Photonic Crystals Imitate Gravitational Effects on Light

A group of researchers has replicated the way that light would behave if it were subject to gravity, supporting a recent scientific theory about pseudogravity.

According to the theory, pseudogravity, a phenomenon replicating the effects of gravity, can be achieved by deforming crystals in the lower frequency region.

Photonic crystals have been shown to bend light as though it were under the influence of gravity. Courtesy of Tohoku University.

Researchers from Tohoku University, in collaboration with other institutions including Osaka University, set out to determine whether lattice distortion in photonic crystals could produce the effects of pseudogravity. They experimentally demonstrated pseudogravity in the terahertz range.

The team’s findings could be significant for the fields of optics and materials science, and for the development of 6G communications.

Photonic crystals are constructed by arranging two or more optical materials periodically. The periodic arrangement of these materials forms a structure that affects the propagation of light. The materials have varying abilities to interact with and slow down light in a regular, repeating pattern.

Photonic crystals have the unique ability to manipulate and control the behavior of light within crystals. Moreover, photonic crystals have demonstrated pseudogravity effects caused by adiabatic changes.

A conceptual image of the distorted photonic crystal (DPC) and photonic crystal. Courtesy of K. Kitamura et al.

To achieve pseudogravity, the researchers modified photonic crystals by introducing lattice distortion. They disrupted the grid-like pattern of the photonic crystals through the gradual deformation of the regular spacing of elements.

This disruption affected the photonic band structure of the crystals, resulting in a curved beam trajectory in-medium — much like a light ray passing by a massive celestial body. The spatially distorted photonic crystals (DPCs) were able to deflect lightwaves, due to the pseudogravity caused by lattice distortion.

The researchers used a silicon DPC with a primal lattice constant of 200 μm and terrahertz waves for their research. In experiments, they verified the deflection of electromagnetic waves in the terrahertz range by pseudogravity in DPCs.

“We set out to explore whether lattice distortion in photonic crystals can produce pseudogravity effects,” said professor Kyoko Kitamura of Tohoku University. “Much like gravity bends the trajectory of objects, we came up with a means to bend light within certain materials.”

The experimental results, with the transmission difference between ports B and C clearly showing the beam bending in a distorted photonic crystal. Courtesy of K. Kitamura et al.

Einstein’s theory of relativity established long ago that the trajectory of electromagnetic waves, including light and terrahertz electromagnetic waves, can be deflected by gravitational fields. Using a photonic crystal, Kitamura and her colleagues demonstrated that electromagnetic waves can follow a gravitational field.

Pseudogravity caused by lattice distortion could lead to new approaches to achieving control of the on-chip trajectory of light propagation in photonic crystals.

“Such in-plane beam steering within the terahertz range could be harnessed in 6G communication,” said Masayuki Fujita, a professor at Osaka University. “Academically, the findings show that photonic crystals could harness gravitational effects, opening new pathways within the field of graviton physics.”

The research was published in Physical Review A (www.doi.org/10.1103/PhysRevA.108.033522).