Boffins map passage of light through 'tiny holes'

Use of terahertz radiation key to breakthrough

An international team has succeeded for the first time in mapping the process by which light passes through a tiny hole.

Dr Aurele Adam and Professor Paul Planken, of Delft University of Technology, in conjunction with South Korean and German research groups, have managed to provide insight into this complex process.

Measurements were conducted using terahertz radiation, a far-infrared light with an approximate frequency of 1012Hz.

This type of radiation allows the researchers to measure the force of the penetrating light's electrical field near the hole and not, as is usual, the intensity of the penetrating light.

The electrical field's values revealed much more about how light behaves in such situations. Measurement of the strength of the electrical field is done by measuring the refractive-index of a crystal near the hole using a laser beam.

The crystal's refractive index varies very slightly when in a variable electrical field. By measuring the variations in the refractive index, conclusions can be drawn about the strength of the light's electrical field near the hole.

"This process has never been mapped properly, mainly because the technology was not available to do so," said Professor Planken.

The experiments largely confirm, for the first time, what is known as the Bouwkamp model, named after a Dutch researcher who worked at Philips and created a theoretical model in 1950 for the way in which light passes through small holes.

For instance, the strength of the electrical field, as predicted by Bouwkamp, is greatest at the edge of the holes, and the field's strength gets smaller with the decreasing frequency of the terahertz light used.

The researchers also discovered that, even if the hole is up to 50 times smaller than the wavelength used, sufficient light can pass through to allow measurements near the hole, an extremely difficult task using other methods.

The technique has also enabled the researchers to record the entire process, allowing them to observe, slowed down a thousand billion times, how the light exits the hole and subsequently how the light waves move outwards in the same way as ring-shaped ripples caused by a stone thrown into a pond.