Opaque lens

Some materials such as milk, paint, and biological tissues strongly scatter any light that penetrates them. For thin samples, the transmitted light is both dim and diffuse; adaptive optics cannot unscramble the emerging light. Thicker samples are visually opaque. (For more on diffusing light, see Physics Today, March 1995, page 34 .) Two physicists at the University of Twente in the Netherlands have now focused a beam of coherent light passing through such a strongly scattering medium. Ivo Vellekoop and Allard Mosk first sent the light through a 10-μm-thick sample of paint tinted with rutile (TiO₂) and, with a CCD camera, recorded the highly attenuated light that emerged (left image). Next, in front of the sample, they inserted a phase modulator that had more than 3000 adjustable segments. By fine tuning each segment to get the brightest transmission at a target spot behind the sample, the physicists constructed a scrambled incident wave-front that interfered constructively—in phase—within the sample and emerged focused into a spot 1000-fold brighter than the diffuse background (right image). The researchers also obtained focused enhancements using 40-μm daisy petals, 430-μm chicken eggshells, and 1.5-mm human tooth samples. ((I. M. Vellekoop, A. P. Mosk, Opt. Lett. 32 , 2309, 2007 http://dx.doi.org/10.1364/OL.32.002309 .)