# Surface Topographical Control of a Liquid Crystal Microlens Array Embedded in a Polymer Network
## 1 Introduction  
Liquid crystals (LCs) have been widely used in recent decades owing to their giant optical nonlinearity, which is nine orders of magnitude larger than ordinary materials [1; 2; 3]. This optical nonlinearity is attributed to the orientational change in LC molecules when irradiated with a polarized light source at a low light intensity of 100 W/cm\({}^{2}\), which is relatively lower than the materials used in the past. Therefore, this photoinduced reorientation of LC molecules upon irradiation with a light source is classified as a nonlinear optical (NLO) effect that has recently attracted much attention owing to their high sensitivity and significant change in refractive index [4; 5; 6]. Many scientists have been intrigued by finding different innovative approaches to enhance the optical nonlinearity of LC materials and observed that this can be accomplished by doping specific dye molecules in the LC materials [7; 8]. [PERSON] et al. first reported that an anthraquinone dye could serve as a photo-functional dye to significantly enhance the sensitivity of molecular reorientation [9; 10; 11]. Since then, many more approaches have been established and proposed to improve this optical nonlinearity of dye-doped LC molecules, for instance, by introducing polymer-stabilized LC (PSLC) systems [12; 13]. Previous research reports have shown that polymer stabilization can enhance the thermal and electro-optical properties of LC molecules [14; 15]. [PERSON] et al. reported that polymer stabilization improved the orientational optical nonlinearity by 56%; that is, an improvement of six orders of magnitude of threshold intensity to cause a photoinduced molecular reorientation [12]. The photoinduced molecular alignment of PSLC systems was further enhanced by manufacturing hybrid systems containing homogeneously aligned LC molecules on one side and homeotropically aligned LC molecules on another side [16],increasing the polymer concentration [13], optimizing the silane coupling concentration that induces the homeotropic alignment of LC molecules through surface anchoring [17], and changing the incident beam to collimated and focused depolarized laser beam [18; 19]. Recently, [PERSON] et al. reported that the nonlinear optical reorientation of oligothiophene-doped PSLC systems could be applied to bendable optical limiters [20]. Therefore, more applications using PSLC systems have been under investigation, such as the fabrication of a microlens array. There is a growing interest in producing more compact optoelectronic systems that require more investigation to deliver more miniaturized optical elements. A microlens array is a two-dimensional array consisting of small lenses whose diameters are about a millimeter or smaller. Many research scientists have used melting mechanisms to prepare patterned microlens materials through photoresist [21], wet etching [22], and laser pulse ablation [23]. Microlenses play an essential role in many fields, including biomedical imaging systems [24], optical communication [25], and 3D imaging [26]. Therefore, research has been implemented in recent decades to improve the optical properties of a microlens array through surface topographical changes under the application of an external stimulus [27; 28]. In this work, we establish an innovative approach for fabricating a microlens array with a tunable surface topography and focal length under heat treatment. In our group, a microlens array is fabricated by the light intensity-dependent molecular reorientation of LCs with subsequent photopolymerization [29; 30]. However, the fabricated microlens array does not possess focal length tunability, and the surface topographical structure change under heat has not been explored. Therefore, the microlens array in this study is fabricated through the photoinduced molecular reorientation of oligothiophene dye-doped LCs embedded in a polymer network.