Normally, when light goes through a slit smaller than its wavelength, it fans out in all directions. But last year researchers managed to create a focused beam by sending light through a sub-wavelength slit in a corrugated metal film. In the 25 April PRL they present a detailed theoretical analysis of the effect. They conclude that the beaming is caused by waves of electrons on the metal surface that emit light when they scatter off the grooves. The light from the scattering cancels the more widely-spread components of light emerging from the slit. Similar corrugated structures could be useful for many optical devices, especially ones requiring small light sources.
According to standard theory, light spreads when it goes through an aperture smaller than the wavelength, and you can’t get around this property: The harder you try to squeeze the light by narrowing the slit, the more it spreads out. This puts a limit on the size of light beams created by optical devices.
But researchers have overcome this limitation in recent years. Thomas Ebbesen of the Louis Pasteur University in Strasbourg, France, and his colleagues found last year that a series of parallel grooves surrounding the aperture on the output side of a metal film produced a beam that spread to an angle of only 3 degrees . They also controlled the direction of the beam by adjusting the depth and spacing of the grooves, and the entire device was no more than a cubic micrometer.
In their new study the researchers have developed a full theory that they say matches experimental data extremely well. “By doing the calculation, we were able to pin down a simplified model,” says team member Luis Martín-Moreno of the University of Zaragoza in Spain. According to the theory, when light emerges from the aperture, it excites waves of “sloshing” electrons known as plasmons that move out along the metal surface toward the grooves. At each groove, plasmons scatter and radiate some light, while some plasmon energy remains to travel to the next groove. With the right groove spacing and depth, the pattern of light radiated from the scattering plasmons exactly cancels most of the light that sprays away from the central region, leaving only a narrow beam emerging from the slit.
The theory describes how the beam properties depend on the wavelength and the groove spacing and depth. The researchers show that their calculations of the beam profile for several groove configurations agree with their experimental data. Using the theory, the team plans to improve the design and study other groove arrangements, says Martín-Moreno.
The device could be valuable in many devices, according to the team. One example is multiplexers, which split multicolored light into many beams of single wavelengths and steer them in different directions. Other examples include data storage, miniature microscopes, and devices that translate data from fiber optic lines into electronic form for computer chips.
“It’s quite a remarkable development,” says John Pendry of Imperial College in London. “Quite frankly, the scale of the wavelength of [visible] light doesn’t fit with many of the things we want to do with it,” but the new results get around that problem, he says. “By shaping the structure on the far side, they can control where the light goes. It’s really like a headlamp.”
Ernie Tretkoff is a freelance science writer in Santa Cruz, California.
- H.J. Lezec et al., Science 297, 820 (2002).