Seeing through opaque materials

BY SPECIAL ARRANGEMENT

Using a novel imaging technique, a team of researchers from the Netherlands and Italy has succeeded in making sharp pictures of objects hidden behind an opaque screen. This breakthrough in research has been published in the journal Nature.

Materials such as skin, paper and ground glass appear opaque because they scatter light. In such materials, light does not move in a straight line but travels along an unpredictable and erratic path. As a result, it is impossible to get a clear view of objects hidden behind such materials. Powerful methods have been developed to retrieve images through materials in which a small fraction of the light follows a straight path. To date, however, it has not been possible to resolve an image from light that has been completely scattered.

Allard Mosk and his team of researchers from the MESA+ Institute for Nanotechnology at the University of Twente in the Netherlands have now succeeded in doing this. The scientists scanned the angle of a laser beam that illuminated an opaque diffuser. At the same time, a computer recorded the amount of fluorescent light that was returned by a tiny object hidden behind the diffuser. While the measured intensity of the light cannot be used to form an image of the object directly, the information needed to do so is in there, but in a scrambled form, Mosk points out. According to him, two young scientists, who are the first authors of this paper, had the brilliant idea to see whether that scrambled information was sufficient to reconstruct the image. And they found a way to do so, he says.

Their method involves a computer program that initially guesses the missing information, and then tests and refines the guess. They succeeded in making an image of a hidden fluorescent object just 50 micrometres across, the size of a typical cell. The test object used was the Greek letter , written in fluorescent ink and 100 times smaller than the one shown in the figure. The test object was covered by a strongly scattering ground-glass diffuser that completely hid it from view. The image was reconstructed from the diffuse speckled pattern of the returned fluorescence for each angle of the laser.

The researchers expect their work to lead to new microscopy methods capable of forming razor-sharp images in a strongly scattering environment. This will be very useful in nanotechnology, says Mosk. We would like to bring structures to light that are hidden inside complex environments like computer chips. They also dream of extending their method so that objects under the human skin can be examined non-invasively. But for the moment, he says, our method is too slow for that.