Photonic Crystals Using Self-Assembly of Metallo-Dielectric Nanospheres

Courtney Roby, University of Colorado

Photonic bandgap structures are artificial structures with a periodicity which forbids certain frequency bands of incident light from propagating within them. Such structures can be used in many ways; for example, they may be exploited for their capacity for very high dispersion or used as light-trapping cavities. If a structure exhibits a bandgap for all polarizations and propagation directions of incident light, this is known as a complete photonic bandgap (CPBG). These are difficult to engineer at normal optical frequencies without using a three-dimensionally periodic structure. Such structures, however, are difficult to fabricate, particularly at optical frequencies which require small features.

One interesting way of fabricating a three-dimensional photonic crystal is to use self-assembly of spheres. Spheres like to pack themselves into a periodic structure, and so a crystal can be formed as a colloidal suspension of nanospheres. I am interested in the behavior of the electric field in such a structure. I approach this problem first analytically, using Lorenz–Mie theory to investigate the fields involving one sphere consisting of multiple concentric shells. I then expand to multiple spheres and ultimately to the full periodic structure, for which I plan to use FDTD. This method seems like the best-suited for this problem, because of the structure's closed, close-packed geometry. Later on, I would like to see the effect of introducing different forms of disorder, whether intentional (such as engineered cavity defects) or accidental (such as nonuniformity in the fabrication of the spheres themselves).

Structures built on the abovementioned principles have a fair degree of design flexibility, since we are at some liberty to choose sphere size and composition. Eventually I hope to have a model which can be used to exploit this flexibility and design such structures intelligently to meet the user's needs.

Abstract Author(s): Courtney Roby