In a newly published paper in Nature Physics, researchers at CUNY ASRC detail a breakthrough experiment in which they were able to observe time reflections of electromagnetic signals in a metamaterial.
Have you ever wondered how we see ourselves in the mirror? It’s a common phenomenon that we take for granted. The reflection we see is produced by electromagnetic light waves bouncing off the mirrored surface, creating a spatial reflection. This process is not limited to visual perception alone; spatial reflections of sound waves also occur. The echoes created by these reflections carry our words back to us in the same order we spoke them, allowing us to hear our own voice. It’s fascinating to think about the intricate ways in which our surroundings interact with us, allowing us to experience the world in various ways.
For over six decades, scientists have put forward the idea that it may be possible to witness a distinct type of wave reflections called temporal, or time, reflections. Unlike spatial reflections that occur when light or sound waves hit a surface such as a mirror or wall at a particular point in space, time reflections occur when the entire medium through which the wave travels suddenly and drastically alters its properties across all space. During such an occurrence, a section of the wave becomes time-reversed and its frequency is transformed into a new frequency.
Until now, scientists had never observed this particular phenomenon in electromagnetic waves. The reason for this lack of evidence is rooted in the fundamental properties of materials and their optical properties.
In a recent development, a team of researchers from the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) have published a paper in the renowned scientific journal, Nature Physics, highlighting a significant breakthrough in their experiments. The team has successfully observed time reflections of electromagnetic signals in a specially designed metamaterial, marking a major milestone in the field of electromagnetic research.
“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,” adds corresponding author Andrea Alù. “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 performance allowed a considerable fraction of the broadband signals going through the metamaterial to be time-reversed and frequency transformed in real-time.
This remarkable feat has created a bizarre echo effect where the final part of the signal is reflected before the initial segment. As a consequence, if you were to observe yourself in a time mirror, you would witness an inverted reflection, revealing your back instead of your face. In the case of acoustics, this finding generates a sound resembling the rewinding of a tape.
The researchers also showed that broadband frequency conversion caused the time-reflected signals’ duration to be extended. Hence, if we could see the light signals, the hues would suddenly shift, with red becoming green, orange becoming blue, and yellow becoming violet.
The researchers employed engineered metamaterials to do their experiments.
Their approach involved injecting broadband signals into a meandered strip of metal that measured approximately 6 meters long and was printed on a board. This strip was loaded with a dense array of electronic switches that were connected to reservoir capacitors. The researchers triggered all the switches simultaneously, causing the impedance along the line to double suddenly and uniformly. This quick and substantial change in electromagnetic properties created a temporal interface that resulted in the measured signals carrying a time-reversed copy of the incoming signals with remarkable accuracy.
The experiment proved that it is feasible to create a time interface by successfully reversing time and altering the frequency of broadband electromagnetic waves. For the most extreme wave control, these two techniques provide additional degrees of freedom. The accomplishment may open the door for innovative wireless communications uses as well as the creation of compact, low-power wave-based computers.
“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,” adds co-first author Gengyu Xu. “It is very difficult to change the properties of a medium quick 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.”
“The exotic electromagnetic properties of metamaterials have so far been engineered by combining in smart ways many spatial interfaces,” remarks co-first author Shixiong Yin. “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.”
The new metamaterial platform can combine time interfaces in a powerful way, making it possible to make electromagnetic time crystals and time metamaterials. The finding has the potential to open new possibilities for photonic technology as well as new methods to improve and manage wave-matter interactions when combined with customized spatial interfaces.
Image Credit: Andrea Alu