The Kinematic View of Brillouin Spectroscopy

the energy transfer between photons and phonons is very small (5x10 Hz/5xl0 Hz=10 ) and hence ki and ks have the same length. Provided, acoustic attenuation is small (Tosc f2) the phase-sound velocity can be calculated from simple geometric arguments. Equations (15, 16) yield Eq. (17), where n is the refractive index of the sample, for the sound velocity v for the longitudinal mode. Equivalent equations hold for other polarizations. 

Knowing the mass density p of the sample, the longitudinal elastic modulus c, related to the wave vector q, can be calculated according to Eq. (18). This equation is most important since it provides the key for the determination of the mechanical properties of materials at hypersonic frequencies from BriUouin spectra in a nondestructive manner. 

---