Nanostructures enable chip-scale light propagation

Ashley N. Paddock, ashley.paddock@photonics.com

New optical nanostructures can slow down photons and fully control light dispersion, a big step forward in figuring out how to carry information on photonic chips without losing control of the phase of the light.

Researchers at Columbia University’s Engineering School have shown that it is possible for light to propagate from point A to point B without accumulating any phase. They engineered and observed a metamaterial with a zero refractive index. They discovered that light disperses through the material as if the material were completely missing in space, and that the oscillatory phase of the electromagnetic wave does not advance the way it would in a vacuum. This is what they termed zero-phase delay.

This is the first time that zero-index observations and simultaneous phase have been made on the chip scale and at the infrared wavelengths.

The nanofabricated superlattices, which consist of alternating stacks of negative-index photonic crystals and positive-index homogeneous dielectric media, have zero phase delay despite the varying physical path. Courtesy of Columbia Engineering School.

The optical phase’s exact control is based on a combination of positive and negative refractive indices. All natural known materials have a positive refractive index. By sculpturing these artificial subwavelength nanostructures, the scientists controlled the light dispersion so that a negative refractive index appeared in the medium. Next, they cascaded the negative index medium with a positive refractive index medium so that the complete nanostructure behaved as one with a zero index of refraction.

The scientists’ findings, a major breakthrough for telecommunications, appeared online July 10 in Nature Photonics (doi: 10.1038/nphoton.2011.129).

“With full control of the dispersion, these engineered thin-film materials can aid in developing highly dispersive phase and amplitude control, selective optical filters, highly directive antennas, self-focusing light beams and, even potentially, an approach to reduce the scattering cross section of objects, at least in the small scale or a narrow band of frequencies currently,” said Chee Wei Wong, associate professor at Columbia Engineering.

Wong said his group is examining the potential of these materials for the design of optical filters and highly dispersive phase and amplitude control as well as near-field measurements at the quasi-zero-index regimes.