Light Mapped Through Holes

The question of how light squeezes through small holes has been answered by a research team successful in accurately mapping the process for the first time.

The researchers -- Aurele Adam, PhD, and professor Paul Planken of Delft University of Technology, in conjunction with two South Korean teams and one German group -- said their work also promises significant improvements in the new imaging technique of terahertz microscopy as well as terahertz microspectroscopy, a technique for identifying tiny quantities of substances using light.

According to the laws of physics, it is particularly difficult to pass light through a hole smaller than half the wavelength of the light used. The researchers managed to provide insight into this process by conducting measurements using terahertz (THz) radiation, or far-infrared light.

THz radiation, a type of electromagnetic radiation, is gaining popularity as a way to create images because many materials -- such as paper, plastics and clothing -- that are opaque in visible light become transparent under THz radiation.

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 reveal much more about how light behaves in such situations than intensity can. Measurement of the strength of the electrical field is done with great precision 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 on 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 Planken.

The experiments largely confirm, for the first time, what is known as the Bouwkamp model, named after a Dutch researcher at Philips who, in 1950, created a theoretical model for the way light passes through small holes. As predicted by Bouwkamp, the strength of the electrical field is greatest at the edge of the holes and the field’s strength decreases as the frequency of the THz light used decreases.

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. 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, resembling the ring-shaped ripples that form on the water's surface after a rock has been thrown into a pond.

Planken said in the long term he hopes to use the tiny holes as an improved source of THz light for applications such as THz microscopy and microspectroscopy, because the smaller these source holes become, the sharper the images that can be created, and the easier it will be to measure small quantities of substances.

Improving the sharpness of THz microscopes, coupled with more sensitive detectors, will improve the viability of creating images of biological cells using the radiation, the researchers said.

Their findings appear in the May 12 issue of the journal Optics Express.

For more information, visit: www.english.tudelft.nl