Scientists demonstrate time reflection of electromagnetic waves

(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. (c) Illustration of the experimental platform used to realize time reflections. A control signal (in green) is used to uniformly activate a series of switches distributed along a metal stripline. As the switches close/open, the electromagnetic impedance of this tailored metamaterial abruptly decreases/increases, causing a forward-propagating broadband signal (in blue) to be partially reflected in time (in red), with all of its frequencies converted. (Adapted from natural physics). Photo credit: Andrea Alu

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 time or time reflections for more than six decades. 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.

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 natural physicsresearchers from the Advanced Science Research Center at 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 was really exciting to see given how long ago this counter-intuitive phenomenon was predicted and how differently time-reflected waves behave compared to space-reflected ones,” said the paper’s corresponding author Andrea Alù, Distinguished Professor of Physics at The City’s Graduate Center University of New York and Founding Director of the CUNY ASRC Photonics Initiative. “Using sophisticated metamaterial design, we were able to realize the conditions to temporally change material properties both in leaps and bounds and with great contrast.”

This feat resulted in a significant fraction of the broadband signals transmitted in the metamaterial being instantaneously time-reversed and frequency-converted. The effect forms a strange echo where the last part of the signal is reflected first. So 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 sounds similar to those emitted when a cassette tape is rewound.

The researchers also showed that the duration of the time-reflected signals was stretched in time due to the broadband frequency conversion. As a result, if the light signals were visible to our eyes, all of their colors would be abruptly converted, turning red into green, orange into blue, and yellow into violet.

To achieve their breakthrough, the researchers used man-made metamaterials. They injected broadband signals into a 20-foot-long coiled strip of metal printed on a printed circuit board and populated with a dense array of electronic switches connected to reservoir capacitors. All of the switches were then tripped simultaneously, suddenly and uniformly doubling the impedance along the line. This rapid and large change in electromagnetic properties created a temporal intersection, and the measured signals faithfully contained 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 operations offer new degrees of freedom for extreme wave control. Success may pave the way for exciting applications in wireless communications and for the development of small, low-power, wave-based computers.

“The main hurdle preventing time reflections in previous studies was the belief that it would require large amounts of energy to create a temporal interface,” said Gengyu Xu, co-first author of the paper and a postdoctoral fellow at CUNY ASRC. “It is very difficult to change the properties of a medium quickly enough, smoothly and with enough contrast to reflect electromagnetic signals in time because they oscillate very quickly. Our idea was not to change the properties of the host material and instead create a metamaterial “in which additional elements can be added or removed abruptly by quick switches.”

“The exotic electromagnetic properties of metamaterials have so far been evolved through the intelligent combination of many spatial interfaces,” added co-first author Shixiong Yin, a graduate student at CUNY ASRC and City College of New York. “Our experiment shows that it is possible to add time interfaces to the mix, thereby expanding the degrees of freedom to manipulate waves. Filter technology for electromagnetic signals.”

The presented metamaterial platform can efficiently 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.

More information:
Andrea Alù, Observation of temporal reflection and broadband frequency translation at photonic time interfaces, natural physics (2023). DOI: 10.1038/s41567-023-01975-y.

Journal Information:
natural physics

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