Scattering makes it impossible to focus light within or through an optically diffuse medium by conventional means. The consequences of this are most notably evident on an overcast day. Diffuse scattering by clouds causes shadows to disappear — a bane for landscape photographers — and makes it difficult even to locate the Sun in the sky. Nonetheless, Ivo Vellekoop and colleagues show that for a disordered solid, fine control over the phase of an array of hundred of channels of incident light onto a sample makes it possible to compensate for this diffuse scattering, and to direct light within it (Opt. Express 16, 67–80; 2008).

Credit: IVO VELLEKOOP

The key to the authors' approach is the fact that, unlike clouds, the microscopic structure of most disordered solids does not change with time. Consequently, although complex, the way in which light is scattered will be well-defined and deterministic. And so they figure it should be possible to make up for this scattering with an appropriately engineered optical field. To investigate the feasibility of this they embedded individual 300-nm-wide fluorescent spheres to various depths within an opaque 32-μm-thick layer of white zinc oxide pigment grains (with an average diameter of 200 nm). As expected, simply scanning focused laser light across the layer in an attempt to stimulate the fluorescent spheres results in a barely detectable fluorescent signal (left panel of figure). But passing the laser through a computer-controlled spatial light modulator composed of an array of 640 individual elements to finely adjust the distribution and phase of the incident light field, and using a feedback process to optimize this field, they find that they can indeed focus the light enough to stimulate an embedded sphere (right panel of figure). Moreover, they find that the success of this approach is independent of the depth of the sphere within a layer.

The results demonstrate the feasibility of using multiple optical channels to characterize the complex propagation of light through a disordered material without prior knowledge of its structure. The authors suggest this could prove useful in biomedical imaging, by enabling light to be selectively focused onto fluorescent probes embedded within a sample of biological tissue.