Can 3D light localization be reached in "white paint"?

TL;DR

This study investigates whether 3D light localization can be achieved in white paint, revealing that fluorescence effects, rather than localization, explain previous observations and challenging the feasibility of strong localization in such materials.

Contribution

The paper provides evidence that fluorescence processes, not localization, explain earlier experimental results in TiO₂ samples, questioning the possibility of reaching 3D light localization in white paint.

Findings

Fluorescence explains long time tails in ToF and TP data.

Localization interpretation is challenged by fluorescence effects.

Strong localization regime is unlikely in polydisperse TiO₂ white paint.

Abstract

When waves scatter multiple times in 3D random media, a disorder driven phase transition from diffusion to localization may occur [Anderson 1958 Phys. Rev. 109 1492-505; Abrahams et al 1979 Phys. Rev. Lett. 42 673-6]. In "The question of classical localization: A theory of white paint?" P.W. Anderson suggested the possibility to observe light localization in TiO$_2$ samples [Anderson 1985 Phil. Mag. B 52 505-9]. We recently claimed the observation of localization effects measuring photon time of flight (ToF) distributions [St\"orzer et al 2006 Phys. Rev. Lett. 96 063904] and evaluating transmission profiles (TP) [Sperling et al 2013 Nat. Photonics 7 48-52] in such TiO$_2$ 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 TiO$_2$ samples, since the disorder parameter can hardly be increased any further in such a "white paint" material.