Is a surface plasmon an EM wave?

[Aziza]

From my understanding, surface plasmons (SPs) are oscillations of electrons caused by incident EM waves. A photon from the EM wave can couple with a plasmon to create a surface plasmon polariton (SPP). Some books/websites use the words SPP and SP interchangeably, so I am confused as to whether they really are the same thing or not. I am not sure what this 'coupling' really is. My friend assures me that SPPs are actually EM waves since they are coupled with photons, but I think they are still just mechanical waves since they cannot propagate through vacuum and are longitudinal, at least to my understanding. Does anyone know what exactly is coupling, and if this really does cause something to act as an EM wave, if the coupling is done with a photon??

 

[DrDu]

Any EM wave propagating in a medium will couple to electronic excitations. These effects are usually subsumed in the refractive index n of the medium. The relative proportion of energy stored in the em field and in the medium varies with frequency and wavenumber. Usually there is only a small range of frequencies, where both mix considerably, the avoided crossing region. In books this is mostly discussed for phonon polaritons, but the principle is the same for surface plasmon polaritons.
See:
http://en.wikipedia.org/wiki/Polariton

 

[Aziza]

Any EM wave propagating in a medium will couple to electronic excitations. These effects are usually subsumed in the refractive index n of the medium. The relative proportion of energy stored in the em field and in the medium varies with frequency and wavenumber. Usually there is only a small range of frequencies, where both mix considerably, the avoided crossing region. In books this is mostly discussed for phonon polaritons, but the principle is the same for surface plasmon polaritons.
See:
http://en.wikipedia.org/wiki/Polariton

but then we cannot call them pure EM waves, correct? because they do need matter through which they can propagate

 

[DrDu]

You are right, they are certainly not pure electromagnetic waves. However, nobody claims this.

 

[sardar]

This is a great question and one that is often debated in the scientific community. To answer your main question, yes, surface plasmons can be considered as electromagnetic (EM) waves. This is because they are a combination of both an oscillation of electrons and a photon, which is the fundamental unit of EM waves. When a photon interacts with a metal surface, it can excite the electrons in the surface, causing them to oscillate and create a surface plasmon. This coupling of the photon and electron creates a hybrid state known as a surface plasmon polariton (SPP).

The confusion may arise because some sources use the terms SPP and SP interchangeably. While they are closely related, they are not exactly the same thing. SPs refer to the oscillations of electrons on a metal surface, while SPPs are the result of the coupling between a photon and SPs. However, both can be considered as EM waves due to their combination of photon and electron components.

Regarding your question about the nature of coupling, it is essentially the transfer of energy between the photon and electron. When the photon interacts with the metal surface, it creates an electric field that can excite the electrons, causing them to oscillate and create a surface plasmon. This transfer of energy between the photon and electron is what is known as coupling.

In terms of whether SPPs can be considered as "true" EM waves, it's important to note that the definition of an EM wave is a propagating disturbance of electric and magnetic fields. While SPPs cannot propagate through vacuum, they can propagate along the metal surface, which can be considered as a waveguide. Additionally, SPPs exhibit both electric and magnetic field components, making them a type of EM wave.

In summary, surface plasmons can be considered as EM waves due to their combination of photon and electron components. While there may be some confusion about the terminology and nature of coupling, it is clear that SPPs exhibit characteristics of EM waves and play an important role in the field of plasmonics. I hope this helps clarify any confusion you may have about the topic.