Measuring light through small holes

For the first time, a team of Dutch, German and South Korean scientists have shown how light can squeeze through any hole regardless of its size using Terahertz (THz) radiation, their findings,
which were recently published in the Optics Express journal, are expected to contribute to the development of terahertz (THz) microscopy and THz microspectroscopy.

For a long time, physicists have been stumped over the complexity of how light can pass through a hole smaller than half the wavelength of the light used. ‘This process has never been mapped
properly, mainly because the technology was not available to do so,’ explained co-author Professor Paul Planken of the Netherlands-based Delft University of Technology.

Using the so-called ‘THz radiation’, which is far-infrared light with a frequency of about 1,000 billion Hz , the researchers were able to show that measurements near a hole are possible when
enough light can pass through it even if it is up to 50 times smaller than the wavelength used.

THz radiation measures the force of the penetrating light’s electrical field near the hole, not, as is usual, the intensity of the penetrating light. The values of the electrical field have the
capacity to offer researchers more information about how light behaves in such situations than the values of intensity can.

The strength of the electrical field is measured by calculating the refractive-index of a crystal near the hole using a laser beam. Slight changes occur in the crystal refractive index when it
is in a variable electrical field. By measuring the differences in refractive index, researchers can make estimates about the strength of the light’s electrical field near the hole.

These findings are expected to bring improvements to terahertz microscopy in the long term, a potentially interesting new imaging technique, and THz microspectroscopy, a technique for
identifying tiny quantities of substances using light.

This latest research confirms what is known as the ‘Bouwkamp model’, named after a Dutch researcher who worked at the Dutch electronics company Philips some 58 years ago. Not only did Bouwkamp
predict that the strength of the electrical field is greatest at the edge of the holes, but he said the field’s strength diminishes when the frequency of the THz light used also drops.

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