# Surface Topographical Control of a Liquid Crystal Microlens Array Embedded in a Polymer Network
## 3 Results and Discussion
This was due to the Gaussian light intensity distribution of the laser beam, where the center of the laser beam had greater light intensity than the outer side. Therefore, the 5 CB molecules within the center of the irradiated area remained isotropic (disordering), while the 5 CB molecules located in the outer region of the laser beam remained homogeneously aligned. The molecular reorientation of 5 CB was stabilized through the photopolymerization of monomers inside the irradiated area as the sample was exposed to high light intensities for long periods, triggering the photoinitiator to initiate the photopolymerization. Owing to the monomer concentration difference between the irradiated and unirradiated areas with the laser beam, the monomer diffuses from the unirradiated area to the irradiated area, causing a high content of polymers inside the irradiated region [28,29]. This caused the microlens array to gain a mountain-shaped topography due to the polymer accumulation inside the microlens. Hence, the microlens array consisted of disordered 5 CB molecules with an accumulation of polymers. The fabricated microlens array was further stabilized by irradiating with a UV light source at 1 mW/cm\({}^{2}\) for 1 h across the whole area. The  
Figure 8: Surface topographical characteristics of the microlens array with a change in (**a**) height and (**b**) width at various temperatures. molecular reorientation of the microlens was again probed with the He-Ne laser beam, as shown in Figure 9b-2. The focal length is an essential property of a microlens array, and different approaches to tune the focal length through an external stimulus were explored. The focal length of the microlens array in different temperatures was measured (Figure 10). At room temperature, the focal length of the microlens array was 640 \(\upmu\)m and remained constant until 30 \({}^{\circ}\)C because the 5 CB molecules remained in the nematic phase. As the temperature was increased to 31 \({}^{\circ}\)C, the focal length slightly decreased until it reached a plateau of 634 \(\upmu\)m at 34 \({}^{\circ}\)C. This decrease in focal length was due to the surface topographical change in the microlens array and the nematic-to-isotropic phase transition of 5 CB. The focal length gradually increased when the temperature reached 35 \({}^{\circ}\)C and remained unchanged after 36 \({}^{\circ}\)C. After 37 \({}^{\circ}\)C, the molecules in the microlens array became isotropic and the surface topography remained roughly unchanged. However, the focal length became smaller compared to the focal length at room temperature because 5 CB in the microlens array became isotropic. The microlens array was cooled to room temperature and the focal length was reversible. This is comparable to the reversibility of topographical change under heat treatment, which also induces the change in focal length. Even though the microlens only showed the focal length tuning by 8 \(\upmu\)m, this could be useful to fabricate thermally tunable microlenses in the future. Figure 9: Photoinduced molecular reorientation by irradiating with a polarized laser beam. **(a)** Polarized laser beam irradiated to the sample at 10 W/cm\({}^{2}\), resulting in concentric diffraction rings. After 10 min of irradiation, the blue laser beam was removed and a red probe beam was used instead. Diffraction ring patterns of the red laser beam before **(b-1)** and after **(b-2)** photopolymerization. The blue and red arrows represent the polarization direction of the blue and red laser beams, respectively.