The concept of time reflection symmetry in physics implies that if you reverse time, the behavior of the physical system remains the same. Scientists have been trying to understand the time-reversal symmetry in electromagnetic waves for decades. Recently, researchers have conducted experiments demonstrating the time reflection of electromagnetic waves. These experiments involve sending a short burst of light through a special crystal that has been engineered to produce a time-reversed replica of the original signal.
For more than six decades, scientists have speculated about the possibility of observing a type of wave reflection known as temporal, or time, reflections. Researchers describe a groundbreaking experiment in which they were able to observe time reflections of electromagnetic signals in a tailored metamaterial.
We are accustomed to seeing our own faces in the mirror. The reflected images are created by electromagnetic light waves bouncing off the mirrored surface, resulting in the common phenomenon known as spatial reflection. Similarly, spatial reflections of sound waves create echoes, which carry our words back to us in the order we spoke them.
For more than six decades, scientists have speculated about the possibility of observing a different type of wave reflection known as temporal, or time, reflections. Time reflections occur when the entire medium in which the wave is traveling suddenly and abruptly changes its properties across all of space, as opposed to spatial reflections, which occur when light or sound waves hit a boundary such as a mirror or a wall at a specific location in space. A portion of the wave is time reversed and its frequency is converted to a new frequency during such an event.
Our experiment demonstrates that time interfaces can be added to the mix, increasing the degrees of freedom to manipulate waves. We were also able to create a time version of a resonant cavity, which can be used to realize a new type of electromagnetic signal filtering technology.
Shixiong Yin
This phenomenon had never been observed before for electromagnetic waves. The fundamental reason for this dearth of evidence is that a material’s optical properties cannot be easily changed at a rate and magnitude that causes time reflections. Researchers at the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) detail a breakthrough experiment in which they were able to observe time reflections of electromagnetic signals in a tailored metamaterial in a newly published paper in Nature Physics.
“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 through the metamaterial to be instantaneously time reversed and frequency converted. The effect creates a strange echo in which the last part of the signal is reflected first. As a result, if you looked 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 tape rewinding.
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.
The researchers used engineered metamaterials to achieve their breakthrough. They injected broadband signals into a 6 meter-long meandered strip of metal printed on a board and loaded with a dense array of electronic switches linked to reservoir capacitors. The switches were then all triggered at the same time, doubling the impedance along the line abruptly and uniformly. Because of the rapid and large change in electromagnetic properties, a temporal interface was formed, and the measured signals faithfully carried a time-reversed copy of the incoming signals.
The experiment demonstrated that it is possible to realize a time interface, producing efficient time reversal and frequency transformation of broadband electromagnetic waves. Both these operations offer new degrees of freedom for extreme wave control. The achievement can pave the way for exciting applications in wireless communications and for the development of small, low-energy, wave-based computers.
“The key roadblock that prevented time reflections in previous studies was the belief that creating a temporal interface would require a large amount of energy,” said Gengyu Xu, the paper’s co-first author and a postdoctoral researcher at CUNY ASRC. “Because electromagnetic signals oscillate so quickly, changing the properties of a medium quickly, uniformly, and with enough contrast to time reflect them is extremely difficult. Instead of changing the properties of the host material, we devised a metamaterial in which additional elements can be abruptly added or subtracted via fast switches.”
“The exotic electromagnetic properties of metamaterials have thus far been engineered by combining many spatial interfaces in smart ways,” said co-first author Shixiong Yin, a graduate student at CUNY ASRC and The City College of New York. “Our experiment demonstrates that time interfaces can be added to the mix, increasing the degrees of freedom to manipulate waves. We were also able to create a time version of a resonant cavity, which can be used to realize a new type of electromagnetic signal filtering technology.”
The introduced metamaterial platform enables electromagnetic time crystals and time metamaterials by powerfully combining multiple time interfaces. The discovery, when combined with tailored spatial interfaces, has the potential to open up new avenues for photonic technologies and new ways to enhance and manipulate wave-matter interactions.
