From diffusive to ballistic-like transport in absorbing media

[S.F. Liew et al., Phys. Rev. B, 89, (2013)]

Intuitively, absorption of light is detrimental for imaging as it reduces the intensity of the image we see. On the other hand, scattering is also a known obstacle for imaging as it mixes light sending it in all the directions. In the present paper, S.F. Liew and his collaborators from Yale University (CT, USA) and the University of Twente (The Netherlands) show that, contrary to appearances, absorption can in fact help light to follow a direct path through disordered media.

Without absorption, spatial information of an object transmitted through an opaque material is totally mixed and difficult to recover. The reason is that the photons are multiply scattered, hence their propagation directions are randomized at every scattering event. In their recent numerical calculation study, the authors noticed that when absorption becomes strong, the transport of light occurs via much straighter paths.

They calculated in simulation the transmission and reflection matrices of disordered waveguides for many configurations of the disorder. They then compared the channels of maximum transmission, minimum absorption, and minimum reflection for different amounts of absorption.

Among other phenomena studied in this article, the scientists show that the channels for which the transmission is higher tend to avoid absorption by going along straight paths. In other words, information can be transmitted with less scrambling as illustrated in Figure 1.

Figure 1. (a) Artistic rendering of an opaque medium that does not absorb light (top), and an opaque medium that absorbs all colors except red light (bottom.) It is illustrated that the underlying pattern can not be seen through an opaque medium. (b) Calculated spatial distribution of the light intensity inside an opaque medium. Light enters the medium from the left. As a result of multiple scattering, the light paths become random walks, as symbolized by the intertwined arrows, and light exits randomly, thus precluding imaging. (c) In an absorbing opaque medium, it appears that the transport of light occurs via straighter paths, resulting in less scrambling of information.

Such predictions can be applied in experiments using wavefront shaping techniques. By applying the suitable input phase mask to couple energy into the maximal transmission channel of a disordered system, the authors expect that one can improve imaging through turbid media such as biological tissues, which absorb many colors of light.