Physicists have observed the reflection of an electromagnetic wave in time for the first time

Now researchers from the Center for Advanced Scientific Research at the CUNY Graduate Center (CUNY ASRC), in the United States, publish in the journal ‘Nature Physics’ a revolutionary experiment in which they were able to observe temporary reflections of electromagnetic signals in a custom-made metamaterial.

When we look into a mirror, the reflected images are produced by electromagnetic waves of light bouncing off the mirror’s surface, creating the common phenomenon called spatial reflection. Similarly, the spatial reflections of sound waves form echoes that send our words back to us in the same order in which we uttered them.

Unlike spatial reflections, which occur when light or sound waves strike a boundary, such as a mirror or a wall, at a particular location in space, temporal reflections occur when the entire medium in which the light travels wave suddenly and sharply changes its properties throughout space. At that moment, a part of the wave is reversed in time and its frequency is converted to a new frequency.

To date, this phenomenon had never been observed in electromagnetic waves. The fundamental reason for this lack of evidence is that the optical properties of a material cannot be easily changed at a rate and magnitude that induce temporary reflections.

Now, the pioneering new experiment has made it possible to observe temporal reflections of electromagnetic signals in a custom-made metamaterial.

“It has been really exciting to observe this phenomenon, because this counterintuitive phenomenon was predicted a long time ago and because of how differently reflected waves behave in time compared to those reflected in space,” acknowledges Andrea Alù, author of the article, Professor of Physics at New York University Graduate Center and Founding Director of the CUNY ASRC Photonics Initiative Using sophisticated metamaterial design, we were able to realize the conditions for changing material properties over time both abruptly and with great contrast.”

This feat caused a significant portion of the broadband signals traveling through the metamaterial to instantly reverse in time and become frequency converted. The effect forms a strange echo in which the last part of the signal is reflected first. As a result, if we were to look into a temporary mirror, our reflection would be reversed and we would see our back instead of our face. In the acoustic version of this observation, you would hear a sound similar to that made when rewinding a tape.

The researchers also demonstrated that the duration of time-reflected signals was lengthened over time due to wideband frequency conversion. As a result, if the light signals were visible to our eyes, all their colors would be abruptly transformed, so that red would turn to green, orange to blue, and yellow to violet.

To achieve their breakthrough, the researchers used artificial metamaterials. They injected broadband signals onto a snaking strip of metal about 20 feet long, printed on a plate and loaded with a dense array of electronic switches connected to backup capacitors.

All the switches were then turned on at the same time, suddenly and evenly doubling the impedance along the line. This large and rapid change in electromagnetic properties produced a temporal interface, and the measured signals faithfully carried a time-reversed copy of the incoming signals.

The experiment demonstrated that it is possible to realize a temporal interface, producing efficient time reversal and frequency transformation of broadband electromagnetic waves. Both operations offer new degrees of freedom for extreme wave control. This achievement may pave the way for exciting applications in wireless communications and the development of small wave-based computers with low energy consumption.

“The main obstacle preventing temporal reflections in previous studies was the belief that large amounts of energy would be required to create a temporal interface,” explains Gengyu Xu, co-author of the paper and a postdoctoral researcher at the CUNY ASRC.

“It is very difficult to change the properties of a medium with enough speed, uniformity and contrast to reflect electromagnetic signals over time, because they oscillate so fast,” he continues. “Our idea was to avoid changing the properties of the host material and, in Instead, create a metamaterial in which additional elements could be abruptly added or subtracted via quick switches.”

Co-first author Shixiong Yin, a graduate student at CUNY ASRC and The City College of New York, notes that “the exotic electromagnetic properties of metamaterials have so far been achieved by cleverly combining many spatial interfaces.”

“Our experiment shows that it is possible to add temporal interfaces to the mix, expanding the degrees of freedom to manipulate the waves,” he stresses. “We have also been able to create a temporal version of a resonant cavity, which can be used to make a new way of electromagnetic signal filtering technology”.

The introduced metamaterial platform can powerfully combine multiple temporal interfaces, making it possible to create electromagnetic temporal crystals and temporal metamaterials. Combined with bespoke spatial interfaces, the discovery offers the potential to break new ground for photonic technologies and new ways to enhance and manipulate the interactions between waves and matter.