Universal Features of Terahertz Absorption in Disordered Materials
  Using an analytical theory, experimental terahertz time-domain spectroscopy data and numerical evidence, we demonstrate that the frequency dependence of the absorption coupling coefficient between far-infrared photons and atomic vibrations in disordered materials has the universal functional form, C(omega) = A + B*omega^2, where the material-specific constants A and B are related to the distributions of fluctuating charges obeying global and local charge neutrality, respectively. 
S.N. Taraskin, S.I. Simdyankin, S.R. Elliott, J.R. Neilson and T. Lo