Light Propagation in Colloidal Glass: enhancement of scattering at reduced coordination number

We measure the propagation of light through random films of strongly-scattering microspheres as a function of the mean number of contacts per particle (the coordination number, $Z)$. Two kinds of colloidal spheres are mixed to prepare dried films with random structure. Latex spheres coated with a high-index ZnS are mixed in various ratios with PMMA spheres and the PMMA spheres are then dissolved by acetone. The transport mean-free path of light $l$* is then extracted from measurements of coherent backscattering of light from the films. We found a minimum of $l*$ (maximum of scattering) occurs around $Z$=4, not in a close-packed film (Z$\sim $11), which is counterintuitive. In a simple mean field model, decreasing $Z$ reduces the local average refractive index and enhances the optical contrast of each scattering sphere with the effective background, thus reducing $l$*. These results may guide our understanding of the propagation of waves in random media in general and may lead to new photonic materials. This work is supported by the NSF-sponsored UMASS MRSEC. A.D.D is a Cottrell Scholar of the Research Corporation.