View Full Version : Q: optical phonon
Andrew Resnick
Apr7-04, 09:05 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resize=yes,status=no,wi dth=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nCould someone please refresh my memory of what these are? I recall it\nhas to do with a particular branch of a solution, the other being\nacoustic phonons, but I can\'t find anything much more than that. Is it\nsimply phonons with energies close to "visible" (using the term loosely)\nlight?\n\nThanks in advance.\n--\nAndrew Resnick, Ph. D.\nNational Center for Microgravity Research\nNASA Glenn Research Center\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Could someone please refresh my memory of what these are? I recall it
has to do with a particular branch of a solution, the other being
acoustic phonons, but I can't find anything much more than that. Is it
simply phonons with energies close to "visible" (using the term loosely)
light?
Thanks in advance.
--
Andrew Resnick, Ph. D.
National Center for Microgravity Research
NASA Glenn Research Center
Danny Ross Lunsford
Apr7-04, 09:22 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Andrew Resnick wrote:\n> Could someone please refresh my memory of what these are? I recall it\n> has to do with a particular branch of a solution, the other being\n> acoustic phonons, but I can\'t find anything much more than that. Is it\n> simply phonons with energies close to "visible" (using the term loosely)\n> light?\n>\n> Thanks in advance.\n\nOptical phonons originate in lattice vibrations induced by an applied\nelectromagnetic field, because of small-scale (on the order of lattice\ncenters) time-varying polarization.\n\n-drl\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Andrew Resnick wrote:
> Could someone please refresh my memory of what these are? I recall it
> has to do with a particular branch of a solution, the other being
> acoustic phonons, but I can't find anything much more than that. Is it
> simply phonons with energies close to "visible" (using the term loosely)
> light?
>
> Thanks in advance.
Optical phonons originate in lattice vibrations induced by an applied
electromagnetic field, because of small-scale (on the order of lattice
centers) time-varying polarization.
-drl
Roland Franzius
Apr7-04, 09:22 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Andrew Resnick wrote:\n> Could someone please refresh my memory of what these are? I recall it\n> has to do with a particular branch of a solution, the other being\n> acoustic phonons, but I can\'t find anything much more than that. Is it\n> simply phonons with energies close to "visible" (using the term loosely)\n> light?\n\nOptical phonons are quantized modes of lattice vibrations when two (or\nmore) charged particles in a primitive cell move in opposite directions\nleaving the center of mass at rest. This mode has highest energy for\nwavelength infinity or k=0 when the two sublattices move rigid against\neach other. The dispersion curve k->E(k) is nearly constant at k=0 where\nit has a maximum.\n\nFor the long wavelengths modes at k~0 nearly equal to an optical\nwavelength (some 100 lattice constants) this optical charge polarising\nphonon modes couple strongly to the electromagnetic field modes.\n\nIn effect at the crossing point of both modes ( E(k)=const phonons\nhorizontal, E(k)= c |k| photons nearly vertical) a second order\nperturbation calculation removes the degeneracy and the level crossing.\nk=0 optical phonons become photons for higher k and k=0 photons become\nphonons for higher k. The crossing of the lines is replaced by two\nnearly touching hyperbolas.\n\n--\n\nRoland Franzius\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Andrew Resnick wrote:
> Could someone please refresh my memory of what these are? I recall it
> has to do with a particular branch of a solution, the other being
> acoustic phonons, but I can't find anything much more than that. Is it
> simply phonons with energies close to "visible" (using the term loosely)
> light?
Optical phonons are quantized modes of lattice vibrations when two (or
more) charged particles in a primitive cell move in opposite directions
leaving the center of mass at rest. This mode has highest energy for
wavelength infinity or k=0 when the two sublattices move rigid against
each other. The dispersion curve k->E(k) is nearly constant at k=0 where
it has a maximum.
For the long wavelengths modes at k~0 nearly equal to an optical
wavelength (some 100 lattice constants) this optical charge polarising
phonon modes couple strongly to the electromagnetic field modes.
In effect at the crossing point of both modes ( E(k)=const phonons
horizontal, E(k)= c |k| photons nearly vertical) a second order
perturbation calculation removes the degeneracy and the level crossing.
k=0 optical phonons become photons for higher k and k=0 photons become
phonons for higher k. The crossing of the lines is replaced by two
nearly touching hyperbolas.
--
Roland Franzius
Pieter Kuiper
Apr7-04, 09:25 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nAndrew Resnick <andy.resnick@NOSPAM.grc.nasaDOTgov> wrote:\n\n> Could someone please refresh my memory of what these are? I recall it\n> has to do with a particular branch of a solution, the other being\n> acoustic phonons, but I can\'t find anything much more than that. Is it\n> simply phonons with energies close to "visible" (using the term loosely)\n> light?\n\nOptical phonons occur in lattices with more than one atom. In the\noptical branches, the different atoms do not vibrate in phase. If the\natoms have different charges, this implies an oscillating dipole, which\ncouples to electromagnetic waves. But the frequencies are still in the\nfar infrared.\n\n--\n"Electrons damage the brain," said Farish. (Donna Tartt)\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Andrew Resnick <andy.resnick@NOSPAM.grc.nasaDOTgov> wrote:
> Could someone please refresh my memory of what these are? I recall it
> has to do with a particular branch of a solution, the other being
> acoustic phonons, but I can't find anything much more than that. Is it
> simply phonons with energies close to "visible" (using the term loosely)
> light?
