# Surface Topographical Control of a Liquid Crystal Microlens Array Embedded in a Polymer Network
## 3 Results and Discussion  
The mixture showed a uniform and transparent yellow color in the LC cell owing to the dopant of the guest dye, as shown in Figure 5a. A microlens array was fabricated through irradiation with a laser beam at a light intensity of 10 W/cm\({}^{2}\) for 10 min at each spot. The fabricated microlens array could be observed with the naked eye, as shown in the inset of Figure 5a, and was distributed in a pattern structure with equal distance in the x and y directions. The LC cell containing the microlens array was observed under POM, as shown in Figure 5b. Following photopolymerization, the surrounding areas of the microlens array showed a polydomain structure due to molecular disordering of 5 CB, indicated by different birefringence, as shown in the green, yellow, and red areas. The surface topography of the microlens array was evaluated by carefully opening the LC cell and placing the glass substrate containing the microlens array in a confocal laser microscope. The 3D and top views of the surface topographical structure of the microlens array are shown in Figures 6 and 7, respectively. At room temperature, the microlens array had a mountain-shaped topographical structure with a non-uniform size. As the temperature was increased above the \(T_{\text{NI}}\) of the host 5 CB, the area surrounding the microlens array became lighter. This suggests that the 5 CB molecules encapsulated within a polymer network were transitioning from the nematic phase to the isotropic phase. Interestingly, the microlens also became smaller and more uniform in shape with the increasing temperature. Additionally, as soon as the sample was cooled down to room temperature, the surface topographical structure of the microlens array returned to its original state, showing shape reversibility. Figure 5: Characterization of the fabricated microlens array. (**a**) Photograph of the LC cell containing the microlens array. (**b**) POM image of the fabricated microlens array. The blue arrow indicates the polarization direction of the 488 nm laser beam used to fabricate the microlens array. increase in height and decrease in width might have been due to the thermal expansion of the polymer network and the nematic-to-isotropic phase transition of 5 CB in the microlens array, respectively. The microlens array was formed through the photoinduced molecular reorientation of 5 CB when irradiated with a Gaussian beam. The photoinduced molecular reorientation, which was proportionally related to the change in refractive index, was observed through the appearance of concentric diffraction rings, as shown in Figure 9a. Before the irradiation with a laser beam, the 5 CB molecules were homeotropically (out-of-plane) aligned. After increasing the light intensity of the laser beam above a certain threshold intensity, the 5 CB molecules began to align parallel to the polarization direction of the laser beam, inducing a homogeneous (in-plane) alignment of the molecules. The direction of molecular alignment of 5 CB usually returns to that of the original homeotropic alignment when the light source is removed from the sample. However, the homogeneous alignment became permanent when the sample was irradiated with a laser beam for 10 min, stabilizing the molecular reorientation of 5 CB. The stabilized molecular reorientation of 5 CB was probed with a 633 nm linearly polarized He-Ne laser beam ([PERSON], 05 LHP 151, Pneum, Saitama, Japan), where the sample did not show any absorbance at that wavelength, showing memorized concentric diffraction rings, as shown in Figure 9b-1. The memorized diffraction rings did not possess any polarization dependency stating that the microlens was fabricated through thermal distortion, i.e., order-to-disorder molecular reorientation. In other words, the center of the microlens showed the molecular disordering of 5 CB, while the outer section showed the molecular ordering of 5 CB.