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Can a laser only excite optical modes? The name appears to have historical significance in terms of the sodium ion, but I presume it must be true when looking at the dispersion relationship (the dispersion for light would only cross the optical modes).

The overall context of the question is: when exciting with a laser, is energy eventually dumped into all phonon modes?

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You are correct: in the bulk of a perfect crystal, only the optical modes can simultaneously satisfy conservation of energy and momentum. That means that direct excitation by IR radiation can excite only the optical modes.

However, the acoustic modes can be excited at surfaces, interfaces, or impurities where momentum conservation is relaxed.

When exciting with a laser, lots of things can happen. Of course, there is reflection, refraction, and transmission, so if absorption is low, some light can pass right through. If light is absorbed, then some of the energy can be re-emitted at lower energies (luminescence). If not, then the energy will end up in phonons ... eventually acoustic phonons.

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  • $\begingroup$ What is the physics of the relaxation at surfaces? I.e. excitation of acoustic modes in 2D like graphene? $\endgroup$
    – Fire
    Commented May 30, 2014 at 19:12
  • $\begingroup$ One has to take care that assumptions, models, and approximations are understood. Here I'll assume that in the bulk of an ideal crystal, electronic excitations and phonons are independent. (Truth is that there is mixing, but the excitations of the mixed states are independent.) But this assumption does not hold at a surface. (an edge, for graphene). When light interacts near the surface, there is a transient re-arranging of electrons, and this re-arrangement "drags" ions along, exciting acoustic modes. There's no need to conserve $k$ vector. The surface provides "infinite momentum". $\endgroup$
    – garyp
    Commented May 30, 2014 at 20:55

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