No holes barred

A recent experiment gives scientists new insights into radiation

Published: Thursday 30 April 1998

any elementary physics text book will tell you that a metallic plate with holes will not allow any radiation to pass through provided the holes are of a smaller diameter than the wavelength of the radiation. This principle is used in many devices, such as microwave ovens, where the glass door has a metallic mesh.

But now, T W Ebbesen and his collaborators at the nec Research Institute in Princeton, usa, have shown that metallic grids are not totally impervious to radiation. Instead, they may selectively transmit radiation, with wavelengths greater than the hole diameter. The researchers used a quartz substrate on which a thin film (of thickness about 0.2 micrometres) was deposited by evaporation ( Nature , Vol 391, No 6660).

Arrays of holes were then fabricated on the film by using an ion beam through a process known as sputtering. The hole diameter was varied between 0.1 micrometre (millionth of a meter) to 1 micrometre while the gap between the holes was between 0.6 and 1.8 micrometre. The spectrum was then observed using a spectrophotometer.

The observations were quite unexpected. When using a film with a spacing of about 0.6 micrometres, the film transmitted a strong radia-tion with a wavelength was around 0.96 micrometre.

This surprised the physicists as the standard theory predicts that at wavelengths larger than the gap between the holes, there should be very little or no transmission. Furthermore, the researchers also found that the strength of this transmission increases when the thickness of the film is increased from 0.2 micrometre to 0.5 micrometre.

The phenomenon of selective transmission which Ebbesen and his team have observed is unique. And though the whys and the hows of this phenomenon are not quite clear yet, there is a good possibility of the effect being used in novel photonic devices.

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