Time-resolved ring structure of backscattered circularly polarized beams from forward scattering layered structures

The backscattering of circularly polarized light at normal incidence to a half-space composed of two index matched layers with different absorption coefficients is studied using the Electric Field Monte Carlo method. The top layer, of thickness L ₁ = 2.5[l _s], where l _s is the scattering length, is non-absorbing and is composed of particles suspended in water with anistropy factor g = 0.8. The bottom layer, of thickness L ₂ = 25[l _s], is composed of absorbing particles with g = 0.8. The backscattered light with the same helicity (co-polarized) as the incident beam emerging from the top surface is analyzed in the time domain as absorption in the second layer increases from 1% to 10% of the scattering coefficient, μ _s. For the case of a homogenous half-space, composed of non absorbing particles with anisotropy factor g = 0.8, a ring-peak is known to be observed in the time-resolved co-polarized backscattered light intensity. For the two layer geometry tested here, a similar ring structure is found and used to determine the path length of photons traveling in the second layer. In recent studies, the ring-peak was postulated to be comprised of photons undergoing semi-circular trajectories as a result of near forward scattering events in the forward scattering media. This ideal picture of photon trajectories is tested and found to be an accurate characterization of photon trajectories in forward scattering media. Specifically, it is shown that time-sliced measurements of the backscattered co-polarized intensity at the ring-peak and path lengths of photons determined from the segment of arc of their idealized semi-circluar trajectories in the second layer can be used in conjunction with Beer's law to reproduce the known absorption coefficient of the second layer. This is a first indication that photons contributing to the ring-peak in co-polarized backscatter follow semi-circular trajectories. Moreover, it demonstrates that ring-structure can be used to determine subsurface features such as absorption coefficients in layered structures.