Mechanism and fabrication of MLAs
Figure 1a depicts the schematic illustration of adopting MLAs for integral imaging realized by selectively wetting the hydrophobic interface with oxygen plasma. The core idea is to adopt two MLAs to capture the target and then reproduce the light field so as to realize an autostereoscopic 3D display. Taking a leaf for example, arrayed images of the object are captured by the first MLA, processed by the display system and then projected by the second MLA before it enters human eyes. Hence, the properties of the MLA, such as uniformity, surface roughness and resolution, play a vital role in determining the performance of the display system. Figure 1b illustrates the image of the experimentally fabricated MLA with diameter of 100 µm. We manufactured the MLAs by using blade coating method, where the curable liquid NOA-73 sticks to the selective wetting surface (hydrophilic area, as indicated in Fig. 1c) and forms into monoconvex lens, due to the surface tension and the chemical bonding force between the interfaces.
Instead of utilizing conventional mold to provide physical boundary for the curable liquid [23], both hydrophilic and hydrophobic surfaces are created to provide strong restriction for the curable liquid in this study. As shown in Fig. 1d, oxygen plasma is adopted to modify the hydrophobic interface and selectively create a well-defined hydrophilic area. The sample with sacrificial layer is put into the chamber of the reactive ion etching machine. The ionized oxygen plasma impinges upon the hydrophobic surface in the chamber to induce its transition to hydrophilic surface. In our experiment, an amorphous fluoropolymer Hyflon is adopted as hydrophobic material on the glass substrate, because they show near-zero extinction coefficient and approximately constant refractive index (with ± 0.005 variance during wavelength of 400 nm ~ 660 nm) in the visible and near infrared range (see the ellipsometer measurement results in Figure S1), suggesting the negligible material loss and weak dispersion. To understand the microscopic change in surface chemical properties before and after O2-plasma modification, X-ray photoelectron spectroscopy (XPS) is employed (see Figure S2 for more details in supporting information). This suggests the enhancement of surface activity induced by the surface defluorination, and thus the surface hydrophilicity. As the binding force caused by chemical bonds on the solid (Hyflon)-liquid (NOA-73) interface is much stronger than traditional solid-liquid binding by van der Waals force [31], more binding is produced in the locally modified region, thus supporting the formation of selective-wetting-dominated microdroplet array during the blade coating process. Hence, compared with MLAs relying on conventional mold [19, 32], the hydrophilic layer may support MLA with larger height, implying that this method may lead to MLAs with sharper curvature and higher numerical aperture (NA). Moreover, the chemical restriction method helps abandon the mold relying on high-precision nanofabrication, and becomes a rather cost-effective method to fabricate the MLAs as blade-coating or slit-coating can be applied.
We first consider curable liquid NOA-73 as fundamental material to form the lens. Figure 2a shows the weak material dispersion as its refractive index is approximately constant as 1.54 in the visible and the negligible extinction coefficient, making it ideal for broadband and high-efficiency optical lens. The curable NOA-73 coated onto an array of well-confined circular hydrophilic area will show self-assembly and morph into MLAs to the size of hydrophilic areas (with a diameter of 1 mm shown in Fig. 2). We want to highlight that previous methods in this step rely on a lens-shaped mold, posing a stringent requirement on the surface quality and machining accuracy of the mold for high-quality MLAs, let alone to the additional cost [32]. In striking contrast, our study tailors the surface tension and chemical bonds between the droplet and modified areas to provide strong viscous force to restrict the morphology of the MLAs, opening a path toward the mold-free and cost-effective fabrication of large-area high-quality MLAs.
Figure 2c shows the relationship between the volume of NOA-73 and contact angle of the MLA using Stylus Profilometer (Dektak XT, Bruker) and Optical Goniometer (JC-2000D1, Powereach). When increasing the volume, the contact angle changes from 18° to 76° (see Fig. 2c). To test the 3D surface profiles of the MLAs, we use Optical Microscope (OM) (CSUC-200C, Hours) and 3D Surface Profilometer (DCM8, Leica). The 2D images of the cross sections of six lenses are displayed in Fig. 2d. It can be observed that the lenses are well-confined inside the region where the Hyflon is modified. As NOA-73 increases from 0.027 µL to 0.235 µL, the height of the resulting MLA morphs from 0.08 mm to 0.25 mm accordingly (see the inset in Fig. 2d) accompanied by the increase of contact angle up to 76°. Interestingly, we note that the diameter of the MLAs remains the same in this process, suggesting the configuration of the curvature and NA of the MLA by controlling the dose of NOA-73. Ideally, single microlens in the MLA can be considered as a spherical cap composing of NOA-73. If the circular-shaped hydrophilic area has a diameter d and the dose of the liquid is assumed as V, then we have V = πH(d2/8 + H2/6), where H is the height of the microlens. Hence, the curvature of the MLA can be configured according to demand by tuning the dose of the NOA-73. Moreover, we find in the experiment that the curvature and NA of the MLA can also be scaled by tuning the time and the dose of the oxygen plasma during modifying the hydrophobic interface. Although the contact angle is expected to saturate after certain threshold volume (0.25 uL in our case) due to the crash of liquid by gravity, the further engineering of hydrodynamic properties of fundamental materials may enable a larger contact angle, which would be interesting for future study.
Figure 3a,b show the OM image and the 3D surface profile of a MLA with periodicity of 200 µm, respectively. The height of the MLA is approximately 6 µm, while the diameter is measured to be 100 µm, which matches with the design. These results indicate that the MLA has an excellent uniformity. It can also be noticed that no liquid overflows into the hydrophobic region, rendering the MLA neat. To test the surface roughness of the MLA, we use the atomic force microscope here. Figure 3c depicts the surface roughness of a microlens randomly selected from Fig. 3a, where an average roughness of 0.34 nm is estimated. The polishing procedure which is usually employed in traditional fabrication techniques is no longer necessary [33] here. MLAs with such smooth surfaces are promising for those applications where the highly uniform MLAs with even dense arrays and ultrasmooth surface are demanded.