MadSci Network: Physics
Query:

Re: Can EM radiation interact with high energy sound waves?

Date: Thu Apr 10 10:12:29 2003
Posted By: Phil Marsden, Post-doc/Fellow
Area of science: Physics
ID: 1048608208.Ph
Message:

The simple answer is, yes. Electromagnetic radiation can indeed interact
with sound waves in materials. This is a very well known phenomenon in
solid state physics, described in any textbook on the subject. See, for
example, Introduction to Solid State Physics by Charles Kittel
and many many other good books on the subject. I restrict myself to solid
state materials since this was specified in the question.

Sound waves in materials, which are described as phonons (a particle
representation of the lattice vibrations), interact with both directly with
the photons (polariton coupling) and indirectly via the electrons (polaron
coupling). In both of these cases the phonons, or sound waves, can be seen
to be interacting with light. There are many many technical articles about
polarons and polaritons on the net and the interested reader can search for
them with their favourite search engine.

However, let us think about the sound waves for a moment. The question
specified high energy sound waves and this is very important because we
must make the distinction between high power and high energy sound waves.
High energy means that each phonon (lattice vibration) will have a very
high energy rather than having many low energy vibrations. To generate
optical transitions it will probably be necessary to have a very high
energy indeed.

To calculate how high the energy has to be we can initially consider how
much energy the lattice already has. Phonons not only transmit sound, but
also heat. If a material is at room temperature then, from Boltzmann
statistics, we can assign it an approximate energy of kB * T (where kB is
the Boltzmann constant). If our material is at 20 degrees celcius (an
average laboratory temperature maybe) then the Boltzmann energy is (in the
convenient unit of electronvolts) 0.025eV. (1eV = 1.6e-19 Joules). This may
not seem like a lot of energy, but let us now think about the energy in a
sound wave. Typically, ultrasound systems (see for example this page)
can have a frequency up to about 10GHz. 10GHz can easily be converted to
an energy by the Einstein relation, E=hf, where h is the Planck constant
and f is the frequency of the wave. Multiplying 10GHz by h gives an energy
of approximately 0.00004eV, a factor of 625 times smaller than the
"background" energy of the phonons. To get to a serious amount of sound
energy would mean increasing the acoustic frequency by about 1000 times.
This would be extraordinarily hard to do, and is probably why we don't see
acoustic absorption modulators on the market today for, if the sound waves
could reach this level, they could certainly be used to stimulate
transitions between energy levels (quantum states) within the material.
Effectively excite or de-excite the material.

However, all is not lost since acoustics are often used in two important
types of modulator. In both cases though there is no transfer of energy
between states, but an elastic deformation which changes the refractive
index within the material. The first is the acousto-optic modulator and the
second is the photo-elastic modulator. The first device acts like a
diffraction grating and the second acts as a modulating optical
phase-retarder. More information about these devices can be found at:

Acoustooptic
modulators
Photoelastic modulators



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