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I'm trying to better understand what exactly happens when a laser beam hits a surface (apart from reflection). Say I have a thick gold surface (could be coated with gold nanoparticles if that helps) and I point a 50 mW CW red laser onto that surface:

My understanding is that the photons excite plasmon modes and that the plasmon modes can decay by exciting phonon modes -- since all three are somewhat coupled. In the end, all absorbed energy ends up in phonon modes (per this discussion).

My questions: First of all, is this picture correct? And second, what aspects of this can be measured experimentally? E.g. can I measure the extent to which my laser excites plasmon modes and/or phonon modes?

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Sure, that picture can occur. A lot of the time, plasmons are not excited because you need to get the momentum difference from somewhere. But suppose plasmons are excited. If we’re ignoring reflection, then ultimately some of the energy will be re-radiated as light (doesn’t count as reflection), and the rest will end up as heat (phonons).

The excitation of plasmons can be measured by this re-radiation, which can be from a different location than the reflected beam. Also, there will be less reflection than normal (see the Kretschmann configuration, for example). The excitation of phonons can be inferred by heat. Also, by looking at the reflection spectrum, you can measure the linewidth of the plasmons, which can tell you their lifetime.

Your picture, focused on plasmons, is missing some things, however. There are nonlinearities ($\chi^{(3)}$, photoemission, etc.). There’s also momentum! Radiation pressure acts on the metal, although how that actually happens on a microscopic level remains an open question.

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