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Light, electrons and energy in the mix

Recent discoveries show previously unknown ways to change the very nature of materials by coupling the light-induced movement of electrons to energy transitions inside them. Something very strange is formed - neither pure energy nor physical motion. “It’s hard to grasp the full depth of what this means,” says professor Alexandre Dmitriev.

There are plenty of electrons in metals. Since the electrons move around when the electric field is applied, the metals are conductive. It’s also possible to use ordinary light to make these electrons move and oscillate collectively, since part of light is an electric field that just oscillates very fast.

When electrons are made to move by light in this manner, they form so-called plasmons, which then function as electronic oscillations. There is a vast area of physics and materials science that deals with plasmons. They can be very helpful in boosting various processes and devices where light is involved, like photocatalytic reactions, lasers and other light emitters, optical sensors and detectors to name a few. What makes plasmons so special is that they also spill the light field outside the metals, and as a result they enhance how light interacts with materials in catalysis, for example.

Shuffling electrons between energy levels
There is another process that electrons in metals (and other materials like semiconductors) are involved in – called electronic transitions. When light is absorbed in metals in this way, electrons will shuffle from one energy level or a band of levels to another. It follows that these transitions in metals are often ‘interband’, where electrons are hopping between the groups of energy levels, or bands.

Now, imagine if the motion of electrons, induced by light (plasmons) and the connected electromagnetic field outside of metal, can couple to these energy (interband) transitions inside the metal – something very strange is formed that is neither a pure energy nor a physical motion or a field. These curious things are called polaritons.

Surprising discovery
“We found plasmon-interband polaritons in nickel, which is a metal, a magnet and a catalyst at the same time, several years ago,” says professor Alexandre Dmitiriev. “However, this discovery was so surprising and hard to imagine, that we for a long time wanted to look closer at the phenomenon, with very high resolution in both energy and space. And now we’ve done it!”

In close collaboration with researchers at DTU (group of Søren Raza), the exclusive details of these exotic plasmon-interband quasi-particles (polaritons) have been successfully mapped with the help of advanced high-resolution transmission electron microscope spectroscopy, so called ‘electron energy loss spectroscopy’ or EELS.

We looked at various sizes of Ni nanoparticles and Ni films and could confirm that these plasmon-interband polaritons are everywhere, as soon as the light hits the surface of Ni. This is important to know since many such nanoparticles are used in catalysis and as potentially magnetic memory storage units (like with Ni alloys). Our finding shows that the polaritons can have new, unexpected and exciting roles, which we still need to disentangle, says Alexandre Dmitriev.

Since electronic transition is something given by nature to the material, by using these polaritons we’re changing the fundamental nature of a material itself by moving these transitions in energy and maybe even ‘spilling’ them out of the material. This opens up astonishing new possibilities - like how can there be electronic transitions without electrons and material? It’s hard to grasp the full depth of what this means, but we continue searching for answers!”

The work has recently been published as a front cover-story of research journal Advanced Optical Materials.

Original works
Z. Pirzadeh, T. Pakizeh, V. Miljkovic, C. Langhammer, and A. Dmitriev, Plasmon: Interband Coupling in Nickel Nanoantennas, ACS Photonics 1, 158 (2014)

A. Assadillayev et al: Nanoscale engineering of optical strong coupling inside metals, Advanced Optical Materials 11, 2201971 (2022)”

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