Time reflection of electromagnetic waves demonstrated in a pioneering experiment

Researchers have conducted an experiment that demonstrates the time reflection of electromagnetic waves, which has potential implications for wireless communications and optical computing.

The discovery establishes the foundation for revolutionary applications in wireless communications and optical computing.

When we look in a mirror, we are used to seeing our faces looking back at us. The reflected images are produced by electromagnetic light waves bouncing off the mirrored surface, creating the common phenomenon called spatial reflection. Similarly, spatial reflections of sound waves form echoes that carry our words back to us in the order in which we spoke them.

For over six decades, scientists have assumed the possibility of observing another form of wave reflections, known as temporallyor time, reflections. Unlike spatial reflections, which occur when light or sound waves strike a boundary such as a mirror or a wall at a specific location in space, time reflections occur when the entire medium in which the wave is traveling suddenly and abruptly changes its properties across. all space. In such an event, part of the wave is reversed in time and its frequency is converted to a new frequency.

Conventional spatial reflections

(a) Conventional spatial reflections: When a person sees their face when they look into a mirror or when they speak, the echo comes back in the same order. (b) Time reflections: The person sees their back when they look into a mirror and they see themselves in different colors. They hear their echoes in reverse order, similar to rewinding a tape. Credit: Andrea Alu

To date, this phenomenon had never been observed for 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 induces time reflections. But now in a recently published paper i Natural physicsresearchers at the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) describe a groundbreaking experiment in which they were able to observe time reflections of electromagnetic signals in a tailored metamaterial.

“This has been really exciting to see because of how long ago this counterintuitive phenomenon was predicted and how different time-reflected waves behave compared to space-reflected ones,” said the paper’s corresponding author Andrea Alù, Distinguished Professor of Physics at The City University of New York Graduate Center and founding director of the CUNY ASRC Photonics Initiative. “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.”

This feat caused a significant portion of the broadband signals traveling in the metamaterial to be instantly reversed and frequency converted. The effect creates a strange echo, where the last part of the signal is reflected first. As a result, if you were to look into a time mirror, your reflection would be reversed and you would see your back instead of your face. In the acoustic version of this observation, you will hear sound similar to that emitted while rewinding a tape.

Time reversed electromagnetic waves

Illustration of the experimental platform used to realize time reflections. A control signal (in green) is used to uniformly activate a set of switches distributed along a metal strip. Upon closing/opening the contacts, the electromagnetic impedance of this tailored metamaterial is abruptly decreased/increased, causing a forward broadband signal (in blue) to become partially time-reflected (in red) with all its frequencies converted. (Adapted from Nature Physics.) Credit: Andrea Alu

The researchers also demonstrated that the duration of the time-reflected signals was stretched in time due to broadband frequency conversion. As a result, if the light signals were visible to our eyes, all their colors would suddenly be transformed, such that red would become green, orange would become blue, and yellow would appear violet.

To achieve their breakthrough, the researchers used manipulated metamaterials. They injected broadband signals into a coiled metal strip about 20 feet long, printed on a board and filled with a dense array of electronic contacts connected to reservoir capacitors. All contacts then tripped at the same time and suddenly and uniformly the impedance along the line doubled. This rapid and large change in electromagnetic properties produced a temporal interface, and the measured signals faithfully carried a time-reversed copy of the incoming signals.

The experiment showed that it is possible to realize a time interface that produces efficient time reversal and frequency transformation of broadband electromagnetic waves. Both of these operations provide new degrees of freedom for extreme wave control. The achievement could pave the way for exciting applications in wireless communication and for the development of small, low-energy, wave-based computers.

“The key roadblock preventing temporal reflections in previous studies was the belief that creating a temporal interface would require large amounts of energy,” said Gengyu Xu, the paper’s co-first author and a postdoctoral researcher at CUNY ASRC. “It is very difficult to change the properties of a medium fast enough, uniformly and with enough contrast for time to reflect electromagnetic signals because they oscillate very quickly. Our idea was to avoid changing the properties of the host material and instead create a metamaterial in which additional elements can be abruptly added or subtracted through quick switches.”

“The exotic electromagnetic properties of metamaterials have so far been engineered by combining many spatial interfaces in clever ways,” added co-author Shixiong Yin, a graduate student at CUNY ASRC and at The City College of New York. “Our experiment shows that it is possible to add time interfaces to the mix, which expands the degrees of freedom to manipulate waves. We have also been able to create a time version of a resonant cavity, which can be used to realize a new kind of filtering technology for electromagnetic signals.”

The introduced metamaterial platform can effectively combine multiple time interfaces, enabling electromagnetic time crystals and time metamaterials. Combined with tailored spatial interfaces, the discovery offers the potential to open new directions for photonic technologies and new ways to enhance and manipulate wave-matter interactions.

Reference: “Observation of temporal reflection and broadband frequency translation at photonic temporal interfaces” by Hady Moussa, Gengyu Xu, Shixiong Yin, Emanuele Galiffi, Younes Ra’di and Andrea Alù, 13 March 2023, Natural physics.
DOI: 10.1038/s41567-023-01975-y

This research was supported in part by the Air Force Office of Scientific Research and the Simons Foundation.

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