Technique Provides Transmission Insight

Ken Zetie

In 1998, Thomas Ebbesen of the NEC Research Institute in Princeton, N.J., discovered that light can pass through holes 10 times smaller than its wavelength, but the explanation for this phenomenon remained a mystery. Now theory has caught up. John B. Pendry at Imperial College, University of London, has two explanations, opening the door to new uses for the discovery.

With colleagues J. Antonio Porto, now at the Ecole Centrale in Paris, and Francisco J. Garcia-Vidal at the Universidad Autónoma de Madrid in Spain, Pendry ran computer models to investigate the electric field created when light is directed at a metal surface with very narrow slits. This arrangement, similar to a diffraction grating, is different from the holes used by Ebbesen, but it was easier to model.

The researchers found that the presence of the slits alters the energy levels for surface plasmons, collective motions of electrons in the metal surface. These plasmons excite the electrons on the far side of the metal, setting them moving in the same way and causing them to emit light as if it had passed straight through the metal. Pendry suggests that the wavelength and bandwidth transmitted depend on the depth of the slit as well as the spacing.

However, the theoreticians have discovered a second possible escape route. If there are many holes per wavelength, the slits can act as a waveguide. The light causes currents to flow in the walls of the slits, which re-emit the light in a forward direction with very high efficiency.

The two processes produce quite different results. Coupled surface plasmons could well allow the diffraction limit to be overcome for photolithography masks. In addition, the grating can be tuned to pass a very narrow bandwidth, making an electromagnetic filter with high efficiency and no diffraction. If the slits are deep enough to act as a waveguide, it will act again as a filter that transmits light regardless of the incident angle.

Pendry pointed out another use: "The concentration of electromagnetic energy in very small volumes gives access to nonlinear phenomena for much lower power input than normal."