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nanotimes News in Brief

11-09 :: September 2011

Optics // Researchers at Harvard Create Bizarre Optical Phenomena, Defying the Laws of Reflection and Refraction © Text: Harvard University

E

xploiting a novel technique called phase discontinuity, researchers at the Harvard School

of Engineering and Applied Sciences (SEAS) have induced light rays to behave in a way that defies the centuries-old laws of reflection and refraction.

The discovery has led to a reformulation of the mathematical laws that predict the path of a ray of light bouncing off a surface or traveling from one medium into another – for example, from air into glass.

“Using designer surfaces, we‘ve created the effects of a fun-house mirror on a flat plane,” says co-principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “Our discovery carries optics into new territory and opens the door to exciting developments in photo- nics technology.”

While studying the behavior of light impinging on surfaces patterned with metallic nanostructures, the researchers realized that the usual equations were insufficient to describe the bizarre phenomena ob- served in the lab.

The new generalized laws, derived and experimen- tally demonstrated at Harvard, take into account the Capasso group‘s discovery that the boundary between two media, if specially patterned, can itself behave like a third medium.

“Ordinarily, a surface like the surface of a pond is simply a geometric boundary between two media, air and water,” explains lead author Nanfang Yu, a research associate in Capasso‘s lab at SEAS. “But now, in this special case, the boundary becomes an active interface that can bend the light by itself.”

The key component is an array of tiny gold anten- nas etched into the surface of the silicon used in Capasso‘s 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 (dubbed “phase discontinuity”) to the crests of the light wave crossing it.

Each antenna in the array is a tiny resonator that can trap the light, holding its energy for a given amount of time before releasing it. A gradient of different

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