The photonic axion insulator in 3D

Imagine a world where light, instead of merely bouncing off surfaces or passing through transparent materials, is controlled in a completely new and unexpected way. Scientists have recently taken a major step ¹ in this direction by discovering what is called a “photonic axion insulator.” While this term might sound complicated, the underlying idea is both fascinating and potentially revolutionary. This new material could transform how we manipulate light and lead to advances in communication, imaging, and even fundamental physics.

To understand this breakthrough, let’s start with something simple: materials and how they conduct electricity. Some materials, like metals, allow electricity to flow freely. Others, like rubber or glass, prevent this movement and are called insulators. But in recent years, scientists have discovered unusual types of insulators—materials that do not conduct electricity inside but have special conductive properties on their surfaces. These materials, known as topological insulators, have sparked great interest in physics and technology.

Now, scientists have extended this idea beyond electricity and into the world of light, discovering a material that affects light in a way similar to how topological insulators affect electrons. This is the essence of the photonic axion insulator.

The role of axions

The term “axion” originates from particle physics, where it was proposed to solve the strong CP problem in quantum chromodynamics (QCD). These hypothetical particles are also leading candidates for dark matter, a mysterious substance that makes up approximately 85% of the universe’s total mass yet does not emit, absorb, or reflect light. If axions exist, they would interact extremely weakly with electromagnetic fields, making them difficult to detect directly.

Photonic axion insulators mimic key properties of axions in condensed matter systems, allowing researchers to explore axion-like phenomena in a controlled laboratory setting. In these materials, the electromagnetic response is governed by an axion-like field, described mathematically by a topological term in Maxwell’s equations. This results in unusual electromagnetic effects, including topologically protected edge states and non-reciprocal propagation of light.

Implementing an axion insulator in three dimensions

A crucial breakthrough in this research is the successful implementation of a photonic axion insulator in three dimensions. Unlike previous studies that primarily focused on two-dimensional analogues, this new achievement realizes a fully three-dimensional system where axion-like behaviour can be observed in a bulk medium. This allows for a more comprehensive study of axion physics in a photonic setting and opens new avenues for engineering topological photonic states in real-world applications.

In this 3D photonic axion insulator, the bulk remains insulating, while the surface supports unidirectional light propagation. The key distinction of the three-dimensional implementation is that it provides a direct photonic analogue to theoretical axion electrodynamics, where the electromagnetic response is described by a quantized axion field. This groundbreaking achievement strengthens the connection between condensed matter physics and high-energy axion physics, making it possible to experimentally probe topological effects that were previously only theorized.

What makes a photonic axion insulator special?

In most materials, light behaves predictably: it reflects off surfaces, bends when entering a new medium (like when you see a straw appear bent in water), or gets absorbed. However, in a photonic axion insulator, light interacts with the material in a much stranger way.

1. Surface control: The bulk of the material remains insulating, while the surface supports unidirectional light propagation, similar to the edge states in electronic topological insulators. This occurs due to the topological nature of the photonic modes, which are protected against scattering from disorder.
2. Electromagnetic axion coupling: The defining characteristic of a photonic axion insulator is its axion electrodynamics, described by the modified Maxwell equations including a topological θ-term. This introduces an effective magnetic response even in non-magnetic materials, leading to novel optical effects such as magneto-optical-like rotation without requiring an external magnetic field.
3. Non-reciprocal light propagation: A particularly intriguing property is that light can be made to travel in only one direction along the surface, meaning that it cannot scatter backward. This phenomenon, reminiscent of the quantum Hall effect in electronic systems, arises due to the broken time-reversal symmetry in the system’s effective description.

Very good, but, where is my money?

This new class of material could have significant applications. Here are a few potential areas where photonic axion insulators might make an impact:

• Telecommunications: Optical fibres carry information using light, but current technology sometimes loses signal due to unwanted reflections. A material that forces light to travel in only one direction could make communications faster and more reliable.
• Topological photonics: The ability to engineer robust, disorder-immune photonic states could lead to new optical devices that function with unprecedented stability and efficiency, paving the way for next-generation optical circuits.
• Quantum computing: Future quantum computers may leverage photonic qubits, and the unidirectional propagation of light in photonic axion insulators could enhance quantum coherence and reduce losses in quantum networks.

A step toward new physics

Beyond practical applications, this discovery also deepens our understanding of fundamental physics. By creating a material that behaves like a theoretical axion, scientists can study its properties in a controlled environment. This could provide insights into how axions—if they exist—might behave in the real world.

Moreover, this research connects different fields of physics, from condensed matter (the study of materials) to electromagnetism and even high-energy physics, where scientists search for new particles. It represents a bridge between theory and experiment, helping us explore ideas that were once purely mathematical.

Additionally, if axions do turn out to be a component of dark matter, studying materials like the photonic axion insulator could help physicists develop new experiments to detect these elusive particles. The ability to replicate axion-like behaviour in a lab setting could bring us closer to answering one of the most profound questions in cosmology: what is dark matter made of?

The future of photonic materials

While we are still in the early stages of understanding and applying photonic axion insulators, this discovery is a major milestone. It suggests that by carefully designing materials at the microscopic level, we can create entirely new ways of controlling light—something that was once thought impossible.

In the coming years, researchers will continue to refine these materials, explore their properties, and find new ways to incorporate them into technology. Much like how the discovery of semiconductors led to the modern computing revolution, the study of photonic materials like this one could lead to technological advances we can barely imagine today.

The discovery of the photonic axion insulator is a testament to human curiosity and ingenuity. It challenges our understanding of light and materials while opening doors to practical innovations. Whether in advanced communication, sensing, or even answering deep questions about the universe, this new material has the potential to change how we interact with the world of light. Furthermore, its connection to axion physics may offer an unexpected window into the nature of dark matter, potentially bridging condensed matter physics and cosmology in a way that was previously thought to be purely theoretical. This is a perfect example of how fundamental scientific research can lead to both exciting discoveries and real-world applications that benefit us all.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.