Fundamental Differenc Ebetween the Role of Absorption and Inelastic Scattering in Random System

A few years ago a new interest1–9 in propagation of classical waves in random systems took place, mainly due to the possibility of weak3–6 and strong1,2 localization effects. Many of the ideas that were developed in solid state physics for transport of electrons in random materials found8,10 their counterpart in optical waves. In the spirit of finding analogies between quantum (electron) waves and classical waves, it was generally assumed2,7,11,12 that absorption for classical waves plays a similar role to inelastic scattering for electrons. It is the purpose of this paper to examine this assumption and to demonstrate that it is unjustified. The role of absorption is fundamentally different from the role of inelastic scattering. The difference arises from the simple fact that absorption is an amplitude effect whereas inelastic scattering affects the phase of the wave. This leads to fundamentally different effects in almost all observed phenomena such as: transmission, coherent backscattering, spectral auto correlation function, renormaliza-tion of the diffusion constant near resonance and the optical Anderson transition.