As a tunable core optical device, liquid crystal microlens array is widely used in 2D/3D switchable glasses-free 3D display, adaptive optics and other fields. However, the design and fabrication of liquid crystal microlens arrays is difficult and costly, and usually involves precision machining technologies such as photolithography, making it difficult for large-scale mass production.
In view of this, Yanjun Liu’s team from Southern University of Science and Technology and Yanqing Lu’s team from Nanjing University proposed a technology for preparing liquid crystal microlens arrays based on photopolymerization-induced phase separation. This technique requires only a single exposure step to fabricate large-area, high-quality liquid crystal microlens arrays. Experiments show that the liquid crystal microlens array has good focusing and imaging effect, and can be used to expand the imaging depth of integrated imaging 3D display. This technology is expected to greatly reduce the difficulty and production cost of liquid crystal microlens arrays.
At present, this work is presented as “Optically Anisotropic, Electrically Tunable Microlens Arrays Formed via Single-Step Photopolymerization-Induced Phase Separation in Polymer/Liquid-Crystal Composite”. Materials” was published in Light: Advanced Manufacturing. Cai Wenfeng, a Ph.D. student at Southern University of Science and Technology, is the first author of the paper, and Professor Lu Yanqing of Nanjing University and Associate Professor Liu Yanjun of Southern University of Science and Technology are the corresponding authors.
Microlens arrays are widely used in optical systems, such as light diversion, wavefront detection, fiber coupling, three-dimensional display, etc. Liquid crystals, on the other hand, are anisotropic materials that endow them with features that traditional microlens arrays do not have, such as adjustable focal length and beam deflection. Compared with mechanical zoom lenses, liquid crystal microlens arrays are more light and thin, conducive to integration, short response time and good stability, and are widely used in 2D/3D switchable naked-eye 3D display, automatic focus imaging system, automatic projection system, etc., which has great application value. However, the fabrication of liquid crystal microlens arrays usually involves precision machining techniques such as photolithography, and the preparation process is relatively complex and costly.
In view of the above problems, the researchers proposed a technique to fabricate large-area liquid crystal microlens arrays based on Polymerization induced phase separation (PIPS), as shown in Figure 1. In this method, Phase-separated composite films (PSCOF) were formed in liquid crystal cells by single-step ultraviolet exposure. The microstructure topography of the composite film is controlled by a grayscale mask above. In the picture, the top is a grayscale mask with a microlens array pattern. Below is a liquid crystal box with a composite structure of a polymer layer and a liquid crystal layer with the morphology of the microlens array.
Figure 1: Schematic diagram of the preparation of a liquid crystal microlens array
The process of photopolymerization-induced phase separation to form a liquid crystal microlens array is shown in Figure 2. First, the prepolymer is poured into the liquid crystal cartridge. Prepolymers are made from a homogeneous mixture of liquid crystals and polymerizable monomers. Since the prepolymer has a strong absorption of UV light, the UV light intensity is distributed in a gradient in the direction of the vertical liquid crystal cell. Under the action of ultraviolet light, the monomer in the prepolymer first undergoes a polymerization reaction on the side close to the light source, and the molecular weight of the prepolymer increases. When a critical value is reached, the mutual solubility of the liquid crystal and the prepolymer decreases until phase separation occurs and gradually precipitates. In order to maintain the concentration equilibrium between the monomer and the liquid crystal molecule, the liquid crystal molecule will move to the lower layer, and the monomer will move to the upper layer, which is the process of diffusion. This process continues until the liquid crystal and polymer are completely separated, resulting in the formation of a polymeric phase separation composite film (PSCOF). Finally, when the temperature drops below the clear point, the liquid crystal is oriented by the orientation layer of the lower layer.
Due to the grayscale mask, the UV light intensity is distributed in a periodic gradient parallel to the LCD cell, as shown in the purple area in Figure 2. The monomer first undergoes a polymerization reaction in the region with strong light intensity, so the morphology of the final polymer layer is related to the gray distribution of the reticle. By designing the grayscale distribution of the reticle, the researchers successfully prepared a liquid crystal microlens array.
Figure 2: The process of ultraviolet photopolymerization phase separation to form an array of liquid crystal microlenses
The research team prepared a 5 cm × 5 cm liquid crystal microlens array sample, as shown in Figure 3. Under a polarizing microscope, the sample exhibits a rich variety of interference colors. The color of each microlens unit gradually transitions from red in the center to blue at the edges, indicating that the thickness of the liquid crystal layer gradually changes from the center to the edges.
Figure 3: Sample plot of a liquid crystal microlens array
In addition, the research team characterized the optical properties of liquid crystal microlens arrays. The results show that the prepared liquid crystal microlens array has a uniform focusing effect. In addition, the liquid crystal microlens array is close to the diffraction limit and has a good focusing effect. Due to the polarization-dependent and electrically adjustable nature of liquid crystals, researchers also use these two methods to control the focusing and imaging effect of the prepared samples. Videos 1 and 2 show the focusing effect of a liquid crystal microlens array under polarization modulation and voltage modulation, respectively.
Finally, the research team prepared a liquid crystal microlens array for integrated imaging 3D display, as shown in Figure 4. Among them, the light source is combined with the mask as the three-dimensional display source. The researchers took advantage of the adjustable focal length of the liquid crystal microlens array to adjust the position of the central depth imaging plane and expand the imaging depth of the three-dimensional display. When a sufficiently large voltage is applied, the focusing effect of the liquid crystal microlens array disappears. This enables 2D/3D switchable glasses-free 3D display.
Figure 4: Integrated imaging 3D display based on liquid crystal microlens array
In summary, this technique requires only a single exposure step to fabricate large-area, high-quality liquid crystal microlens arrays. Therefore, this technology is expected to greatly reduce the difficulty and production cost of liquid crystal microlens arrays. In addition, this technology can be used as a general platform for the fabrication of liquid crystal micro-optical devices with other functions, such as liquid crystal column lens arrays, liquid crystal gratings, etc. (Source: Advanced Manufacturing WeChat public account)
Related Paper Information:https://doi.org/10.37188/lam.2023.028
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