The 'HOLE'Y light

A French scientist working in Japan had problems communicating in Japanese. He had asked the technician in the lab to drill 100 holes in square centimeter of a metal sheet, but the technician drilled 10000 holes in the same area. Then the French scientist performed the following experiment –

He shined light of intensity A through the perforated metal film and obtained light of intensity B at the other end. To his surprise, intensity B was far greater than intensity A (B >>>> A) This is the remarkable story of Thomas W. Ebbesen, who discovered the extraordinary transmission through sub-wavelength apertures.

What is a sub-wavelength aperture ?

It means that the diameter of the hole drilled in the metal film is lesser than the wavelength of the incident light. See the picture above…

Why does this extraordinary transmission happen?

This amazing effect is due to the interaction of the light with electronic resonances in the surface of the metal film, and they can be controlled by adjusting the size and geometry of the holes.

This knowledge is opening up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical sensing and biophysics. If the output surface surrounding the aperture is also corrugated, a surprisingly narrow beam can be generated, having a divergence of less than a few degrees, which is far smaller than that of the single apertures. This is because the light emerging from the hole couples to the periodic structure of the exit surface and to the modes existing in the grooves—which in turn scatter the surface waves into freely propagating light. This then interferes with the light that has travelled directly through the hole generating the focused beam.

What’s the use of this ?

In the field of opto-electronics for instance, studies are being carried out to extract more light from light-emitting devices. The metal electrodes of such devices, which are normally a source of loss, can be structured with holes to help extract the light from the diode. The need for ever-smaller features on electronic chips is pushing photolithography to use shorter wavelengths, with the associated increased costs and complications. The use of extraordinary optical transmission could perhaps circumvent this problem by using plasmon-activated lithography masks. Thes holes might find use in quantum optics. For instance, hole arrays are promising tools in the study of the physical nature—quantum versus classical—of plasmons as collective excitations when implemented in quantum entanglement experiments. They can also be harnessed for bio-detection where the molecule of interest can be specifically illuminated with a subwavelength aperture. The high optical contrast of these holes, their small sizes and their simplicity make them ideal candidates for integration on biochips as sensing elements. As in all plasmon enhanced phenomena, both the input and output optical fields can be strengthened, with the additional feature that the structure can potentially focus the signal towards a detector.

Recently, an opposite trend has also been observed. Say, you have very thin metal film which is so thin (nanometer thickness) that it is transparent. If you incident light through this film, most of the light passes through it, which is quite obvious. Now, if you perforate same kind of holes as mentioned above in them, then the intensity of the out coming light is drastically decreased! This observation has further led to excitement in the field…

It’s indeed amazing that how the everlasting light continues to awe us…