  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.
