Light propagation in strongly scattering, random colloidal films: The role of the packing geometry

We study the propagation of light through randomly packed films of micron-sized spheres. Dried films consist of strongly scattering core-shell particles mixed with polymer spheres, which are then dissolved to tune the number of contacts, Z, among the remaining scatterers. The transport mean free path l* is measured from the width of the coherent backscattering cone; l*=2.1 mu m when Z similar to 4-5, but increases twofold (scattering weakens) in a film with Z similar to 9-10. The results contradict the standard diffusive transport model, but are explained by accounting for optical coupling of contacting spheres.