Zig-zag light
Light waves emanating from a lamp travel uniformly in all directions. When such waves strike an object, such as a mirror, light reflects. When traveling from air to water, light refracts. These familiar facts form the foundation for how light travels and interacts with everyday matter, and are deeply rooted in the theory of electrodynamics that underpins all modern-day telecommunications.
Now a team led by physicist Professor Baile Zhang at the Nanyang Technological University (NTU) Singapore has demonstrated behavior that extends beyond this conventional intuition. Writing in Science, they report having engineered a medium in which light travels unidirectionally (not reflecting when encountering an object) along complex three-dimensional zig-zag networks. This behavior simulates an unconventional form of electrodynamics called axion electrodynamics, which arises when light interacts with an axion – an as-yet-undetected candidate particle for dark matter.
Using crystals to steer zig-zag light
When light propagates through the vacuum of space, its speed is a constant (about three hundred thousand kilometers per second) irrespective of its frequency (i.e. color). However, when it travels through synthetic structures called photonic crystals, its behavior can be engineered. For example, the speed of light through a photonic crystal can be dramatically slowed down; the propagation velocity at one frequency can even be made distinct from another by customising the structure of the photonic crystal.
Even more extreme are synthetic topological photonic crystals, where light propagation is completely blocked in the centre of the synthetic structure but travels efficiently along the edges. Such topological photonic crystals are most readily constructed in two-dimensions, e.g., a square. Light flows along the edge of the square in a unidirectional fashion, with the direction of flow (clockwise or anticlockwise) called “chirality” (e.g., left handed or right handed).
To achieve a unidirectional flow in three dimensions, the team constructed a stack of two-dimensional topological photonic crystals joined together. By stacking layers with alternating chirality, they engineered an unusual three-dimensional material, in which light propagation is blocked in the centre of the structure and only flows unidirectionally along “hinges” on the outer surface of the photonic crystal (see Figure 1). The light waves traveling on these hinges do not get reflected, even when encountering an obstacle.
“Light from a source does not penetrate our 3D structure,” says Professor Baile Zhang “Instead, it is transmitted along the hinges creating an unusual pattern in three dimensions.” This 3D structure is termed a Photonic Axion Insulator.
“In a dark room, light from a lamp shines uniformly in all directions” Dr Guigeng Liu a research fellow at NTU and the first author of the study adds, “Light from a source at the edge of our photonic axion insulator takes an unusual unidirectional path, darting around only on the edges. It’s as if the light acts like the flash running in a zig-zag pattern”.
Figure 1: (Left) image of the artificial photonic axionic insulator constructed by the NTU team (Right) electromagnetic waves are only transmitted along hinges at the edge of the structure forming an unusual zig-zag pattern.
Synthetic Axions
The zigzag trajectory of light in the team’s photonic crystals can be understood to arise from an unusual interaction between electric and magnetic fields. In the vacuum of space, static electric and magnetic fields do no interfere with each other. However, the team’s photonic crystal simulates a special kind of coupling between electric and magnetic fields that mimics a hypothesised interaction between photons and a theoretical particle called an axion.
First proposed in the 1970s, axions are now thought to be a leading candidate to form dark matter (invisible matter thought to comprise a significant portion of the all the matter in the universe). Despite this, real axions have yet to be detected.
Professor Yidong Chong, an author on the study, says: “Our team used our expertise in photonics to design an artificial medium that makes light behave as though axions are present. These results highlight the importance of interdisciplinary approaches to tackle fundamental questions in physics.”
At present, the team’s structure simulates an axion coupling that is constant in space and time. In contrast, real axions are thought to be dynamical, with an axion coupling that oscillates in space and time. Even so, the effect of such constant synthetic axion coupling is dramatic – creating unusual light flows such as the zigzag transmission patterns observed in the team’s experiment.
Looking forward, the team is optimistic that their work could help in the detection of real axions in future experiments.
“The findings from our new crystal structures give us more confidence that we could one day use such crystals to detect real axions,” said Prof Zhang. “Since axions are promising candidates for dark matter, our research might lay the groundwork for unraveling some of the universe’s greatest mysteries.”
Read more about it on the NTU News Page: https://www.ntu.edu.sg/news/detail/discovery-poised-to-help-detect-dark-matter-and-pave-the-way-to-unravel-the-universe-s-secrets
Read more in: “Photonic axion insulator”, published in Science, 10 Jan 2025. DOI: 10.1126/science.adr5234.