Silver foils the diffraction limit

26 Jun 2002

French scientists reveal that fine patterns on metal films could lead to smaller photonic and electronic devices.

The incredible optical characteristics of textured metal films are prompting physicists to rethink the laws of optics. An international team of physicists led by Henri Lezec and Thomas Ebbesen of the Louis Pasteur University in Strasbourg has shown that large amounts of light can pass through a sub-wavelength aperture in a patterned metal film without being diffracted. The discovery could lead to smaller photonic and electronic devices by overcoming the so-called diffraction limit (Science to appear).

According to the theory of diffraction, only a tiny amount of light can pass through a hole that is narrower than the wavelength of the light. Also, the light that is transmitted is diffracted in all directions. These effects limit the minimum size of many optical devices and techniques, such as the creation of features on semiconductors by optical lithography and the efficient coupling of light into optical fibres.

Now the Strasbourg-led team has found a way to shine more light through a tiny aperture, and to channel it into a collimated beam. Lezec and co-workers created a sub-wavelength aperture in a thin silver film and etched a periodic pattern of grooves around it using a focused ion beam.

This corrugated metal surface supports the excitation of surface waves known as plasmons that soak up the incident light. Previous theoretical studies have suggested that these plasmons squeeze through the hole and are converted back into light on the other side. This enhances the optical transmission of the film.

The researchers found that the wavelength of the transmitted light depends on the spacing of the grooves in the film. By patterning the reverse side of the film, they also discovered that the light emerges from the hole as a tightly focused beam that can propagate with very little divergence. The team then found that the direction of the transmitted light could be controlled by changing the symmetry of the periodic pattern.

The technique may be useful in a variety of nanoelectronics applications, including optimizing near-field devices for microscopy or data storage, and improving optical devices such as light-emitting diodes (LEDs) and semiconductor lasers.

Ebbesen told PhysicsWeb that the group now plans to understand the subtleties of the physics of the thin films, but remains tight-lipped about prototype devices under investigation.

Author
Valerie Jamieson is Features Editor of Physics World