TU Delft demonstrates for the first time how light squeezes through small holes

‘This process has never been mapped properly, mainly because the technology was not available to do so,’ says 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 who in 1950 created a theoretical model 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 indeed decreases in with decreasing frequency of the THz light used. In their experiments, the researchers also discovered that even if the hole is up to fifty times smaller than the wavelength used, sufficient light can pass through to allow measurements near the hole; an extremely difficult task using other methods. This technique has also enabled the researchers to record the entire process, allowing them to observe, slowed down a thousand billion (1012) 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.

Applications

The findings of Planken and his colleagues are not just significant from the point of view of fundamental science. They can help develop the use of Terahertz microscopy (THz). In the long term, Planken wishes to use the tiny holes as an improved source of THz light. The smaller these source holes become, the sharper the images that can be created using this technique and the easier it will be to measure small quantities of substances.

Terahertz radiation (with a frequency of about 1012 Hz) is a type of electromagnetic radiation which is increasingly used to create images. After all, many materials, such as paper, plastics and clothing, are transparent to THz radiation, while they block visible light.

Terahertz microscopes do not yet provide such sharp images. The development of stronger and smaller sources and more sensitive detectors will improve the viability of creating images of, for example, biological cells using THz radiation.