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I am a little bit confused here. When a metallic nanoparticle is impinged with light at its resonance, the electric field is greatly enhanced at the surface along the polarisation of the wave(suppose its enhanced East-west). But the heat that is generated is not from these hotspots, in fact in North-South directions. Can somebody explain why does it happen? The text says it is because the electrons can move freely there but I don't understand it completely.

Mapping Heat Origin in Plasmonic Structures. G Baffou, C Girard, and R Quidant. Phys. Rev. Lett. 104, 136805 (2010).

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I am not familiar with nanoparticle science by any means, but I did read through the paper and give an attempt at an answer.

To explain the spatial mismatch between optical hot-spot and heat source hot-spot, they present Figure 3 (comparison between experiment and simulation) and make this comment:

Optical hot spots usually come from tip effect and charge accumulation at the metal interface [Figs. 3(k) and 3(l)], whereas heat arises on the contrary from areas where charges can freely flow [Figs. 3(m) and 3(n)].

In metals, electrons make the dominant contribution to thermal conductivity (phonon contribution becomes more significant for impurity samples or insulators) [Kittel, Introduction to Solid State Physics]. And Equation (2) makes it clear that heat source density is related to electric current density.

Now if you compare Fig 3(k) and 3(m) for longitudinal polarization, you see that current density is focused in the middle, because the strong surface charges at the middle gap, and the left and right extremities inhibit free current. On the other hand, electric field is very strong at the "gap" region due to the (+) and (-) surface charges, so optical hot-spot appears in this "gap" region. They use a two-photon process which is nonlinear, so the signal will be very sensitive to the strength of the electric field.

In summary, I think it all has to do with the accumulation of surface charges, which can give rise to strong local electric field (good nonlinear optical signal), but which inhibits free flow of electrons, the dominant heat carrier.

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  • $\begingroup$ I think you are right, with the small clarification that current is the dominant heat generator, from Joule heating (which is worse along the edges of the particle). It’s not so much about the heat conductivity. And as you mention, the current goes to zero at the ends of the particle. High electric field -> “hot spot”; high current -> heat. These don’t occur at the same place. $\endgroup$
    – Gilbert
    Commented Jul 29, 2018 at 1:36
  • $\begingroup$ @IamAstudent I had more problem in understanding the heat generation in the case of spherical nanoparticle (not a part of this paper). As, all the electrons will be oscillating along the polarisation of the Electric field, in what sense some electrons are free to move? $\endgroup$
    – mollie
    Commented Jul 29, 2018 at 4:26

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