MadSci Network: Physics |
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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