Photons can collide as if they were massive objects thanks to metamaterials known as time interfaces that undergo abrupt changes in their optical properties. This is the finding of researchers at the City University of New York, US, who say their work could have applications in wireless communications, imaging and energy harvesting technologies.

When two objects bump into each other, they collide and their kinetic energy is either conserved, lost or increased, depending on their mechanical properties. In contrast, two photons (electromagnetic waves) generally pass right through each other, although their encounter may manifest itself via wave phenomena such as interference. However, the situation is very different when they scatter off a lossy structure as they interfere. In this case, energy can pass between them so that it is entirely transmitted or absorbed depending on the photons’ relative amplitude and phase. Such “coherent wave control”, as it is known, can be exploited to create phenomena such as perfect absorption – that is, the destructive interference of waves as their energy is completely dissipated – and to tailor the amount of absorption as required.

Researchers led by physicist and engineer Andrea Alù have now shown they can create a new form of control over energy exchanges between photons. In the process, they realized the photonic analogue of a mechanical collision for electromagnetic waves using metamaterials that can undergo abrupt and large changes in their electromagnetic properties. These variations allowed the team to create a structure known as a time interface.

“When two waves propagating in opposite directions experience such an interface while they are overlapping, they experience extremely fast energy exchanges, as if they were colliding objects,” Alù explains. “The relative phase of the two waves can control the nature of this collision, which can either conserve energy, dissipate it or amplify it.” In this form of temporal coherent wave control, the waves reflected off the time interface destructively interfere with refracted waves. Under suitable conditions, this allows one or even both waves to be cancelled out.

Shaping electromagnetic pulses

The researchers got the idea for their new work after they wondered whether it might be possible to erase an unwanted mechanical wave, such as a tsunami or a seismic wave, by “throwing” another, similar, wave against it to counteract it. “While such an outcome is impossible in conventional wave physics, we knew it was possible, in principle, with a temporal metamaterial,” says Emanuele Galiffi, a postdoctoral fellow in Alù’s lab and the lead author of a study in Nature Physics on the work. “Our experiment allowed us to demonstrate this concept in action for electromagnetic waves.”

This photonic analogue of mechanical collisions could also be used to shape electromagnetic pulses by colliding them against each other. The researchers have demonstrated such sculpting for electromagnetic waves in the microwave regime and are now aiming to achieve this at higher frequencies by using devices such as high-speed graphene transistors instead of time interfaces.

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