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Designer surfaces rewrite laws of refraction and reflection

Photonics Spectra
Nov 2011
Ashley N. Paddock, ashley.paddock@photonics.com

CAMBRIDGE, Mass. – A new technique called phase discontinuity helps light break the age-old laws of reflection and refraction – and could lead to the development of flat lenses that can focus images with no aberrations.

For centuries, it has been recognized that light travels at different speeds through different media. Reflection and refraction occur whenever light encounters a material at an angle, because one side of the beam is able to race ahead of the other. As a result, the wavefront changes direction.


(Above) Clockwise from left: Patrice Genevet, Nanfang Yu, Federico Capasso, Zeno Gaburro and Mikhail A. Kats. (Below) A simulation of the image that would appear in a large mirror patterned with the team’s new phase mirror technology. Courtesy of Nanfang Yu and Eliza Grinnell, Harvard SEAS.


Conventional laws in physics predict the angles of reflection and refraction based only upon the incident (incoming) angle and the properties of the two media. However, researchers at Harvard School of Engineering and Applied Sciences (SEAS) have discovered that the boundary between the two media, if specially patterned, can itself behave as though it were a third medium.

Using designer surfaces, the researchers created a fun-house mirror effect on a flat plane, according to Federico Capasso, professor and senior researcher. The key component is an array of gold nanoantennae etched into the surface of the silicon used in the SEAS lab. The array is structured on a scale much thinner than the wavelength of the light hitting it. This means that, unlike in a conventional optical system, the engineered boundary between the air and the silicon imparts an abrupt phase shift – or phase discontinuity – to the crests of the light wave crossing it.


An array of nanoscale resonators that are much thinner than a wavelength creates a constant gradient across the surface of the silicon. In this visualization, the light ray hits the surface from below at a perpendicular angle. The resonators on the left hold the energy slightly longer than those on the right, so the wavefront (red line) propagates at an angle. Without the array, it would be parallel to the surface. Courtesy of Nanfang Yu.


Each antenna in the array acts as a tiny resonator that can trap the light, holding its energy for a given amount of time before releasing it. A gradient of different types of nanoscale resonators across the surface of the silicon can effectively bend the light before it even begins to propagate through the new medium.

The resulting phenomenon breaks the old rules, creating beams of light that reflect and refract in arbitrary ways, depending upon the surface pattern.

“This amazing design freedom allows us to shape the reflected and refracted beams in arbitrary ways so that reflection and refraction phenomena that never occur in nature can be observed,” Capasso said. “For example, a light coming in at an angle can be reflected toward the light source, and, paradoxically, there is an incident angle beyond which light is not reflected.”


Harvard researchers have created strange optical effects, including corkscrewlike vortex beams, by reflecting light off a flat, nanostructured surface. Courtesy of Nanfang Yu.

The team is working on a variety of planar optical components based on the concept of interfacial phase discontinuities such as planar lenses, which could focus an image without curving it. This would eliminate the need for compound lenses to correct aberrations, and birefringent interfaces.

The discovery appeared online Sept. 1 in Science (doi: 10.1126/science.1210713).


GLOSSARY
resonator
A volume, bounded at least in part by highly reflecting surfaces, in which light of particularly discrete frequencies can set up standing wave modes of low loss. Often, in laser work,the resonator contains two facing mirrors that may either be flat (Fabry-Perot resonator) or have some spherical curvature, which together bind the lasing material that is referred to as the gain medium, and hence the optical cavity of a laser is where lasing occurs.  
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