Optical phonons occur in lattices with more than one atom. In the
optical branches, the different atoms do not vibrate in phase. If the
atoms have different charges, this implies an oscillating dipole, which
couples to electromagnetic waves. But the frequencies are still in the
far infrared.
--
"Electrons damage the brain," said Farish. (Donna Tartt)
Repeating Rifle
Apr7-04, 09:27 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\n\nin article 20040405132750437-0400@newsread.grc.nasa.gov, Andrew Resnick at\nandy.resnick@NOSPAM.grc.nasaDOTgov wrote on 4/6/04 7:08 AM:\n\n>\n>\n> Could someone please refresh my memory of what these are? I recall it\n> has to do with a particular branch of a solution, the other being\n> acoustic phonons, but I can\'t find anything much more than that. Is it\n> simply phonons with energies close to "visible" (using the term loosely)\n> light?\n>\n> Thanks in advance.\nAcoustic phonons are those that have momentum proportional to frequency near\nzero frequency. Optical photons are those that cannot have zero frequency at\nzero momentum. They usually are of high frfequency even at zero momentum\n(wave number).\n\nBill\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>in article 20040405132750437-0400@newsread.grc.nasa.gov, Andrew Resnick at
andy.resnick@NOSPAM.grc.nasaDOTgov wrote on 4/6/04 7:08 AM:
>
>
> Could someone please refresh my memory of what these are? I recall it
> has to do with a particular branch of a solution, the other being
> acoustic phonons, but I can't find anything much more than that. Is it
> simply phonons with energies close to "visible" (using the term loosely)
> light?
>
> Thanks in advance.
Acoustic phonons are those that have momentum proportional to frequency near
zero frequency. Optical photons are those that cannot have zero frequency at
zero momentum. They usually are of high frfequency even at zero momentum
(wave number).
Bill
Igor Khavkine
Apr7-04, 09:28 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\n\nAndrew Resnick <andy.resnick@NOSPAM.grc.nasaDOTgov> wrote in message news:<20040405132750437-0400@newsread.grc.nasa.gov>...\n> Could someone please refresh my memory of what these are? I recall it\n> has to do with a particular branch of a solution, the other being\n> acoustic phonons, but I can\'t find anything much more than that. Is it\n> simply phonons with energies close to "visible" (using the term loosely)\n> light?\n>\n> Thanks in advance.\n\nIf you look at a simple system like a 1D chain of masses\nconnected by springs. But you have two different masses\nalternate, so the unit cell has two masses in it. If you\nsolve for the excitation modes, you get two branches in\nthe frequency wave-vector dispersion relation.\n\nThe lower branch vanishes at zero wave-vector (long wavelength).\nThis is the so-called acoustic phonon branch. The upper branch\ndoes not vanish for zero wave-vector. This is the so called optical\nphonon branch. Similar things happen in more than 1D, but you get\nmore branches, one for each polarization.\n\nAcoustic phonons are called so because they look like like this\n\n\n*** ***\n** ** ** **\n* * * * ...\n** ** **\n***\n\nAll the masses are being displaced "in phase". This mode corresponds\nto the mode of two coupled oscillators moving in phase. This kind of\nphonon could be excited by a sound wave.\n\nAn optical phonon looks like this\n\n\n(+) * * *\n* * * * * *\n* * * * ...\n* * * * * *\n(-) * * * * * *\n\nAdjacent masses are moving "out of phase". This mode corresponds to\nthe mode of two couple oscillators moving out of phase. If one mass\nwas say a positive Na+ ion and the other a negative Cl- ion, this\nkind of phonon could be excited by an electromagntic wave, hence the\nname.\n\nHope this helps.\n\nIgor\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Andrew Resnick <andy.resnick@NOSPAM.grc.nasaDOTgov> wrote in message news:<20040405132750437-0400@newsread.grc.nasa.gov>...
> Could someone please refresh my memory of what these are? I recall it
> has to do with a particular branch of a solution, the other being
> acoustic phonons, but I can't find anything much more than that. Is it
> simply phonons with energies close to "visible" (using the term loosely)
> light?
>
> Thanks in advance.
If you look at a simple system like a 1D chain of masses
connected by springs. But you have two different masses
alternate, so the unit cell has two masses in it. If you
solve for the excitation modes, you get two branches in
the frequency wave-vector dispersion relation.
The lower branch vanishes at zero wave-vector (long wavelength).
This is the so-called acoustic phonon branch. The upper branch
does not vanish for zero wave-vector. This is the so called optical
phonon branch. Similar things happen in more than 1D, but you get
more branches, one for each polarization.
Acoustic phonons are called so because they look like like this
*** ***[/itex]
** ** ** **
* * * * ...
** ** **
***
All the masses are being displaced "in phase". This mode corresponds
to the mode of two coupled oscillators moving in phase. This kind of
phonon could be excited by a sound wave.
An optical phonon looks like this
(+) * * *
* * * * * *
* * * * ...
* * * * * *
[itex](-) * * * * * *
Adjacent masses are moving "out of phase". This mode corresponds to
the mode of two couple oscillators moving out of phase. If one mass
was say a positive Na+ ion and the other a negative Cl- ion, this
kind of phonon could be excited by an electromagntic wave, hence the
name.
Hope this helps.
Igor
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