Time reflection of electromagnetic waves demonstrated in a groundbreaking experiment

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

The discovery forms the basis for revolutionary applications in wireless communications and optical computing.

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

Scientists have suspected the possibility of observing another form of wave reflections known as for over six decades temporalor Time, reflections. In contrast to spatial reflections, which occur when light or sound waves hit a boundary such as a mirror or wall at a specific point in space, time reflections occur when the entire medium in which the wave is traveling suddenly and erratically its Properties changes everything place. 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: A person sees their face when looking in a mirror, or when speaking, the echo comes back in the same order. (b) Time Reflections: The person sees their back when looking in a mirror and sees themselves in different colors. You hear their echoes in reverse order, similar to a rewound tape. Photo credit: Andrea Alu

This phenomenon has never been observed with electromagnetic waves before. The basic reason for this lack of evidence is that the optical properties of a material cannot easily be changed at a rate and magnitude that produces time reflections. But now in a newly published paper in

“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 instantaneously time reversed and frequency converted. The effect forms a strange echo in which 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 flipped, and you would see your back instead of your face. In the acoustic version of this observation, you would hear sound similar to what is emitted during the rewinding of 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 stripline. Upon closing/opening the switches, the electromagnetic impedance of this tailored metamaterial is abruptly decreased/increased, causing a broadband forward-propagating signal (in blue) to be 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 be abruptly transformed, such that red would become green, orange would turn to blue, and yellow would appear violet.

To achieve their breakthrough, the researchers used engineered

The introduced metamaterial platform can powerfully 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 time interfaces” by Hady Moussa, Gengyu Xu, Shixiong Yin, Emanuele Galiffi, Younes Ra’di and Andrea Alù, 13 March 2023, Nature Physics.
DOI: 10.1038/s41567-023-01975-y

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

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