SHORT-RANGE INTERFERENCE EFFECT IN THE DIFFUSION OF LIGHT IN RANDOM-MEDIA

We study the interference effect of multiply scattered waves in the di ffusive light propagation in random media. We take porous glass sample s of various pore radii a as an example of random media, and measure t he transport mean free path l* and the diffusion constant D for variou s light wavelengths lambda over a wide range of the size parameter a/l ambda=0.05-0.85. A crossover is observed from the Rayleigh scattering region to the geometrical optics region. The transport velocity upsilo n(E)=3D/l* determined from the data is found to decrease monotonically with a/lambda, falling well below the light velocity in glass. We als o present a framework of theoretical analysis of wave energy diffusion in a most general setting, taking explicit account of the broadening of light dispersion and paying attention to the requirement of Ward-Ta kahashi identity. We calculate diffusion parameters as functions of a/ lambda for a model of spatially fluctuating dielectric constant, using two types of self-consistent scattering approximation to the self-ene rgy and the scattering kernel. Comparison of the experiment against th e calculation with and without interference terms of multiple scatteri ng reveals the importance of short-range interference effects in the d iffusive propagation of light.