Weak Localization and Coherent Backscattering of Photons in Disordered Media

Coherent backscattering of waves by a disordered scattering medium is responsible for weak localization. We have directly observed this effect for the first time using visible light and concentrated aqueous suspensions of submicron-size polystyrene spheres. The scattered intensity is found enhanced by up to 75% within a narrow cone centered at the backscattering direction. As predicted by theory, the aperture of the cone is inversely proportional to the light mean-free path; the latter was controlled by the concentration of spheres. The importance of light polarization and particle size is discussed.