Physicists Build First Photonic Crystal From “Einstein” Tile and Discover It Bends Light in Unique Ways

Physicists at the University of Tokyo have developed a photonic crystal using a non-repeating 13-sided shape that bends light based on its spin direction, according to a study published July 29 in the journal Nature Communications. The structure, based on a geometry known as the “einstein” tile, allows for a type of circular polarization dependence that ordinary crystals cannot produce.

Yuto Moritake, an experimental physicist at the University of Tokyo, created the device by arranging nanoscale holes into a pattern that never repeats. While most photonic crystals rely on regular grids to steer light for optical sensors and lasers, Moritake used the “Smith hat” tile to test how a non-repeating arrangement would affect light scattering.

The “einstein” tile—a pun on the German phrase “ein stein” meaning “one stone”—solves a long-standing mathematical puzzle regarding whether a single shape could cover a flat surface indefinitely without creating a repeating pattern. This geometry was discovered in 2023 by David Smith and his collaborators, following earlier work in the 1970s by Roger Penrose, who proved that two different shapes could achieve a similar non-repeating effect.

Manufacturing the Nanoscale Photonic Crystal

To translate the mathematical shape into a physical material, Moritake and his team used electron beam lithography and etching. These precision techniques allowed them to punch hundreds of thousands of holes into a thin film of silicon nitride, a ceramic material frequently used in the production of computer chips.

Each hole in the structure has a radius of 100 nanometers, making them roughly 500 times thinner than a human hair. The resulting chip measures approximately half a millimeter across, which is roughly the width of a pencil tip.

When a laser was shined at the chip, the light diffracted into a pinwheel-shaped pattern of bright spots known as Bragg peaks. Moritake first documented these patterns using the long-exposure mode on an iPhone before moving to specialized cameras for precise measurement. The consistency of these peaks confirmed the material is a quasicrystal, meaning its holes follow an orderly but non-repeating pattern, unlike the grids found in diamonds or table salt.

Chirality and Light Polarization

The most significant finding involves the tile’s lack of mirror symmetry, a property called chirality. Because the “Smith hat” shape does not look the same as its own reflection, the light scattering pattern it produces is also asymmetrical.

This asymmetry affects how the crystal interacts with circularly polarized light, which spins either clockwise or counterclockwise as it travels. Moritake found that the crystal responded differently depending on the direction of the light’s spin.

This structure can have some kind of circular polarization dependence.

According to the study, ordinary quasicrystals possess mirror symmetry and are therefore unable to produce this specific effect. Moritake noted that this outcome was not something he initially expected to find during the project.

Applications for Optical Computing

Moritake intends to move beyond studying light that bounces off the surface of the crystal to controlling light that travels inside a photonic chip.

This development could be applied to optical communications and optical computing. These technologies aim to use light instead of electricity to process and transmit information, according to Moritake.