Manipulating electromagnetic waves to cause head-on collisions

Researchers from the Advanced Science Research Centre at the CUNY Graduate Center have shown that it is possible to manipulate photons so that they can collide, interacting in new ways as they cross paths. The discovery, detailed in Nature Physics, will allow scientists who develop technologies rooted in electromagnetic wave propagation to make advances in telecommunications, optical computing and energy applications.

Andrea Alù, the founding director of the CUNY ASRC Photonics Initiative, said the researchers worked on building a series of experiments that show how we can create metamaterials with unique properties that emerge from abrupt time variations of their electromagnetic properties. These variations allow researchers to manipulate wave propagation in ways not seen in nature. “This newest work shows that we can use abrupt temporal changes in tailored metamaterials — known as time interfaces — to make waves collide as if they were massive objects. We were also able to control whether the waves exchanged, gained or lost energy during these collisions,” Alù said.

Typically, when two electromagnetic waves cross paths, they move right through each other without interacting. This is different from what happens when two objects, like two balls, bump into each other. In the latter case, the particles collide and their mechanical features determine whether the energy is conserved, lost or increased in the collision. While photons would be expected to go through each other without any interaction, by triggering a time interference the scientists were able to demonstrate strong photon-photon interactions and control the nature of the collision.

The research team’s work was inspired by speculation about whether it would be possible to erase an unwanted mechanical wave, such as a tsunami or a seismic wave, by throwing another similar wave against it to counter it. Emanuele Galiffi, the study’s lead author, said that while such an outcome is impossible in conventional wave physics, it is possible in principle with a temporal metamaterial. “Our experiment allowed us to demonstrate this concept in action for electromagnetic waves,” Galiffi said.

The researchers also proposed and demonstrated an application of their concept to shape electromagnetic pulses by colliding them against each other. “This technique allows us to use an additional signal as a mould to sculpt a pulse that we are interested in structuring. We have shown this for radio frequencies and we are now working to realise this sculpting ability at higher frequencies,” Gengyu Xu, co-leading author of the paper, said.

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