Self-assembling microlens array with ultra-smooth surface and high resolution

Microlens arrays (MLAs) have become essential optical components in applications such as optical sensing, lidar, light field cameras, optical microscopy, high-throughput maskless lithography, 3D imaging, and stereoscopic displays due to their support for large field of view and infinite depth of field.

Recently, the team of Professor Zhou Guofu of South China Normal University reported a new method for low-cost, batch manufacturing of MLA, obtaining a large-area MLA with controlled curvature, with a numerical aperture of up to 0.26, which can be precisely adjusted by adjusting the modified intensity or droplet dose. The team demonstrated that the manufactured MLA has a high-quality surface with sub-nanometer roughness and allows high-resolution imaging up to 10328 ppi. This study demonstrates a large-scale, low-cost method for the preparation of high-performance MLA, which is expected to be widely used in the fast-growing integrated imaging industry and high-resolution displays.

The article was published in Light: Science & Applications under the title “Mold-Free Self-Assembled Scalable Microlens Arrays with Ultrasmooth Surface and Record-high Resolution.” Liu Zhihao of South China Normal University and Assistant Professor Hu Guangwei of Nanyang Technological University in Singapore are co-first authors of this paper, and Professor Zhou Guofu, Associate Professor Ye Huapeng and Professor Tang Biao of South China Normal University are the corresponding authors of this paper.

Microlens arrays generally refer to miniature lens arrays with an aperture of less than 1 mm, and their key parameters mainly include numerical aperture, surface roughness, fill factor, field of view, modulation transfer function, depth of field, etc. With its support for large field of view and infinite depth of field imaging, MLA has become an indispensable optical component in a wide range of applications such as optical sensing, lidar, light field cameras, optical microscopy, high-throughput maskless lithography, 3D imaging, and stereoscopic display. With the advancement of modern manufacturing technology, MLA with high resolution and low aberration has become an indispensable component in miniaturization and high-resolution 3D photography and integrated imaging.

At present, the preparation methods of MLA are mainly divided into indirect method and direct method. One of the main challenges of this method is that the preparation of molds or pattern masks is complex and expensive, and that post-processing processes such as peeling and transferring can damage the surface profile of the microlens. The direct method mainly relies on the surface tension of the lens material, which makes the surface topography roughness of the microlens as low as 1 nanometer during the manufacturing process, and can realize large-scale micron droplet arrays, thereby realizing the mass production of MLA. However, the geometry of MLA manufactured by the direct method is controlled by parameters such as temperature, wettability, liquid dose, and processing time, and the precise control of curvature and focal length is still difficult. In order to obtain droplets with large radius of curvature, it is sometimes still necessary to define the physical boundaries of the droplets with the help of templates. At present, large-scale, low-cost preparation of MLA with controlled focal length, smooth surface and high fill factor still faces great challenges, and these features affect the performance of the relevant optical system.

In order to solve the above problems, Professor Zhou Guofu’s team proposed a method to manufacture MLA with high-quality surface and high uniformity, which realizes simple and economical large-area MLA manufacturing by scraping or slit coating process (see Figure 1b), which helps to get rid of the dependence on traditional nanoscale molds. Based on the difference in selective surface wetting, this method strictly confines the droplets to the hydrophilic area during the coating process (see Figure 1c) while maintaining a very high cleanliness of the hydrophobic area. To achieve the ideal selective wetting of the surface, the team used oxygen plasma to locally modify the hydrophobic interface (see Figure 1d), selectively obtaining a hydrophilic region with a high-precision boundary that would produce a chemical bonding force much greater than the van der Waals force in combination with the lens material (see Figure 1c), and precisely control the lens curvature and size by adjusting the modification intensity or droplet dose. The team experimentally successfully validated MLAs as small as 10 micron pore size, high uniformity, large focal length control range, and high-quality surfaces.

Figure 1: Schematic of integrated imaging using MLA and selective surface wetting with oxygen plasma. (a) MLA integrated imaging schematic. (b) MLA manufacturing process based on blade scraping technology. (c) Selective wetting mechanism. (d) Oxygen plasma modification process based on reactive ion etching.

The researchers conducted a series of experiments to verify the superior performance of the designed microlens array. First, the team characterized the mean roughness of MLA (0.34 nm, see Figure 2c) and calculated the variance of the focal light intensity (σ=0.09, see Figure 2f) by atomic force microscopy, demonstrating that MLA has a high-quality surface and high uniformity with sub-nanometer roughness. In addition, the relationship between liquid volume, focal length, and numerical aperture is investigated (see Figure 2g).

Figure 2: Optical characterization of experimentally manufactured MLA. (a) MLA topography with a pore size of 100 μm. (b) Three-dimensional topography of a single microlens in MLA. (c) MLA surface roughness detected by atomic force microscopy. (d) MLA optical performance characterization optical path. (e) Focal spots on the focal plane of the experimental recording. (f) The distribution of light intensity in the x and y directions in the focal plane. (g) Relationship between liquid volume and MLA focal length and numerical aperture NA.

The research team also performed systematic optical characterization of the experimentally prepared MLA samples (see Figure 3). The experimental results show that the samples have good focusing effect and high focusing efficiency at different wavelengths (405 nm, 532 nm and 633 nm). Figure 3a-c shows the focusing effect and half-height width of MLA at different wavelengths; Figure 3D plots the resolution imaging map under blue, green, and red light illumination at the same position in the propagation direction, and the images of these three colors are almost identical to each other, showing good chromatic aberration; Figure 3e depicts the modulation transfer function of MLA, using red, green, blue, and white LED light sources to image the resolution board to obtain the modulation transfer function of line-pair density. The results show that when the spatial frequency exceeds 114 lp/mm, the modulation transfer function is about 22%, and line pairs can still be distinguished even at spatial frequencies up to 228 lp/mm. In addition, the field of view of MLA is about 25.8°, and the focusing efficiency at the three wavelengths is 46.2%, 40.8%, and 46.3%, respectively, which is comparable to the focusing efficiency of dielectric microlenses. (Source: China Optics)

Figure 3: Characterization of the focusing characteristics of MLA under laser illumination at different wavelengths. (a) λ=405 nm。 (b) λ=532 nm。 (c) λ=633 nm。 (d) Resolution images under blue, green and red LED illumination. (e) Modulation transfer function of MLA.

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