In a new study, scientists at the Advanced Science Research Center of the CUNY Graduate Center (CUNY ASRC) describe a groundbreaking experiment in which they were able to observe time reflections of electromagnetic signals in a custom metamaterial.

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

The mirror is a reflection. We are used to seeing our faces when we look in the mirror. The frequent phenomenon known as spatial reflection is caused by electromagnetic light waves bouncing 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, another type of wave reflection. Time reflections occur when the entire medium through which the wave is flowing changes properties quickly and abruptly throughout space, unlike spatial reflections, which occur when light or sound waves hit a boundary such as a mirror or a wall at a specific position in the space. room. A wave component is inverted in time and changes 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 rate 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 was exciting to see because of how long ago this counterintuitive phenomenon was predicted and how differently time-reflected waves behave compared to space-reflected waves.”

“Using an advanced metamaterial design, we realized the conditions to change the properties of the material abruptly and with great contrast over time.”

Thanks to this achievement, a significant portion of the broadband signals moving through the metamaterial were instantly shifted in time and frequency. The result creates a strange echo where the last part of the signal is reflected first. Therefore, if you were to look 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 similar to that produced when a tape is wound.

The researchers also showed that broadband frequency conversion increased the time duration of the reflected signals. As a result, all the hues in the light signals would change abruptly when visible to our eyes, changing red to green, orange to blue, and yellow to violet.

For this experiment, scientists used engineered metamaterials. They placed a dense array of electrical switches connected to reservoir capacitors on a printed circuit board, a meandering strip of metal about 20 feet long, and pumped broadband signals into it.

The impedance along the line was then abruptly and evenly doubled by the simultaneous activation of all switches. This abrupt and significant shift in electromagnetic characteristics created a temporal interface, and the measured signals faithfully carried a time-inverted replica of the incoming signals.

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

Gengyu Xu, the paper’s co-first author and a postdoctoral researcher at CUNY ASRC, said: “The main hurdle preventing time reflections in previous studies was the belief that it would take 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 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 means of 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 developed by cleverly combining many spatial interfaces. Our experiment shows that it is possible to add time interfaces to the mix, increasing the degrees of freedom for wave manipulation. We have also been able to create a time version of a resonance cavity, which can be used to realize a new form of filtering technology for electromagnetic signals.”

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

Magazine reference:

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