What is Plasmon, the Collective Oscillation of Electrons? - The Reason for the Red Color in Stained Glass -

The name "plasmon" might sound like something from Pokémon or Digimon, but it is actually an important scientific term.

A plasmon (English: plasmon) is a quantum of collective oscillation (plasma oscillation) of free electrons in a metal. by Wikipedia

However, I think that is a bit difficult to understand, so I would like to explain it a little more simply.

What is a plasmon?

In metals, there are electrons that can move freely, called free electrons. It is because of these freely moving electrons that electricity flows easily.
When an electromagnetic field, such as light, hits this, an oscillation of the electric field occurs. The electric field affects the free electrons and moves them in the opposite direction.

In a metal, free electrons oscillate as shown in the figure along with the oscillation of electromagnetic waves. The phenomenon where electrons (the yellow particles in the figure) oscillate up and down together due to the influence of these electromagnetic waves is called a plasmon.

What is even more interesting is that although a plasmon is a collective oscillation of electrons, it can be treated as if it were a single particle.

A phenomenon that behaves like a particle even though it is not a real particle is called a quasiparticle. Famous examples include holes, which represent a lack of electrons, and phonons, which represent lattice vibrations, both of which are types of quasiparticles.

Examples of plasmons

Perhaps it feels like plasmons have little to do with our daily lives.
However, many people have probably seen the beautiful colors of stained glass. Even if you haven't seen the real thing, many people must have seen photos of it.

The red color used in this stained glass is actually due to gold.
Although it is called gold, it is slightly different from the golden-shining gold; it is extremely small gold particles (gold colloid). However, the material is exactly the same gold (Au).

What is a colloid? ▼

While it is a metallic yellow in large chunks like gold bullion (gold leaf), when it becomes particles so small that they cannot even be seen with an optical microscope, the effect of plasmons appears.

As a result, tiny gold particles absorb green light, so they appear red. Also, because the effect of plasmons changes depending on the size of the particles, it changes from red to yellow when they become smaller, and from red to blue when they become slightly larger.

What can it be used for?

Safe coloring materials that do not fade
As for what these plasmons can be used for, they are expected to be used as materials that produce color based on principles different from dyes and pigments. Generally, dyes can fade (photobleaching) and pigments can be toxic. However, by using harmless and non-fading plasmon materials, these two issues can be solved at once.

Until now, it was difficult to accurately fabricate fine structures, but recent progress in microfabrication technology has been remarkable, and coloring materials that selectively produce various colors are being created.

Detection of trace molecules
Also, by using surface plasmon resonance that occurs between a pair of metal particles, the light localized between those particles can be enhanced by orders of magnitude.
When molecules enter between these particles, the very weak light emitted from certain molecules can be enhanced and detected. By obtaining signals (scattered light) that could not be easily detected before, we can learn information about the molecules (Surface-Enhanced Raman Scattering, SERS).

It is said that using plasmon resonance enables highly sensitive detection of viruses and proteins, and in principle, it should be possible to realize the detection of current epidemic diseases as well.

Localization of Light
Many people have likely focused light with a magnifying glass when they were children, but theoretically, there is a limit to the size to which this can be focused.
Generally, visible light cannot be focused down to a few nanometers, which is much smaller than its wavelength. Here, by using light localized on metal nanoparticles, it is possible to collect light in a region of a few nanometers.

By using this technology, it might be possible to create an optical microscope that can see much smaller things in detail using light.

Conclusion

Regarding plasmons, the subject is extremely profound, and I myself only have knowledge of a small part of it.
This field related to plasmons is called plasmonics and has the potential to advance optical technology. It is being attempted to be used in combination with photonics, which does not seem to be very popular recently.

Furthermore, its applications are expected not only in engineering but also in fields such as biotechnology and medicine, so I am looking forward to future research.