A group of researchers led by Kyoko Kitamura, a professor of electronic science and engineering at Tohoku University in Japan, are manipulating photonic crystals’ light behavior as if under the influence of gravity in space: pseudogravity.
Photonic crystals can be built by periodically arranging two or more different materials with varying abilities to interact with and slow light down in a repeating pattern. These properties allow researchers to manipulate and control light’s behavior—to essentially act as traffic controllers.
“Light within photonic crystals express Bloch states—a type of wave function for a particle within a periodically repeating environment,” explains Kitamura. “And lattice distortion or pseudogravity effects are defined by adiabatic changes—no heat enters or leaves the system—within the lattice constant or shape.”
Gravity and gravitational fields “are the result of a space-time distortion within the vicinity of massive objects, according to Einstein’s theory of general relativity,” Kitamura adds. “Such space-time distortion may also be observable in the adiabatic change of Bloch states.”
Albert Einstein’s 1915 theory of general relativity states that the trajectory of electromagnetic waves—including light and terahertz electromagnetic waves—can be deflected by gravitational fields. What we perceive as the force of gravity arises from the curvature of space and time.
More recently, researchers theoretically predicted it should be possible to create pseudogravity by deforming photonic crystals’ lower normalized frequency region.
So Kitamura and colleagues modified photonic crystals by introducing lattice distortion: a gradual deformation of the regular spacing of elements to disrupt the grid-like pattern of photonic crystals (see video). To do this, they designed a silicon photonic crystal with a distorted lattice to manipulate the photonic band structure of the crystals that produces a curved-beam trajectory in-medium—akin to a light ray passing by a massive celestial body like a black hole (which warps the fabric of space-time and manifests as gravity).
They put it to the test by hitting their silicon photonic crystal (which has a primal lattice constant of 200 µm) with terahertz waves. And it successfully deflected these waves—revealing that pseudogravity caused by lattice distortion acts in accordance of general relativity.
“Much like gravity bends the trajectory of objects, we came up with a means to bend light within certain materials,” Kitamura says. “This phenomenon has implications for the fields of photon and electron systems in physics.”
Kitamura is working with Masayuki Fujita, an associate professor at Osaka University known for his work with photonic crystals within the terahertz electromagnetic frequency. “If we use this frequency region, we can make micrometer-scale structures,” Kitamura says.
In-plane beam steering within the terahertz range may be “harnessed for 6G communications,” says Fujita. “Our findings show photonic crystals could harness gravitational effects and open new pathways within the field of gravitational physics.”
The group is now “trying to demonstrate more varieties of beam steering via designing lattice distortion,” says Kitamura.
K. Nanjyo et al., Phys. Rev. A, 108, 033522 (Sept. 28, 2023); https://doi.org/10.1103/physreva.108.033522.