Surface plasmon resonance (SPR) is an important phenomena because of which Au and Ag NP exhibit interesting optical properties that are sensitive to size, shape, and dielectric constant.
Does SPR based absorption always lead to excitation of plasmons? Or only when laser light is used as a source will it lead to excitation of coherent motion of free electrons?
When will the dephasing of coherent motion of electrons occur leading to formation of hot electrons?
Most of the studies related to Au NP are based on its absorption properties in visible region? Recently I have come to know that we can excite the plamons and dephase their coherent motion? When/How/At what conditions does the latter happen?
I am not going to try and fully address all your questions as I am not sure I understand everyone of them.
q1) does SPR always lead to excitation of plasmons? in general yes, hence why it is the surface plasmon resonance... this means it is exciting the plasmons. now if there is some other mechanism present this excitation can be suppressed or could to another excitation. In that case it may not be observed.
q1b) excitation only by laser light - no, the source depends on things like how it is excited, but plasmons are often excited by white light sources like a light bulb or can also be excited by elections passing near or through the sample. The latter example is how the modern idea of a plasmon was first proposed by Ritchie. He was examining electron energy loss spectra and looking to explain observed peaks.
q2) something about hot electrons - not sure I understand what you mean here. that said keep in mind that the energy that gets transfered into the plasmon state has to go somewhere. so it can transfer partly into a new photon, and/or partly or wholly into thermal (read internal) energy = phonons.
q3) the reason for this is that when you look at the dispersion relation, the plasmon resonance is closet to the light line in the visible
I will stop there as I need to get back to other writing projects. I hope this helps for the time being.
After 10 femtoseconds the surface plasmons decays in hot electron-hole pairs
As answered above, the excitation of the plasmons depends only on the plasmon resonance wavelength. So if the white light contains that particular wavelength then it'll definitely excite the plasmons. Using laser of just that particular wavelength ensures there's higher (and more efficient) transfer of energy from the photons to the plasmons.
When the plasmons are excited by this resonant light, it creates a non-equilibrium electron distribution that decays through electron-electron scattering. This electron scattering results in the hot electron gas, which consequently transfers the energy into the surrounding medium as phonons.
Dephasing of the plasmon oscillation occurs mostly due to the field enhancement near the nanoparticle. There could be other factors affecting this (two-photon processes, dielectric property of the media, etc.) but I'm not sure about that. Here's a recent paper that talks about reconstructing the plasmon dynamics using a novel technique:
http://pubs.acs.org/doi/full/10.1021/nl101090s
Thank you very much John, Adnen, and Somsubhra for your valuable answers. I am able to comprehend somethings.
I still have some doubts?
@ Somsubhra: What do you mean by non equilibrium distribution. Can you explain? why plasmon excitation leads to non-equilibrium distribution? In the current context what does the term dephasing mean? Is it same as non equilibrium distribution? Does everytime a plasmon gets excited, it results in dephasing or non-equilibrium distribution?
I made AuNPs. They are exhibiting red color due to SPR form the normal day light. Does it mean that even in this case, hot electrons are being generated due to SPR? or only If I use laser light of particular frequency corresponding to SPR, will these phenomena like dephasing, and generation of hot electrons occur?
I hope you are understanding my doubgt though they may be trivial? Because I am not from physics background I was not able to understand these terms.
Thank you again for your time and answers. Awaiting your reply.
This may help u.
http://www.lle.rochester.edu/media/publications/lle_review/documents/v87/87_04_HotElectron.pdf
I doubt that in linear regime the SPR in the metals can generate hot electrons in usual sense (when the temperature of this entity is different from the lattice temperature). But when the SPR heats NP the electron will also be heated. The e-heating effect would be better in doped semiconductor NP, I wonder
The fact that your gold NPs appear red is because they absorb in the blue region. This absorption phenomenon is a consequence of SPR, which frequency depends strongly on the NPs size and shape. As explained in previous answers, electrons are excited even using a polychromatic non-coherent beam, but using a laser tuned at the particular SPR frequency of your NPs will greatly increase the field enhancement at the surface of the NPs, it is only a matter of magnitude regarding the phenomenon...
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