It is possible to manipulate photons, study

No longer ships passing in the night: These electromagnetic waves had head-on collisions.


Coherent wave control takes advantage of the interference between numerous waves impinging on a system to suppress or boost outgoing signals depending on their relative phase and amplitude. In order to tailor the scattering of the waves in this process, energy transfers between the waves and spatial interfaces must be combined.

A recent study investigates the temporal analog of this phenomenon using time interfaces that permit photons to scatter nonconservative in real-time. According to a study by the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC), photons can be made to collide and interact in novel ways as they come into contact.

Andrea Alù, Distinguished Professor and Einstein Professor of Physics at The City University of New York Graduate Center, said, “Our work is building on 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 us to manipulate wave propagation in ways not seen in nature.”

“This newest work shows we can use abrupt temporal changes in tailored metamaterials – known as time interfaces – to make waves collide like massive objects. We could also control whether the waves exchanged, gained, or lost energy during these collisions.”

Two electromagnetic waves often pass through each other when their paths overlap, avoiding any contact. This is very different compared to when two large things, like two balls, collide. In the latter scenario, when the particles contact, it is determined by the mechanical characteristics of the collision whether energy is conserved, lost, or enhanced. 

For instance, when two pool balls come into contact, the system’s overall energy is conserved, unlike when two rubber balls meet, which often results in energy loss. While it would be expected for photons to pass through one another without any interaction, the scientists could establish strong photon-photon interactions and regulate the nature of the collision by causing a time interface.

The question of whether it could cancel out an unwelcome mechanical wave, such as a tsunami or seismic wave, by opposing it with another wave of a similar nature served as the inspiration for the study team’s work.

Emanuele Galiffi, a postdoctoral fellow with Alù’s lab and the study’s leading author, said, “While such an outcome is impossible in conventional wave physics, we knew it was possible in principle with a temporal metamaterial. Our experiment allowed us to demonstrate this concept in action for electromagnetic waves.”

The scientists also suggested and showed their idea to bend electromagnetic pulses through collisions.

Gengyu Xu, a postdoctoral fellow with Alù’s lab and co-leading author of the paper, said“This technique allows us to use an additional signal as a mold to sculpt a pulse that we are interested in structuring. We have shown this for radio frequencies and are now working to realize this sculpting ability at higher frequencies.”

Journal Reference:

  1. Galiffi, E., Xu, G., Yin, S., et al. Broadband coherent wave control through photonic collisions at time interfaces. Nat. Phys. (2023). DOI: 10.1038/s41567-023-02165-6
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