What are the optical phonons and the acoustic phonons in an optical fiber?
Phonons are just lattice vibrations, but we imagine them as particles that carry this vibrational energy in a similar manner to photons i.e. they are discrete and quantized. Consider a lattice with two kinds of atoms - that is lattice with a basis of two atoms in the primitive cell. The allowed frequencies of propagation wave are split into an upper branch known as the optical branch, and a lower branch called the acoustical branch. It can be found that for optical branch (in the long wavelength limit) the two atoms in the unit cell move opposite to each other and the light mass amplitude is greater. For acoustical branch (in the long wavelength limit) the displacement of both atoms has the same amplitude, direction and phase.
Phonons are just lattice vibrations, but we imagine them as particles that carry this vibrational energy in a similar manner to photons i.e. they are discrete and quantized. Consider a lattice with two kinds of atoms - that is lattice with a basis of two atoms in the primitive cell. The allowed frequencies of propagation wave are split into an upper branch known as the optical branch, and a lower branch called the acoustical branch. It can be found that for optical branch (in the long wavelength limit) the two atoms in the unit cell move opposite to each other and the light mass amplitude is greater. For acoustical branch (in the long wavelength limit) the displacement of both atoms has the same amplitude, direction and phase.
How do these two types of phonons affect the scattering of light?
I think this weblink would be helpful for your understanding, http://en.wikipedia.org/wiki/Light_scattering
Or, you can google it to know more detail. There are plenty of Weblinks and articles you'll find on that. All the best.... :)
The way I remember the difference is as so:
An "acoustic" phonon is a vibration in the crystal lattice where all moving components are the same mass. So if you take the crystal's unit cell as a single object, the three different ways the unit cells can move w/r/to each other (two transverse one longitudinal) are the acoustic phonon modes.
The "optical" phonon is so named, I believe, because we now look *inside* the unit cell, where the dissimilar atoms often have slight charge differences (for example, Si-O, where the highly electronegative O has the electrons more often and is thus more negative than the Si). Because of this charge difference, an Electric field could interact with the pair of atoms within this unitcell - in particular an electromagnetic wave would interact with this two-atom dipole, hence the term "optical". (Although I think it's always called "optical" regardless of the charge difference or frequency it interacts with.)
The ways these atoms *within* the unitcell can move are again two transverse & one longitudinal (ie. Towards/away from each other). However, there may be numerous dissimilar atoms in the unitcell, which is why the number of "optical" phonons supported by the crystal increases with number of different atoms.
In optical mode phonon, two atoms in a unit cell vibrate in opposite direction but for accoustic mode phonon the atoms in the unit cell vibrate along the same direction.
Refer to : Semiconductor Optoelectronic devices by Jaspreet Singh.
Does this movement of dissimilar atoms produce any EM waves? What is the effect of optic phonon in piezoelectric material?
In the diatomic linear lattice we can think of each unit cell as containing two atoms of differing mass. It is the characteristic of
crystals with two atoms per unit cell that two types of mode occur. One of these modes is called the acoustic mode. In an acoustic mode, the adjacent atoms as vibrating almost in phase. however, in an optic mode, we think of adjacent atoms as vibrating out of phase. These descriptions of optic and acoustic modes are valid only in the long wavelength limit.
I think , it differs in frequency, acoustic frequency is in the 10-11 GHz range, where as optical phonons in the range of 10-12 THz.
Dear readers
can we to correlate between the distance of acoustic and optical phonons and thermal conductivity.
Is there any particular range for getting acoustic modes in Raman spectrum?
@Md please tell me the difference between first and second phonon emission
The phonons are thermal vibrations resulting from the thermal energy
of the materials. So, they can not absorb or emit electromagnetic radiations. The particles which absorb or emit photons are the electrons in the material. The optical phonons have much greater frequencies than the acoustic phonons and their frequency lies in the optical range therefore they are called optical phonons. The energy pf the phonon= hf where h is the Planck consatnt and f is the frequency of the viberation of the phonon. The average energy of the phonons is the thermal energy kT
with k is the Boltzmann constant and T is the absolute temperature.
Best wishes.
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