Scattered light has the best focus: Can a breath mint make a sharper image than the best lens?

A lens can focus light in transparent materials such as air, glass and pure water, and the size of the focus determines the sharpness of the image made. Although the size of the focus depends on the quality of the lens, even the best lens has its limits because light is a wave phenomenon and, in fact, the focus gets less sharp as the distance from the lens increases. This limit is known as the diffraction limit. In non-transparent materials such as paint, milk or frosted glass the situation is even worse: light is scattered and forms a diffuse blur without any sharp focus. However, researchers at the FOM Foundation, the MESA+ Institute at the University of Twente and FOM Institute AMOLF in Amsterdam have come to a surprising conclusion: this scattering can be used to focus light more sharply. This conclusion is very important for nanotechnology, in which light is used in making and researching minuscule structures. The researchers published their findings online in Nature Photonics on Sunday, 14 February.

It is possible to focus light within a non-transparent material by distorting the incoming light waves in such a way that they bend towards the focus after scattering. Three years ago, the researchers demonstrated for the first time that light could be focused in this way through almost all objects which would normally cause scatter: anything from white paint to eggshells. The instrument that does the distorting is affected – as any optical instrument – by the diffraction limit. If light is focused from a greater distance, the focus is no longer sharp. However, if a scattering material (in this case, a layer of paint) is placed between the instrument and the focus, it suddenly becomes possible to produce a much smaller focus. In other words, the scattered light allows itself to be focused perfectly; much more sharply than light that travels undisturbed through the air. Using this method it is also possible to focus light in places that are normally difficult to reach, which is important for applications in nanotechnology. Clearly, there are potential applications in microscopy, too, where the diffraction limit makes it difficult to produce images of the smallest components of a living cell. Would it be possible to make a sharper image with, for example, a breath mint than with the best available lens? This question is a research challenge open for new undergraduate and graduate students.

For more information, please contact:
Dr Allard Mosk, Dept. of Science & Technology and MESA+ Research Institute,
Telephone +31 53 489 5392 or FOM Communications, Anita van Stel, +31-30-6001208

I.M. Vellekoop, A. Lagendijk, A.P. Mosk
Exploiting disorder for perfect focusing
Nature Photonics, Published online: 14 February 2010

Caption accompanying the diagram: Focussing light with a maze. Materials such as white paint or peppermints act as a maze for light, as they cause the light to scatter and constantly change direction. Because light is a wave phenomenon, we are able to send it into a maze from different directions at the same time. After the light has changed directions a number of times, the light waves leave the maze – again at different places simultaneously. Now, however, the light is extremely sharply focussed. It is precisely the change in direction in the maze that makes the increase in sharpness possible.