Observation of nonclassical optical diffusion

The optical diffusion coefficient D and photon absorption time ${\mathrm{\tau }}_{\mathrm{a}}$ in a sample of titania spheres in air are determined from measurements of picosecond pulse transmission over the wavelength range 514–650 nm. At 514 nm, D=1.45×${10}^4$ ${\mathrm{cm}}^2$/s. This corresponds to an anomalously small value of the effective mean free path l̃==3D/v=250 Å, where v is the velocity of light in the medium. Throughout the wavelength range measured, l̃≤0.15λ. Assuming that D is reduced by coherent backscattering and that the optical mobility edge is given by the Ioffe-Regel criterion, the renormalization factor at 514.5 nm is 2.8.