Second and third harmonic generation in topological insulator-based van der Waals metamaterials

High-order harmonic generation (HHG) — a technique that converts light into much higher frequencies — is an essential tool for exploring otherwise inaccessible parts of the electromagnetic spectrum. But until now, accessing terahertz (THz) frequencies through HHG has been a challenge, largely limited by the symmetrical nature of most materials.

 

Graphene, for instance, has shown promise in this field, but due to its perfect symmetry, it only produces odd harmonics — frequencies that are odd multiples of the original light signal. Even harmonics, which are crucial for a fuller range of applications, remained out of reach.

 

In a new paper published in Light: Science & Applications, a team of scientists led by prof. Miriam Serena Vitiello have has achieved a major milestone in the field of light-based technologies, using exotic quantum materials to unlock previously inaccessible regions of the electromagnetic spectrum.

 

The team turned to topological insulators (TIs), materials that behave like insulators in their interior but conduct electricity on their surfaces. These materials possess unique quantum properties, thanks to strong spin–orbit interactions and time-reversal symmetry. While TIs were theoretically predicted to support more complex forms of harmonic generation, actual experimental evidence had been lacking — until now.

 

Using specially engineered nanostructures known as split ring resonators, researchers embedded thin layers of Bi₂Se₃ and van der Waals heterostructures containing (InₓBi₁₋ₓ)₂Se₃. These resonators dramatically amplify the incoming light, making it possible to observe HHG at both even and odd THz frequencies — a rare feat. The up-conversion was achieved in the range between 6.4 THz (even) and 9.7 THz (odd), revealing the hidden contributions of both the symmetrical bulk and asymmetrical surface of the topological materials.

 

This experimental success confirms long-standing theories and provides a powerful new platform for developing compact terahertz sources, sensors, and ultrafast optoelectronic devices. It also offers new ways to probe the interplay between symmetry, quantum states, and light–matter interactions at the nanoscale.

 

As demand grows for faster, smaller, and more versatile technology, this breakthrough marks a key step toward harnessing the full potential of quantum materials in real-world applications.

 

This breakthrough paves the way for compact, optically pumped tunable terahertz light sources, potentially revolutionizing technologies in high-speed wireless communication, medical imaging, and quantum computing.

---

---