Observation of nonclassical optical diffusion

The optical diffusion coefficient D and photon absorption time τₐ 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×{}10⁴ cm²/s. This corresponds to an anomalously small value of the effective mean free path l ̃ \~{}{}==3D/v=250 A{}, where v is the velocity of light in the medium. Throughout the wavelength range measured, l ̃ \~{}{}{≤}0.15{λ}. 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.