Somewhere (and some time) over the rainbow

Whether a symbol of hope, the path toward a pot of gold, or the result of sunlight refracting on water droplets, rainbows capture the imagination. They also demonstrate the electromagnetic scattering that occurs at the surface between materials with different properties.

Advances in metamaterials open possibilities for specially engineered scattering. Stefanini et al. demonstrated this potential using a boundary-induced time interface to create rainbow scattering. Their advanced optical prism uses a single, monochromatic wave to produce the rainbow, instead of traditional methods that require a beam of white light, simplifying the excitation source.

During a misty day, a raindrop’s refractive index differs from air, causing electromagnetic scattering at the water’s surface and producing a rainbow. A temporal interface is the time-domain counterpart of this spatial interface: when the properties of the space instantaneously change.

However, it is difficult to alter the electromagnetic properties of an entire region. To circumvent this challenge, Stefanini et al. induced the temporal interface by changing the boundary conditions of an electric field inside a parallel plate waveguide.

The authors harnessed the ability to instantly change the conductivity of the metamaterial metallic plate in the parallel plate waveguide. When the metamaterial became conductive, it no longer bounded the electric field, which scattered into free space. Conservation laws forced the monochromatic wave into a set of different frequency waves that propagated in various directions, creating the so-called “temporal rainbow.”

This new rainbow is indicative of the potential of metamaterials. Temporal control of electromagnetic waves is not only beautiful to observe, but it could also improve diagnostic imaging, targeted radiotherapy, and secure telecommunications.

Source: “Temporal rainbow scattering at boundary-induced time interfaces,” by L. Stefanini, D. Ramaccia, A. Toscano, and F. Bilotti, Applied Physics Letters (2023). The article can be accessed at https://doi.org/10.1063/5.0132798 .

This paper is part of the Time Modulated Metamaterials Collection, learn more here .