Observation of nonclassical optical diffusion

Abstract

The optical diffusion coefficient D and photon absorption time ${\mathrm{\ensuremath{\tau}}}_{\mathrm{a}}$ 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\ifmmode\times\else\texttimes\fi{}${10}^{4}$ ${\mathrm{cm}}^{2}$/s. This corresponds to an anomalously small value of the effective mean free path l\ifmmode \tilde{}\else \~{}\fi{}==3D/v=250 A\r{}, where v is the velocity of light in the medium. Throughout the wavelength range measured, l\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.15\ensuremath{\lambda}. 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.