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?' 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 (Störzer et al 2006 Phys. Rev. Lett. 96 063904) and evaluating transmission profiles (TPs) (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.

The color of materials usually originates from a combination of wavelength‐dependent absorption and scattering. Controlling the color without the use of absorbing dyes is of practical interest, not only because of undesired bleaching properties of dyes but also regarding minimization of environmental and health issues. Color control without dyes can be achieved by tuning the material's scattering properties in controlling size and spatial arrangement of scatterers. Herein, calibrated photonic glasses (PGs), which are isotropic materials made by random aggregation of nonabsorbing, monodisperse colloidal polystyrene spheres, are used to generate a wide spectral range of purely structural, angular‐independent colors. Experimental reflectance spectra for different sized spheres compare well with a recent theoretical model, which establishes the latter as a tool for color mapping in PGs. It allows to determine the range of visible colors accessible in PGs as function of size, packing fraction, and refractive index of scatterers. It also predicts color saturation on top of the white reflectance as function of the sample's optical thickness. Blue, green, and red are obtained even with low index, while saturated green, cyan, yellow, and magenta can be reached in higher index PGs over several orders of magnitude of sample thickness.

A fundamental quantity in multiple scattering is the transport mean free path the inverse of which describes the scattering strength of a sample. In this paper, we emphasize the importance of an appropriate description of the effective refractive index n eff in multiple light scattering to accurately describe the light transport in dense photonic glasses. Using n eff as calculated by the energy-density coherent-potential approximation we are able to predict the transport mean free path of monodisperse photonic glasses. This model without any fit parameter is in qualitative agreement with numerical simulations and in fair quantitative agreement with spectrally resolved coherent backscattering measurements on new specially synthesized polystyrene photonic glasses. These materials exhibit resonant light scattering perturbed by strong near-field coupling, all captured within the model. Our model might be used to maximize the scattering strength of high index photonic glasses, which are a key in the search for Anderson localization of light in three dimensions. DOI: 10.1103/PhysRevA.96.043871 Transport phenomena are omnipresent in nature, governing many processes in chemistry, biology, physics, and engineering. Systems as diverse as electrons [1] and ultrasound [2] in condensed matter, mechanical waves in the earth [3], cold atoms in an optical trap [4], and light in disordered photonic materials [5] share the same physical principle [6,7]. Optical experiments are especially appealing because of the absence of photon-photon interaction (unlike in electronic systems) and the existence of relatively high index scattering media such as photonic crystals and glasses. Moreover, optical transport experiments have reached an unprecedented accuracy thanks to the great technological development of sources (e.g., lasers), detectors (e.g., CCDs), and time resolution. All these progresses allow the realization of table-top experiments which highlight the richness of transport phenomena.Wave transport in a diluted disordered suspension of scatterers can be described by the sole far-field properties of the single scatterers. On increasing concentration, however, interference effects due to scatterer-scatterer position correlation need to be taken into account [8]. In this description, the scattering cross section is still the single scatterer one, which, in general, is calculated in the far field. In optics, this approach is expected to fail as soon as the photon scattering mean free path s becomes smaller than a few wavelengths of the light. In this case, the distance between two scattering events is so short that (1) each scattered photon does not reach the far-field limit before being rescattered and (2) the (differential) scattering cross section of each and every scatterer is affected by multiply scattered photons returning to it. A first attempt to describe these near-field effects was recently proposed by Rezvani Naraghi et al. [9], but takes into account only the first point. In this paper, we propose a different light ...

We present wave transport experiments in hyperuniform disordered arrays of cylinders with high dielectric permittivity. Using microwaves, we show that the same material can display transparency, photon diffusion, Anderson localization, or a full band gap, depending on the frequency ν of the electromagnetic wave. Interestingly, we find a second weaker band gap, which appears to be related to the second peak of the structure factor. Our results emphasize the importance of spatial correlations on different length scales for the formation of photonic band gaps.