SEAS Research Points to Optics Breakthrough

In a discovery that could potentially revolutionize the way lenses are constructed, researchers at the Harvard School of Engineering and Applied Sciences have reformulated some of the most basic equations governing the behavior of light.

Cameras and telescopes commonly use compound lenses to “flatten” an incoming curved image. But with the team’s discovery, these bulky compound lenses could possibly be replaced by a single flat lens, which could be used to create very low footprint cameras and telescopes, according to Yu.

“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, who is the co-principal investigator of the study along with Applied Physics Professor Federico Capasso.

The researchers developed generalized optics equations, a discovery which SEAS Research Associate Nanfang Yu, who is the lead author of the study, said was “very straightforward, very basic to the laws of physics.”

High school physics students studying optics have traditionally been taught that the angle of incidence is equal to the angle of reflection—that is, light hits a surface at a certain angle and then bounces back at that same angle.

According to Yu, this standard equation “is not general enough” because it only applies to two media separated by a simple surface, such as the boundary between water and air. The boundary itself is assumed to be “passive,” having no impact on the behavior of the passing light.

“We are generalizing the laws of reflection and refraction,” Yu said.

The researchers’ goal was to generalize the optics laws to apply to both passive boundaries and non-passive boundaries—interfaces that do interfere with the path of a light beam.

To do this, the team designed a boundary of tiny oscillating resonators that functioned as a non-passive medium between silicon and air.

When the researchers directed a beam of light at the engineered surface, the resonators imparted a time delay, or phase discontinuity, that bent the light at angles not predicted by the conventional optics laws.

To account for the discrepancy, the SEAS researchers added a new term to the standard laws of refraction and reflection to represent the phase shift imparted at the engineered interface.

Yu explained his team’s discovery using the classic “lifeguard problem” of physics, which poses the dilemma faced by a lifeguard upshore on the beach who must jump into the ocean to save a drowning swimmer.

Because he moves much faster on land than in water, the lifeguard—representing a beam of ligh—maximizes his traveling distance on land so that he can reach the swimmer as quickly as possible.

But with the SEAS team’s discovery, the problem changes. The lifeguard on the beach is now separated from the water by a jagged cliff—representing the non-passive interface—which he must now factor into his calculations, changing his optimal route.

According to Yu, the lifeguard problem illustrates the increased complexity of adding the new interface.

With or without the engineered surface, Yu said that light “always chooses the smallest traveling time from one point to another,” just like the lifeguard.

The team’s research paper was published Friday in the magazine Science.

—Staff writer Rebecca D. Robbins can be reached at rrobbins@college.harvard.edu.