# Why can't optical phonons travel far?

by joelio36
Tags: optical, phonons, travel
 P: 21 I can't figure this out/find the answer. Why are acoustic phonons very low loss (i.e. earthquake P and S waves), but optical phonons die out rapidly? Thanks, Joel
 Homework Sci Advisor HW Helper Thanks ∞ PF Gold P: 10,942 What mechanisms have you considered? In what sense are phonons "low loss"? What does that mean exactly?
 P: 4 The energy loss is low is because of the low frequency/longer wavelength phonon - which does not coincide with the size of the molecules/atoms. The higher frequencies/shorter wavelength phonon is likely to coincide with the size of the molecules/atoms, and thus losing its energy through resonating the energy through all the molecules/atoms. This is the cause of energy attenuation. Another factor is energy dissipation: By having the size of molecules/atoms coinciding close to that of the phonon, the phonon streams is more likely to be reflected/refracted and thus dissipated. Both of these factors can help to explain why longer wavelength can travel far, low signal loss. Another possible explanation is the phonon density: higher frequencies phonon matches with that of smaller atoms/molecular structures, which occurred at a higher density/number, and thus is able to spread the energy faster. Lower frequencies/longer wavelength need larger molecular structures (or multiple atom forming a macro-structures), which occur at a much lower densities, and thus is less able to spread the energy faster. Another possible dissipative phenomena is electron-phonon coupling, which is more likely to happen for higher frequencies phonons. These are my layman's perspective of what's happening, but from a specialist point of view (beyond me), u can refer to: http://www.iop.vast.ac.vn/theor/conf...roc/35/153.pdf (on resonance) http://ocw.mit.edu/courses/chemistry...23_562ln08.pdf (which correlate the wavelength of the phonon with the size of the molecule/atom) and http://www-ee.eng.buffalo.edu/facult...Papers/115.pdf (on electron-phonon coupling) http://www.iue.tuwien.ac.at/phd/smirnov/node53.html http://www.uni-tuebingen.de/meso/ssscript/phononen.pdf http://ndl.ee.ucr.edu/Paris-Lecture-05.pdf

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