Paper EM-ThP9
Control of Randomness in Microsphere-Based Photonic Crystals Assembled by Langmuir-Blodgett Process

Thursday, October 25, 2018, 6:00 pm, Room Hall B

Photonic structures in biological systems typically exhibit an appreciable degree of disorder within their periodic structures. Such disorder contributes to unique optical properties but has not been fully understood. Towards the goal of improving this understanding, we have investigated Langmuir-Blodgett (LB) assembly of silica microspheres to controllably introduce randomness to photonic structures. We theoretically modeled the LB assembly process and determined a condition for surface pressure and substrate pulling speed that results in maximum structural order. For each surface pressure, there is an optimum pulling speed, and vice versa. Photonic structures fabricated at various conditions were characterized by scanning electron microscopy and light scattering analysis, which confirms the modeled optimum condition. However, along the trajectory defined by the optimum condition, the structural order decreases moderately as the pulling speed increases. This moderate decrease in structural order would be useful for controlled introduction of randomness into the periodic structures. Departing from the trajectory, our experiment reveals that a small change in pulling speed at a given surface pressure can significantly disrupt the structural order. According to these observations, mechanism of forming structural order in LB assembly is proposed. Additionally we also find that, for multilayer LB assembly at a fixed pulling speed, the surface pressure should increase as the number of layers increases to achieve maximum structural order. In summary, this work quantitatively presents the optimum trajectories for n^th layer assembly relating surface pressure and pulling speed.