In a first, scientists show time reflection of electromagnetic waves

The breakthrough experiment could lead to low-energy, wave-based computers and new applications for wireless communications.
Chris Young
Illustration of the experimental platform used to realize time reflections
Illustration of the experimental platform used to realize time reflections

Andrea Alu 

Researchers at the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) performed a breakthrough experiment in which they observed time reflections of electromagnetic signals in a tailored metamaterial.

The scientists, who published their findings in a paper in Nature Physics, were able to successfully cause time reversal as well as frequency conversion of broadband electromagnetic waves in their experiments.

Time reflection versus spatial reflection

Reflections in a mirror are caused by electromagnetic light waves bouncing off mirrored surfaces, creating a common phenomenon known as spatial reflection. Similarly, sound waves are spacially reflected to form echoes that carry our words back to us in the order they were spoken.

More than six decades ago, scientists first hypothesized the possibility that we could observe a different form of wave reflections, known as temporal or time reflections.

Unlike spatial reflections, which occur when light or sound hits an adequate boundary, time reflections happen when the entire medium a wave travels suddenly alters its properties across all of space. When this happens, a portion of that wave is time-reversed, and its frequency is converted to a new one.

This phenomenon had never been observed for electromagnetic waves since the optical properties of a material cannot be easily changed at the speed and magnitude required to cause time reflections.

The behavior of time-reflected waves

Now though, a new paper published in the journal Nature Physics details how the team of scientists from CUNY ASRC was able to observe time reflections of electromagnetic signals.

"This has been really exciting to see, because of how long ago this counterintuitive phenomenon was predicted," corresponding author Andrea Alù, founding director of the CUNY ASRC Photonics Initiative explained in a press statement. "Using a sophisticated metamaterial design, we were able to realize the conditions to change the material’s properties in time both abruptly and with a large contrast."

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The researchers were able to cause a large portion of broadband signals traveling in their engineered metamaterial to instantly time reverse and also change frequency.

They injected broadband signals into a meandered strip of metal roughly 6 meters long. It was printed on a board and featured a dense array of electronic switches linked to reservoir capacitors. All the switches were triggered at the same time for the experiment.

The experiment demonstrated that it's possible to realize a time interface that produces time reversal and frequency conversion of broadband electromagnetic waves. The researchers believe their new discovery can help develop small, low-energy, wave-based computers and exciting new wireless communications applications.

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