricated by the TPP-DLW approach to realize the spontaneous planar alignment of LCs. However, the LC molecules in the periodic structure were aligned uniformly, rather than orientated differently and compartmentally.
In this Letter we utilize the TPP-DLW method to fabricate sparse out-of-plane polymer ribbons, with which LC cells with -shaped and checkerboard-type microstructures are constructed. The polarized optical microscopy (POM) transmission images of the cells show that the compartmentalized alignment states of the LC molecules are achieved with their directors parallel to the local orientation of the polymer ribbons. These ribbons possess surface relief gratings on both sides that are in contact with the LC material. By analyzing the surface topography of these surface relief gratings, we confirmed that the gratings are caused by optical interference between the incident and reflected laser beams used in the TPP-DLW process. The alignment layers are oriented perpendicular to the LC cell substrates, which is very different from traditional LC devices. The sparse distribution of polymer ribbons not only guarantees the compartmentalized LC alignment in pre-designed microstructures, but also increases the fabrication speed significantly.
Figure 1(a) schematically shows the experimental setup used for fabrication of the polymer ribbon patterns. A negative photoresist material (SU-8 3025, supplied by MicroChem Corp.) was spin-coated on a clean ITO-coated glass plate () and then soft baked on a hotplate to evaporate the residual solvent. The plate was then mounted on a computer-controlled 3D translation stage (PI, M-405.CG) capable of providing a scanning velocity of . A Ti:sapphire laser beam (wavelength , 120 fs pulse duration, 1 kHz repetition rate) with an average power of 70 μW was focused to a suitable position near the interface between the SU-8 film and the substrate by an objective lens (, ). In the focal region, the polymerization reaction took place via the TPP process and polymer ribbons were achieved by suitable motion of the translation stage; each polymer ribbon was formed by a single translation stage scan. After exposure, the film was post baked and then processed using a SU-8 developer followed by isopropyl alcohol. This immersion-based developing process produced ribbons that remained oriented perpendicular to the substrate, as depicted in Fig. 1(b).
The substrate bearing the pre-designed out-of-plane polymeric ribbons was covered with another glass plate without any surface alignment layers. The assembled LC cell had a cell gap of 10 μm so that it matched the height of the ribbons. A conventional nematic liquid crystal mixture E7 (Shijiazhuang Chengzhi Yonghua Display Material Co.) was injected into the LC cell in its isotropic phase () and, then, slowly cooled to room temperature. Assembled cells were observed with a polarizing optical microscope.
In the first example, two parallel ribbons with a distance of 20 μm were fabricated to form a -shaped channel. Figures 2(b) and 2(c) show POM transmission images of local LC alignment in the -shaped pattern. Under crossed polarizers, the horizontal part of the channel is dark and the diagonal part is bright [Fig. 2(b)]. After rotating both polarizers for 45°, the bright and the dark regions interchange with each other [Fig. 2(c)]. The phenomena suggest that the LC molecules injected into such a channel were well aligned locally, parallel to polymer ribbons, or along the diagonal direction at the turning points due to the competition of the alignment effects. The corresponding director distributions of LC molecules are illustrated in Fig. 2(a). The LC regions out of the channel are not aligned, because there is no alignment coating on the glass substrates.
With the above described method, one can fabricate arbitrary shaped “channels” with the LC molecules lying parallel to the channel walls in all locations if the distance between the ribbons is smaller than 30 μm, which can be very convenient both for construction of LC-based microfluidic devices and for the assembling of the optical waveguide systems. In addition to their alignment function, the ribbons also act as physical barriers that divide the LC cell into different zones. Consequently the LC orientational profiles in different zones are completely independent from one another. This attribute allows LC patterning to be scaled down to the nanometer regime, which is otherwise difficult to obtain due to the large increase of the LC elastic energy in the interface regions between the zones.
