Precision determination of the line shape for coherently backscattered light from disordered solids: Comparison of vector and scalar theories

We present high-precision measurements of the line shape of coherently backscattered light from a disordered solid which strongly supports the recent vector theory of Stephen and Cwillich, and which cannot be adequately described by current scalar theories. Three different regions have been identified in the experimentally observed line shape which reflect three different types of photon trajectories inside the multiply scattering medium. They are (i) extremely long trajectories which are affected by finite sample size, (ii) trajectories much smaller than the sample dimensions but much larger than the optical mean-free path; for these trajectories the scattering process is essentially two dimensional, and (iii) short trajectories which give rise to an asymptotic falloff which is proportional to the inverse square of the momentum transfer.