# Why can't optical phonons travel far?

1. Jan 31, 2013

### 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

2. Jan 31, 2013

### Simon Bridge

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

3. Feb 2, 2013

### 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/conferences/nctp/proc/35/153.pdf (on resonance)

http://ocw.mit.edu/courses/chemistr...y-ii-spring-2008/lecture-notes/23_562ln08.pdf (which correlate the wavelength of the phonon with the size of the molecule/atom)

and

http://www-ee.eng.buffalo.edu/faculty/mitin/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 [Broken]

http://ndl.ee.ucr.edu/Paris-Lecture-05.pdf

Last edited by a moderator: May 6, 2017