Why can't optical phonons travel far?

joelio36

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

Simon Bridge

What mechanisms have you considered?
In what sense are phonons "low loss"? What does that mean exactly?

tthtlc

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