Opacity to transparency

Theorists have long predicted that scattering materials, such as milk and white paint, contain transparent channels that light can pass right through. Ordinarily, very little of the light in an incident beam enters those so-called open channels, and the materials appear opaque. But it should be possible to create a shaped wave that couples more light into the open channels. In fact, Ivo Vellekoop and Allard Mosk of the University of Twente in the Netherlands have done just that. Last year, they demonstrated a method for shaping a light wave, by tuning the relative phases of segments of their beam, so that much of the light transmitted through their scattering sample would be focused to a point. (See PHYSICS TODAY, October 2007, page 26 .) Now, using the same algorithm with an improved apparatus, they’ve shown that they can increase the total amount of light passing through the material by as much as 44%. However, their device’s performance is still limited by tiny drifts of the sample with respect to the beam. Extrapolating from their results, they confirmed the theory’s prediction that if they could implement their algorithm perfectly, about two thirds of the incident light would be transmitted, regardless of the thickness of the scattering material. (I. M. Vellekoop, A. P. Mosk, Phys. Rev. Lett., in press.) — Johanna L. Miller