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

Photonic structures in biological systems typically exhibit an appreciable level of disorder within their periodic framework. However, how such disorder within the ordered framework renders unique optical properties has not been fully understood. Toward the goal of improving this understanding, we have investigated Langmuir-Blodgett assembly of microspheres to controllably introduce randomness to photonic structures. We theoretically modeled the 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. Along the trajectory defined by the optimum condition, however, 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. For multilayer assembly, we find that, at a fixed pulling speed, the surface pressure should increase as the number of layers increases to achieve maximum structural order. In totality, we quantitatively present the optimum trajectories for the nth layer assembly relating surface pressure and pulling speed.