Novel photonic crystal thin films using the Langmuir–Blodgett approach

Abstract

Photonic crystals prepared via the Langmuir–Blodgett (LB) method are shown to be quite different to those structures prepared by other self-assembly approaches. In LB prepared films the layer spacing is expanded relative to that expected for a true FCC structure while the correlation between layers is reduced. The resulting films are thus best described as (2+1) dimensional in nature, since they do not possess the full three-dimensional (3-D) ordering present in opaline structures. A simple analysis of Bragg reflection peaks for a sample prepared from silica particles of diameter ca. 250nm reveals layer spacing for the LB-grown films of 0.941D, where D is the particle diameter. We account for these phenomena in terms of a model in which each layer of spheres forms within the confines of the LB trough before being transferred as a complete layer to the substrate. Under such conditions the likelihood of the spheres finding the optimum sites for the formation of a true FCC structure is low and the resulting structures, although still consisting of ordered layers, do not adopt the FCC structure and hence they may not be described as ‘opals’. Results are also presented for bilayer (size matched) heterostructure photonic crystals. For the size-matched bilayer systems where the diameter of one set of particles is twice that of the other, it is shown that when sampled in a particular geometry, cooperative effects can occur involving the first-order Bragg reflections from the layer of smaller particles combining with the second-order Bragg reflections from the layer of larger particles. As a consequence an enhancement of the intensity of the first-order Bragg peak from the layer comprised of the smaller particles is observed in the binary heterostructure.