Brillouin scattering of phonons in complex materials

Initially, the theory of propagation of long-wavelength acoustic phonons and Brillouin scattering of laser light in condensed matter is concisely summarized. Then, the case of two relevant classes of complex materials in which Brillouin scattering can be measured is reviewed. First, in low-density, low-dimensional, disordered materials, the crossover between confinement and propagation is discussed on the basis of experimental findings. Moreover, the possibility of measuring the local mechanical properties of these materials at the mesoscale by Brillouin scattering is critically discussed. Second the application of Brillouin scattering to biological materials, a rather hot topic, is presented.