Sub-wavelength apertures once seemed like boring entities to physicists because they were thought to transmit only a very small amount of light. Moreover, the transmitted light emerged from the aperture uniformly in all directions. This so-called diffraction limit has placed a fundamental restriction on the scale with which light can be manipulated, and has therefore limited the development of certain micro-photonics technologies. In recent years, however, things have changed, and the optical properties of sub-wavelength apertures have become one of the most exciting areas of optics research.
In February 1998 Thomas Ebbesen and co-workers at NEC in Princeton found that the transmission properties of sub-wavelength holes change dramatically when they form a 2D array. The researchers studied what happened to light when it was incident on a piece of thin gold foil that was peppered with 100 million identical holes, each with a diameter of 300 nm and separated by 1 µm. For light with a wavelength slightly larger than the period of the array –known as a resonant wavelength – the NEC team discovered that the holes transmitted more than 100% of the light that fell directly on them. According to standard aperture theory, such a metallic film should only let about 0.1% of the incident light through, but it appeared that the apertures were able to “funnel” some of the light that fell on the metal between them.
Four years later the same group, in collaboration with the present author and colleagues at Louis Pasteur University in France and the University of Zaragoza in Spain, demonstrated that you do not need millions of holes to funnel light through the metal foil: one hole will do the job, provided it is properly flanked by a regular pattern, such as a bull’s-eye. Even more surprisingly, when both sides of the foil are patterned, we discovered that the light emerges in a very narrow beam instead of spreading out. In other words, it seems as if the diffraction limit can be overcome by just texturing both the entrance and exit surfaces around a sub-wavelength aperture.
In the June issue of Physics World Francisco Garcia-Vidal at the Universidad Autónoma de Madrid, Spain describes this work in more detail.
