08/19/2025
Observing a special form of light propagation in an atom-thin metal for the first time
Today, it is studied with electron microscopes; tomorrow, it could itself become a microscope: a metal only one atom thick can bend, guide, and confine light in its plane and on an extremely small scale - even at visible wavelengths.
A team from Kiel University has now demonstrated this effect for the first time in borophene. The two-dimensional metal, first synthesized less than ten years ago, could serve as a platform for novel light-matter interactions. The results have been published in Advanced Functional Materials.
How these special light waves arise
The basis of this effect lies in a special interaction between light and electrons. Light waves couple in the material with collective oscillations of electrons - plasmons - to form a "hybrid wave" of light and matter.
In borophene, this takes the form of in-plane hyperbolic polaritons - waves that travel at different speeds and intensities depending on the propagation. It is a bit like cars that can drive freely north-south but must slow to 30 km/h east-west.
Borophene's atomic structure forces electrons into preferred paths. This direction-dependent behaviour, called anisotropy, allows for extreme light confinement and control well below the diffraction limit - far finer than normally possible. This is a decisive advantage when optical systems are becoming ever smaller and more precise. To observe the effect, the Kiel team excited borophene with electron beams inside an electron microscope and analysed the resulting light with nanometre precision.
Borophene joins a rare class of materials
Hyperbolic polaritons have been seen before, but mostly in materials that work in the infrared or terahertz ranges. Borophene, by contrast, operates in the visible range - opening new possibilities for nanophotonics. "Borophene is metallic, atomically thin, and naturally anisotropic," explains Prof. Nahid Talebi of Kiel University, who led the study in collaboration with guest scientist Prof. Yaser Abdi. "These properties make it an entirely new platform for guiding visible light at the nanoscale. We can control light propagation in ways that were simply not possible with other materials."
This research lays the foundation for future technologies: ultra-compact photonic devices, highly sensitive optical sensors, or microscopy techniques that go beyond the classical resolution limit. And because borophene works in the visible spectrum, it is compatible with the wavelengths used in many quantum communication systems.
Source: University of Kiel