The spectrum of a single metal nanoparticle shows a plasmonic peak with a wavelength which depend on the material, shape and size of the nanoparticle.
But the spectrum of a pair of nanoparticles shows a red shift in plasmonic peak comparing to those of a single particle, if the polarization of the incident light is along the connecting axis of two particles. On the other side, if the polarization is perpendicular to the connecting axis, a slight blue shift is observed.
Does anyone have a physical explanation for this phenomena?
Certainly - coupling! Light polarized perpendicular to the connecting axis has a node on this axis. Ob the other hand, coupling between the same modes on both particles for light polarized parallel to the axis leads to a red shift of the coupled mode.
Thanks for your answer, but what do you mean by node?
I am looking for the reason why this coupling happens in one direction and it does not happen in the other. and I know it is related to the way in which the electrons oscillate due to electromagnetic field.
Do you have a good reference for that coupling between the particles?
Just imagine that your two particles is an oddly shaped one particle. A nanorod. A nanorod shows the same behavior.
A node is a point in space where the electric field is essentially zero.
For a reference in general, see e.g.
Article Finite-Difference Time-Domain (FDTD) design of gold nanopart...
I think the most simple way to answer your question is to look at the system in a more simpler way. We can represent the two nanoparticles as two parallel coupled electrical dipoles (because the oscillation of the electrons is basically creating an oscillating dipole) and thus this blue shift can be deduced from the classical problem of the interaction of two electrical dipoles at a certain distance between them.
Just in order to get an idea of the fact that the wavelength is blue shifted when the dipoles are parallel, we can look at the energy of the electrical spins and the Pauli principle... the spins are anti-parallel in order for the energy to be minimal, thus a parallel coupling would have more energy than the independent components.
I hope that this answers your question in a more detailed way than my article above referenced by Thomas.
Almost correct. 2 parallel dipoles attract when they are head to tail and repel when they are side by side. But the analogy to spins is wrong. That is governed by exchange interaction and can be any sign. Hund's rule states that spins like to be parallel, they are parallel in ferromagnetics and paramagnetics like oxygen molecule, but they are antiparallel when forming a bond. Spins do not belong to this space.
Thank you for the correction Igor. Indeed my analogy was a bit off and not very appropriate for the discussion.
Fortunately i did manage to find some literature related more to this topic.
http://www.sciencedirect.com/science/article/pii/S0030401803013579
Thank you all for the answers.
I found the text below from the paper that Christian recommended:
"First, if the driving field is parallel to the long particle pair axis this effect results in a weakening of the repulsive forces for the surface charges. The positive charge of the left particle in Fig. 4(b) faces the corresponding negative charge of the right particle. Due to the attractive forces between these two charge distributions of the different particles the repulsive forces within each particle are weakened, leading to a correspondingly lower resonance frequency."
You can see Fig.4(b) in the attached file.
However, the explanation did not convince me, because I think the attractive forces between two particles may increase the amount of repulsive forces within each particle.
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