Light Disorder: Laws of Reflection, Refraction and the Surface Pattern Loophole…

Scientists at Harvard have discovered new ways of making light dance, along the way changing the laws of reflection and refraction of light. The wonderful world of Nano particles, along with their effects on light’s behavior were explored in order to make these discoveries. Under normal circumstances light is nice and predictable, Mr Consistent, the team from the Harvard School of Engineering and Applied Sciences – SEAS – found a loop-hole though, precise patterns of metallic nano-structures.

Just as in a Carnivals Magic Mirror amusement the nano-structures were able to warp and bend light, unlike the carnival though they did not require bendy mirrors or clever lighting. Instead using Nano structures embedded in precise patterns on the surface of silicon they were able to alter lights behavior. Their findings were published in the scientific journal Science, September 2 and have since led to the reformation of the mathematical laws of reflection and refraction, the predicted path of a ray of light bouncing of a surface or passing from medium to medium, bouncing or bending.

“Using designer surfaces, we’ve created the effects of a fun-house mirror on a flat plane,” said 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.”

Light reflecting off a surface has always relied on one constant that has now disappeared, a flat surface. Flat surfaces reflect light in nice predictable ways, allowing the laws of Reflection and Refraction of light to concentrate on those two aspects of lights interactions, bouncing and bending. Now scientists have discovered by impregnating a reflective surface with tiny – less than the wave length of light – patterns they are able to make light behave in unpredictable ways. One flat mirror demonstrated this best, using a specially developed pattern they were able to bend the light using the pattern, creating the tall man curved mirror effect, on a totally flat piece of mirror.

The team – Patrice Genevet, Nanfang Yu, Federico Capasso, Zeno Gaburro, and Mikhail A. Kats – first realized they were looking at something special while studying the behavior of light impinging on surfaces patterned with metallic nano-structures, the usual equations were insufficient to describe the bizarre phenomena observed. In order to properly describe what they had observed a new generalized set of laws were developed that allow for surface patterns, if no surface gradient or pattern is present the traditional laws may still used. The traditional laws deal with the boundary between the two mediums or materials, take for example if light traveled through the air and bounced off a brick wall, the two mediums are the air and the bricks, the boundary being where the two meet. If there is a Nano sized pattern on the bricks, this is considered a third medium in the equation and is where the new equations become involved.

The key to the discovery is a tiny gold antenna etched into the surface of the material, more specifically metallic nano-structures embedded into a reflective surface that cause phase discontinuity in light hitting the surface. The antenna are far smaller than the wavelength of light striking it causing each antenna to act as a tiny resonator, capturing and releasing the photonic energy striking them. The time the energy is held is the phase discontinuity, the light wave is no longer propagating. The amount of time the antenna holds on to the energy before releasing it can be tuned by altering the spacing and size of the antenna. By tuning regions of a surface with various patterns of antenna the researchers were able to get light to act counter to the normal laws of reflection and refraction. Stretching and altering the color upon demand. The team has even managed to create a helical spiral beam of light using only their patterning techniques.

“By incorporating a gradient of phase discontinuities across the interface, the laws of reflection and refraction become designer laws, and a panoply of new phenomena appear,” said Zeno Gaburro, a visiting scholar in Capasso’s group who was co-principal investigator for this work. “The reflected beam can bounce backward instead of forward. You can create negative refraction. There is a new angle of total internal reflection.”

The effects observed and demonstrated in the laboratory, the changes to lights behavior are normally only seen in complex laser systems or optical communications networks, requiring large amounts of complex hardware. The Harvard research team have been able to achieve similar results; stretch, twist and distort light, using the simplest of flat surfaces. Communications, optics and other fundamental technology of the modern world will all benefit from these discoveries that add to the our understanding of this new Nano world. Welcome back to wonderland, Alice.

Further information Harvard Gazette

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