A New Way To Trap Light

There are several methods to confine light, but a new approach uses two waves of the same wavelength, but exactly opposite phases, to cancel each other out, allowing light of other wavelengths to pass freely.

Beams of light are usually trapped with traditional or advanced dielectric mirrors, as well as exotic photonic crystals and devices that rely on a phenomenon called Anderson localization. These methods are used to build optical devices such as lasers, solar cells and fiber optics. In all of these cases, light’s passage is blocked; in physics terminology, there are no “permitted” states for the light to continue on its path, so it is forced into a reflection.

Researchers at MIT have discovered a new technique for trapping light. Light is found to be confined within a planar slab with a periodic array of holes, although the light is theoretically “allowed” to escape. Blue and red colors indicate surfaces of equal electric field. Images courtesy of Chia Wei Hsu.

In the new MIT system, however, that is not the case. Instead, light of a particular wavelength is blocked by destructive interference from other waves that are precisely out of phase.

“It’s a very different way of confining light,” said physics professor Marin Soljacic, who worked with professors John Joannopoulos and Steven Johnson, and graduate students Chia Wei Hsu, Bo Zhen, Jeongwon Lee and Song-Liang Chua on the project.

Devised through computer modeling and then demonstrated experimentally, the system sets up two waves that have the same wavelength but exactly opposite phases — when one wave has a peak, the other has a trough. These waves cancel each other out while the light of other wavelengths can pass through freely. What makes this particular technique so useful is that it can be applied to any type of wave — from sound to radio.

Light can escape the photonic crystal slab using different channels, but waves in these channels can destructively interfere such that nothing escapes and light remains trapped.

In mathematical terms, the new phenomenon — where one frequency of light is trapped while other nearby frequencies are not — is an example of an “embedded eigenvalue.” While this had been described as a theoretical possibility in 1929 by mathematician and computational pioneer John von Neumann, it had yet to be seen in practice.

“New physical phenomena often enable new applications,” Hsu said. Such applications could include large-area lasers and chemical or biological sensors.

The investigators, however, are putting the practical applications on the back burner to focus on examining the new, unexpected phenomenon.

The work appeared in Nature (doi: 10.1038/nature12289).

For more information, visit: www.mit.edu