Astonishing: Scientists stunned to see time reflections of electromagnetic signals

To date, this phenomenon had never been observed for electromagnetic waves.


In a new study, scientists at the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) detail a breakthrough experiment in which they could observe time reflections of electromagnetic signals in a tailored metamaterial.

They observed photonic time reflection and associated broadband frequency translation in a switched transmission-line metamaterial whose effective capacitance is homogeneously and abruptly changed via a synchronized array of switches.

The mirror is a reflection. We are used to seeing our faces when we glance in a mirror. The frequent phenomenon known as spatial reflection is caused by electromagnetic light waves that bounce off the mirrored surface to form the reflected images.

For more than 60 years, researchers have speculated about the prospect of witnessing temporal, or time, reflections, a different type of wave reflection. Time reflections happen when the entire medium through which the wave is flowing quickly and abruptly changes its properties over all of space, as opposed to spatial reflections, which occur when light or sound waves contact a boundary like a mirror or a wall at a specific position in space. A wave component is time-reversed, changing its frequency during such an event.

To date, this phenomenon has 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 speed and magnitude that induces time reflections.

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, said, “This has been 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.”

“Using a sophisticated metamaterial design, we realized the conditions to change the material’s properties in time abruptly and with a large contrast.”

Due to this achievement, a sizeable fraction of the broadband signals moving through the metamaterial were instantly time- and frequency-shifted. The result creates an odd echo where the final portion of the signal is reflected first. Because of this, if you looked into a time mirror, your image would be reversed, showing your back instead of your face. The acoustic equivalent of this observation would produce a sound akin to that produced when a tape is being wound up.

The researchers also showed that broadband frequency conversion caused the time-reflected signals’ duration to be extended. As a result, all of the hues in the light signals would abruptly change if they were visible to our eyes, turning red into green, orange into blue, and yellow into violet.

For this experiment, scientists used engineered metamaterials. They fitted a dense array of electrical switches connected to reservoir capacitors onto a printed board, meandering strip of metal about 6 meters long, and they pumped broadband signals into it. 

The impedance along the line was then abruptly and uniformly doubled due to the simultaneous activation of all switches. A temporal interface was created by this abrupt and significant shift in electromagnetic characteristics, and the measured signals faithfully carried a time-reversed replica of the entering signals.

The experiment proved that it is possible to create a time interface by successfully reversing time and altering the frequency of broadband electromagnetic waves. These two techniques provide new degrees of freedom for the most extreme wave control. The accomplishment may open the door for innovative wireless communications uses and the creation of compact, low-power wave-based computers.

Gengyu Xu, the paper’s co-first author and a postdoctoral researcher at CUNY ASRC, said, “The key roadblock that prevented time reflections in previous studies was the belief that it would require large amounts of energy to create a temporal interface. It is very difficult to change the properties of a medium quickly enough, uniformly, and with enough contrast to time reflect electromagnetic signals because they oscillate very fast. 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 fast switches.”

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

With the help of the newly developed metamaterial platform, electromagnetic time crystals and time metamaterials are made possible. The discovery has the potential to open new possibilities for photonic technologies and new ways to enhance and manipulate wave-matter interactions when combined with customized spatial interfaces.

Journal Reference:

  1. Moussa, H., Xu, G., Yin, S. et al. Observation of temporal reflection and broadband frequency translation at photonic time interfaces. Nat. Phys. (2023). DOI: 10.1038/s41567-023-01975-y
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