The laws of reflection and refraction may not be a universal law after all.


Top, clockwise from left: Patrice Genevet, Nanfang Yu, Federico Capasso, Zeno Gaburro, and Mikhail A. Kats. Bottom: A simulation of the image that would appear in a large mirror patterned with the team's new phase mirror technology. Photos by Eliza Grinnell and Nanfang Yu. Credit: SEAS



The above fun-house images were produced by researchers at the Harvard School of Engineering and Applied Sciences (SEAS), who have induced light rays to behave in a way that defies the laws of reflection and refraction.

Their discovery will potentially revolutionize the construction of lenses, as it rewrites the mathematical laws that predict the path of a ray of light bouncing off a surface or traveling from one medium into another.

Our discovery carries optics into new territory and opens the door to exciting developments in photonics technology, said co-principal investigator Federico Capasso

Light has been known to travel at different speeds through different media. According to the conventional law of reflection, the angel of reflection and refraction of light are based  solely on the incoming angle and the properties of the two media. However, the researchers in their lab observed bizarre phenomena that cannot cover the usual equations.

They discovered that the boundary between two media can become an ac tive interface that bends the light by itself, behaving like a third medium, explaind Nanfang Yu, a research associate in the lab.

In the lab, an array of tiny gold antennas etched into the surface of the silicon was structured on a scale much thinner than the  incoming light's wavelength.

The engineered boundary between the air and the silicon then imparts an abrupt phase shift (dubbed phase discontinuity) to the crests of the light wave crossing it.

Each antenna in the array works as a tiny resonator to trap the light, hold its energy for a while, and release it. The light was bent at angles not predicted by the conventional laws as the resonators imparted a time delay.

With different types of nanoscale resonators spread across the surface of the silicon could effectively bend the light before it begins to propagate through the new medium, the researchers found. 

The conventional optic laws turned out to be not general enough, for its application to only two media separated by a simple surface. The passive boundary has no impact on the behavior of the light.

However, in the study, the boundary of small oscillating resonators played a role of non-passive medium between silicon and air.

At Harvard, the new generalized optic laws, taking the discovery into account, and were derived and experimentally demonstrated.  The new laws would apply to passive boundaries as well as non-passive boundaries that do not interfere with the path of a light beam.

A new term was added to the equations, representing the gradient of phase shifts imparted at the boundary.

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, says Zeno Gaburro, a visiting scholar 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.

According to SEAS, the frequency (color), amplitude (brightness), and polarization of the light can be controlled as well, making the output is designer beam.

If you need to build a system of lenses, you might be able to avoid some parts of the instruments with this technique, said SEAS Visiting Fellow Zeno Gaburro, the co-principal investigator of the study.

The researchers have already successfully produced a vortex beam (a helical, corkscrew-shaped stream of light) from a flat surface, and are envisioning flat lenses that could focus an image without aberrations.




Nanfang Yu, Zeno Gaburro, Federico Capasso, and colleagues at SEAS have created strange optical effects, including corkscrew-like vortex beams, by reflecting light off a flat, nanostructured surface. Image courtesy of Nanfang Yu.