Controllable transport mean free path of light in xerogel matrixes   embedded with polystyrene spheres
  Xerogel matrices, made by sol-gel techniques, are embedded with polystyrene spheres to promote multiple scattering of light. Varying the concentration of the spheres inside the matrix allows one to adjust the transport mean free path of light inside the material. Coherent backscattering measurements show that a range of transport mean free paths from 90 to 600 nm is easily achieved. The determination of the matrix refractive index permits a direct comparison to multiple scattering and Mie theory. Such tunable diffusive sol-gel derived samples can be further optimized as random laser materials. 
Boris P. J. Bret, Nuno J. G. Couto, Mariana Amaro, Eduardo J.   Nunes-Pereira, and Michael Belsley