As the second example, the compartmentalized LC alignment was generated via a 2D checkerboard-type periodic pattern consisting of sparse polymer ribbon pairs. A unit block of [shown in Fig. 3(a) and designated by the red dashed box in Figs. 3(b) and 3(c)] is formed by four pairs of polymer ribbons; in the image, the upper left and lower right pairs are oriented horizontally, while the upper right and lower left pairs are oriented vertically. The POM images of the resulting LC structure under crossed polarizers are shown in Figs. 3(b) and 3(c), with the crossed polarizers being rotated for 45° in Fig. 3(c). The distance between the two ribbons forming the pair is 20 μm. One can observe that the excellent compartmentalized LC alignment patterns agree well with the periodic microstructure. The LC alignment in the regions inside the ribbon pairs coincide with the local orientation of the ribbons. In the intermediate regions, the LC molecules exhibit a diagonal orientation with respect to the ribbons, which is due to the competition of the alignment effects of the neighboring ribbon pairs. Figure 3(a) illustrates the local LC orientation in the unit block.
From the above POM transmission images, one can see that the LC molecules close to the polymer ribbons aligned well along the ribbons. In order to reveal the mechanism of the alignment effect, a single collapsed ribbon was prepared by rinsing the ribbon with the developing liquids in a perpendicular direction. A scanning electron microscopy (SEM) topographical image of the collapsed ribbon is shown in Fig. 4(a). The presence of a perfect periodic surface undulation is evident. Analysis of surface topography with the atomic force microscopy (AFM) in Fig. 4(b) shows that the depth of the undulation is about 53 nm and the period of the relief grating is 251 nm. These values are consistent with the half-wavelength of the laser light in the SU-8 film. This result suggests that the formation of the periodic surface relief structure on the side walls of the ribbons came from the optical interference between the incident beam and the reflected beam by the ITO film between the glass substrate and the SU-8 film.
To verify the alignment effect, the glass substrate with a single collapsed ribbon was used as the bottom layer of a LC cell with a cell gap of 5 μm [Fig. 5(a)]. The top layer was a standard rubbed polyimide-coated glass plate oriented with the rubbing direction perpendicular to the ribbon direction. Figures 5(b) and 5(c) show POM transmission images of the E7-filled cell at room temperature. When viewed under crossed polarizers, the region above the polymer ribbon appears bright, while the regions far from the ribbon appear dark. An opposite effect occurs when the device is viewed under parallel polarizers. This observation demonstrates that a twisted nematic configuration is established between the side surface of the ribbon and the rubbed polyimide layer, which proves that the LC molecules are well anchored along the surface relief gratings.
Two substrates being paved completely by parallel collapsed ribbons were fabricated. A conventional twisted nematic LC cell with orthogonal orientation of the ribbons on the lower and the upper plates was assembled. The rotation of optical polarization produced by the twisted LC structure was verified by their POM transmission images. The surface anchoring energy of the surface relief gratings was estimated to be through the torque balance method . The anchoring effect of the sidewall relief gratings formed by the TPP-DLW process can be easily controlled by modification of the illumination conditions. For example, the grating period can be changed by tuning the laser wavelength, and the grating depth can be affected by adding a selected reflective or antireflective coating on the glass substrate. It is known that some LC materials can induce plasticization of the polymers, or that LC molecules can diffuse into the polymer material, which can reduce the long term quality and performance of the LC devices. Therefore, aging of surface anchoring should be investigated in further studies.
In conclusion, we demonstrate that the TPP-DLW approach is very suitable for the generation of compartmentalized LC alignment. The sparse out-of-plane polymer ribbons with the surface relief gratings formed by a single scan of the laser through the photoresist can evidently reduce the fabrication time of patterned compartments. Due to their specific features, such compartmentalized alignment assemblies open up many new perspectives and challenges that can possibly be applied in fabrication of various LC-based devices. Two such challenges already mentioned in the text are alignment in arbitrary curved channel-type configurations and downscaling of the patterning to the nanometer regime. Another challenge would be the construction of micro- or even nano-scale compartments with the bi- or multi-stable LC orientational profiles. Such structures are very interesting for fabrication of LCD devices providing permanent images that can be displayed in the absence of an external voltage.
National Natural Science Foundation of China (NSFC) (11174161, 61205035); National Basic Research Program of China (2013CB328702); The 111 Project (B07013); PCSIRT (IRT0149); International S&T Cooperation Program of China (2011DFA52870); National Research Program of Slovenia (P1-0192).
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