The properties of light make it difficult to control on a scale smaller than its own wavelength. But researchers at the Korea Advanced Institute of Science and Technology (KAIST) have made a breakthrough in sub-wavelength optics that makes this easier, providing a way to visualise structures that are too small to see with conventional microscopes.

Image: Professor YongKeun Park

When focusing light with conventional optical tools such as microscope lenses, an effect called diffraction, which spreads light waves, makes it impossible to resolve individual objects that are smaller than about 250 nanometres (nm). If this diffraction limit could be overcome, it would be possible to directly visualise individual molecules within cells, or nanostructures used in manufacturing electronics, for example.

Theoretically, it is possible to overcome the 250 nm diffraction limit and resolve smaller objects by controlling “near-field” light waves. These are created when light reflects off the surface of an object, but doesn’t travel any further from the surface than a distance equal to one wavelength. This is in contrast to “far-field” light waves, which travel long distances from the surface and can be easily controlled with conventional optics. Since near-field waves are confined to within a few nanometres of an object’s surface, they are inaccessible with such optics and so are impossible to control in the same way. But KAIST researchers have found a way to overcome this problem.

The team discovered that by scattering far-field light waves in a controlled way, they could recreate near-field waves to allow focusing of light below the diffraction limit. The light was scattered by passing it through a layer of turbid (cloudy) medium containing nanoparticles: if they selectively manipulated the shape of the far-field light waves that reached this layer, they were able to focus the light at a resolution below the normal limit of diffraction.

The KAIST team showed that the same effect was achieved with a layer of either zinc oxide nanoparticles or simple white spray paint, and they say that any similarly turbid medium would be suitable. Other methods developed to overcome the diffraction limit require specialist equipment, such as a near-field scanning optical microscope or lenses made from artificial materials, so the new approach is cheaper, easier and more versatile. The research was published in Nature Photonics.

For further information contact:

Professor Yong-Keun Park
Department of Physics
Biomedical Optics Laboratory at KAIST
Korea Advanced Institute of Science and Technology
Republic of Korea