Researchers create bizarre optical phenomena, defying the laws of reflection and refraction

Exploiting 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, published this week in Science, 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 photonics technology.”

It has been recognized since ancient times 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.

An array of nanoscale resonators, much thinner than a wavelength, creates a constant gradient across the surface of the silicon. In this visualization, the light ray hits the surface perpendicularly, from below. 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. Credit: Image courtesy of Nanfang Yu.

The conventional laws, taught in physics classrooms worldwide, predict the angles of reflection and refraction based only on the incident (incoming) angle and the properties of the two media.

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

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