LONG-RANGE INTENSITY CORRELATIONS FOR THE MULTIPLE-SCATTERING OF WAVES IN UNORDERED MEDIA

The long-range correlations in the reflected and transmitted fluxes in the case of the coherent transport of waves in an unordered medium wi th discrete inhomogeneities are considered. The correlator and spectru m of the intensity fluctuations are expressed in a general form in ter ms of the one-center scattering amplitude and the propagators of the m ean radiated intensity, The random interference of the waves and the f luctuations of the number of scattering centers in a microvolume of th e medium are taken into account simultaneously. Detailed calculations are performed for two limiting radiation propagation regimes, viz., sp atial diffusion and small-angle multiple scattering. It is shown that the conservation of the total flux upon elastic scattering leads to th e formation of a dip in the spectrum and, accordingly, a negative corr elation between the intensities at large distances. In the case of spa tial diffusion this feature is displayed upon reflection, and in the c ase of small-angle multiple scattering it is displayed upon transmissi on through a slab. The relative roles of the various sources of intens ity fluctuations, as well as the sensitivity of the correlations to fa ctors that influence the wave propagation regime, viz., the finite siz e of the scattering sample, absorption in the medium, and the presence of a frequency shift in the incident waves, are analyzed. We find tha t fluctuations in the distribution of the scatterers show up mast stro ngly in a medium with strong, i.e., ''non-Born,'' centers, especially if they exhibit absorption. (C) 1997 American Institute of Physics.