Groundbreaking Experiment Rewinds Light Signals

Whether we think about them or not, reflections are a part of our everyday life. Light reflections let us see ourselves in the mirror, while sound reflections produce the echo we hear as we walk down an empty corridor. Now scientists have created a new type of reflection called a time reflection: a phenomenon where the wave causing the reflection — such as a light or sound wave — travels in reverse and at a different frequency than emitted. For a light wave, this would look like watching a movie clip of a yellow speeding car play in reverse while the car’s color changes to violet or red.

Time reflections have previously been seen for waves in water, but with this new experiment they were seen for the first time with electromagnetic waves — a form of light. These reflections, first proposed more than 60 years ago, could enhance wireless communications and enable power-saving computers. The results of the experiment were published in Nature Physics on March 13.

“We were very excited when we first saw the time reflections,” says Andrea Alù, lead author on the new paper, who is a Simons Foundation Investigator in Physics and who leads the Simons Collaboration on Extreme Wave Phenomena Based on Symmetries. Alù is also a Distinguished Professor at the City University of New York (CUNY), the Einstein Professor of Physics at the CUNY Graduate Center and the founding director of the CUNY ASRC Photonics Initiative. “This demonstration opens a lot of exciting potential for new applications,” he says.

Time reflections are unlike other reflections, such as echoes or mirrors, which are caused as a wave bounces off a spatial boundary. Instead, time reflections occur when the medium a wave is traveling through suddenly changes properties, causing the wave to reverse direction and change frequency. However, changing the properties of a medium quickly enough to cause a time reflection of light has been a long-standing challenge. With the invention in recent years of new materials with optical properties that are easy to manipulate, there has been revived interest in creating something that could create time reflections.

“The changes to the material must happen very fast and be very large,” Alù says. “We’ve known for years how to make very abrupt changes to the optical properties of a material, but those changes are typically very small, not sufficient to reflect the waves in time. These changes would be like trying to look at your reflection in a pane of glass instead of a mirror.”

The new time-reflection experiment was successful due to a specially designed metamaterial — a growing class of materials engineered to have specific properties. By embedding the metamaterial with electrical switches that can be switched on and off, the scientists were able to change the material’s properties by effectively adding or removing stored electrical energy.

Alù first got the idea to use switches connecting to external reservoirs of stored energy two years ago, but bringing the idea to life took a lot of work.

“You have to make sure the switches are densely packed, and they are all triggered at the same time, without any sensible delay between them,” Alù says. “There are a lot of subtle tricks to making it work.”

In the experiment, an electromagnetic wave signal composed of many frequencies was sent through a long metal strip that snaked around the metamaterial filled with switches. By rapidly changing the metamaterial’s effective capacitance using the switches, the material was able to reverse part of the electromagnetic signal traveling through it.

In addition to the time reflection, when the switches were all synchronously triggered on, the wave’s frequencies were shifted to longer wavelengths. When the switches were all triggered off, the frequencies shifted to shorter wavelengths. The results are a demonstration of new ways light waves can be manipulated. Time reflections could be harnessed to help create smaller, faster and more efficient computers; better wireless communications devices; and improved optical computing. Instead of relying on mirrors and lenses to manipulate light, computers and devices could have integrated metamaterials allowing multiple time reflections to be made in smaller spaces.

“With time reflections, we can manipulate waves much faster and with much less energy,” Alù says.

Alù and his team are already working on improving the theory of time reflections and demonstrating similar phenomena at much smaller wavelengths. They’re also working on designing experiments that could demonstrate time reflections using optical light by using laser beams to change a material’s properties instead of electrical switches.

“The overall vision is to be able to combine space and time interfaces in smart ways to create the ultimate platform for wave manipulation,” Alù says. “It’s a rich field with lots of opportunities coming.”