Dispersion and Thickness Control in Evaporation-induced Self-Assembly of Opal Photonic Crystals

TL;DR

This study investigates how crystal thickness influences the optical properties of opal photonic crystals formed via evaporation-induced self-assembly, establishing a relationship between volume fraction, thickness, and optical band gaps.

Contribution

It provides the first direct correlation between sphere volume fraction, crystal thickness, and optical properties in EISA-formed opal photonic crystals, including criteria for thickness control.

Findings

Maximum crystal thickness converges over evaporation rates.

Optical band gaps shift with crystal thickness and order.

Structural quality is maintained with specific evaporation conditions.

Abstract

Opals are naturally occurring photonic crystals which can be formed easily using low-cost self-assembly methods. While the optical behaviour of opals has received significant attention over the last number of decades, there is limited information on the effect of crystal thickness on the optical properties they display. Here, the relationship between volume fraction and crystal thickness is established with an evaporation-induced self-assembly (EISA) method of formation. The extent to which thickness can be used to manipulate the optical properties of the crystals is explored, focusing on the change in the photonic band gap (PBG). Microscopical structural characterization and angle-resolved transmission spectroscopy are used to examine the quality of the photonic crystals formed using different volume fractions of polystyrene spheres, with thicknesses up to 37 layers grown from volume fractions of 0.125%. This work provides a direct correlation between sphere solution volume fraction and crystal thickness, and the associated optical fingerprint of opal photonic crystals. Maximum thickness is examined, which is shown to converge to a narrow range over several evaporation rates. We identify the criteria required to achieve thickness control in relatively fast evaporation induced self-assembly while maintaining structural quality, and the change to the spectroscopic signature to the (111) stopband and higher order (220) reflections, under conditions where a less ordered photonic crystals are formed.