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?
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.
