Wave optics of white paint
Abstract: Materials such as white paint, milk, paper or biological tissue have spatial inhomogeneities in the refractive index which cause multiple scattering of light. In such opaque materials, most of the light reflects back, hindering the transport of optical energy and information. However, by utilizing the interference of scattered coherent waves, it is possible to prepare optimized wavefronts that completely suppress reflection-a striking phenomenon first theoretically predicted in the context of electron transport through conducting wires [1-4]. In recent years, spatial light modulators (SLMs) have been used to shape light into specific wavefronts that are called transmission eigenchannels [5,6]. By coupling light into transmission eigenchannels, transmittance through the medium can be altered from 0 to 1.
Here, I will present our recent experimental and numerical results on transmission eigenchannels of white paint made of an ensemble of zinc oxide (ZnO) nanoparticles in open wide-slab geometry. We discover that transmission eigenchannels are exponentially localized in the transverse directions . Our results show that high-transmission eigenchannels not only enhance total transmitted power, but also energy density inside and on the back surface of a scattering opaque medium, which is important for applications such as optogenetics and multiphoton imaging that aim for enhancement of light-matter interactions in complex optical systems.
 O. N. Dorokhov, SolidState Commun. 51, 381–384 (1984).