Why can't optical phonons travel far?

  1. 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. jcsd
  3. Simon Bridge

    Simon Bridge 14,909
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    What mechanisms have you considered?
    In what sense are phonons "low loss"? What does that mean exactly?
     
  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/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

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