Can 3D light localization be reached in "white paint"?
Résumé
When waves scatter multiple times in 3D random media, a disorder driven phase transition from diffusion to localization may occur [1]. In "The question of classical localization: A theory of white paint?" P.W. Anderson suggested the possibility to observe light localization in TiO2 samples [2]. We recently claimed the observation of localization effects measuring photon time of flight (ToF) distributions [3] and evaluating transmission profiles (TP) [4] in such TiO2 samples. Here we present a careful study of the long time tail of ToF distributions and the long time behavior of the TP width for very thin samples and different turbidities that questions the localization interpretation. We further show new data that allow an alternative consistent explanation of these previous data by a fluorescence process. An adapted diffusion model including an appropriate exponential fluorescence decay accounts for the shape of the ToF distributions and the TP width. These observations question whether the strong localization regime can be reached with visible light scattering in polydisperse TiO2 samples, since the disorder parameter can hardly be increased any further in such a "white paint" material.