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Physicists perform first measurement of ‘time reflection’ in microwaves

17 Mar 2023
set-up for time reflection demonstration
On reflection: illustration of the experimental platform used to realize time reflections. (Courtesy: Andrea Alù)

Physicists in the US have observed an effect known as time reflection in an electromagnetic wave for the first time. They detected the phenomenon – the temporal counterpart of familiar spatial reflection – by rapidly switching a series of capacitors in a novel type of metamaterial. They say the result could improve wireless communication and ultimately help bring about long-sought-after optical computing.

Everyday reflection involves the transformation of a wavepacket when it meets an interface in a distinct region of space. The process preserves temporal ordering, so that the leading part of the incident wave remains ahead after reflection. This means that objects further from a mirror look more distant in the reflection, while sounds in an echo arrive back in the same order they were emitted.

Time reflection instead involves a wavepacket being transformed as a result of an abrupt change in time that applies equally throughout the medium it is traversing. In other words, the material in question experiences a sudden shift in its properties. This causes the wave to switch direction such that its trailing edge prior to reflection is now at the front. Objects nearer a mirror in the real world would look further away in the reflection, while for an echo the last sound emitted would become the first to arrive back.

The two processes conserve different quantities. A wave bouncing off an object transfers momentum to that object while its frequency is conserved. In contrast, a wave reflected in time must preserve momentum, causing a change in the speed with which it oscillates (its frequency). In other words, the reflected wave maintains its shape but is stretched out in time.

To date, scientists have only observed such temporal reflections in water waves. Seeing the same thing in electromagnetic radiation is complicated by the high frequency of the waves. The trick involves being able to switch a material’s refractive index uniformly at a high enough speed – taking much less time than the wave period – and with a great enough contrast so as to generate a measurable effect.

Time to reflect

Andrea Alù and colleagues at the City University of New York have now succeeded in doing that by devising a new kind of metamaterial. Metamaterials have striking electromagnetic properties, thanks to their large numbers of tiny, precisely arranged engineered structures.

The material in question consists of a 6 m-long strip of metal serving as a microwave waveguide that snakes back and forth 20 times to form a device some 30 cm2. Thirty capacitive circuits are positioned at regular intervals along the length of the strip, but separated from it by switches. The idea is to inject a train of microwave pulses and then switch all the circuits on or off at the same time while the pulses are in transit along the strip – causing a sudden change in the metamaterial’s effective refractive index and impedance. That sudden change temporally reflects the microwave signal.

Alù and colleagues were able to double (or halve) the refractive index in far less time than it took the wave to complete a single oscillation, thanks to their switching circuitry taking a short cut across the snaking waveguide. Injecting a signal consisting of two unequally strong peaks and then connecting the capacitive circuits, they found that a part of the signal arrived back at the input port with the peaks in reverse order and stretched out in time – just as would be expected for a time-reflected wave. The rest of the signal instead returned to the port with the two peaks in their original order, having spatially reflected off the far end of the metamaterial.

According to Alù, the analogue nature of this time reversal mechanism could lead to a number of applications. For example, he says, it might be used to combat distortion in a wireless data channel. Such distortion is often estimated by a receiver station sending back known signals to the transmitter with their temporal profiles reversed. But this usually involves digitising the signals. With time reflections being instead entirely analogue, he says that their use could save time, energy and memory.

Radio engineers may say that they have a new instrument in their toolbox

Simone Zanotto

In the longer term, he says, the scheme might find use in a new generation of analogue optical computers. As he points out, time and energy are sacrificed in current computers as a result of having to convert analogue electrical signals to and from the digital domain. But it turns out that one type of analogue operation that is particularly useful for signal processing and computing is phase conjugation – the transformation that takes place when waves undergo time reflection.

Before this can happen, Alù and his colleagues will try and shrink their metamaterial as far as possible. He says they are currently working on a chip-scale version that would operate at much higher frequencies – in the tens of gigahertz range, rather than the hundreds of megahertz of their current device. They might conceivably get to terahertz and beyond, he says, although at that point they would have to use laser pulses rather than electrical switches.

Chen Shen of Rowan University in the US, who was not involved with the work, reckons that the ability to control the spectra of radio waves could enable applications such as time-reversal medical imaging, temporal cloaking (a counterpart to spatial cloaking) and better estimating channel numbers in wireless communication. “These demonstrations show that time modulation can be added as a new ingredient for wave manipulation,” he says.

Simone Zanotto of the Scuola Normale Superiore in Pisa, Italy, agrees. “Radio engineers may say that they have a new instrument in their toolbox,” he says. “An instrument whose operating principle is well understood and probably further tuneable upon their needs.”

The research is published in Nature Physics.

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