Insights into the dependent-scattering contributions to the extinction coefficient in highly scattering suspensions.

We study theoretically the extinction of collimated light in random systems of highly scattering particles embedded in nonabsorbing media. We aim to provide rough guidelines on the behavior of the extinction coefficient in the so-called dependent-scattering regime. We base our analysis on Keller's second order perturbative approximation to the effective propagation constant. To gain physical insight, we also analyze a simple model based on the physical notion that particles in a dense system scatter light in an effective medium.

Effect of the concentration of magnetic grains on the linear-optical-absorption coefficient of ferrofluid-doped lyotropic mesophases: deviation from the Beer-Lambert law.

In this paper is reported a systematic experimental study of the linear-optical-absorption coefficient of ferrofluid-doped isotropic lyotropic mixtures as a function of the magnetic-grains concentration. The linear optical absorption of ferrolyomesophases increases in a nonlinear manner with the concentration of magnetic grains, deviating from the usual Beer-Lambert law. This behavior is associated to the presence of correlated micelles in the mixture which favors the formation of small-scale aggregates of magnetic grains (dimers), which have a higher absorption coefficient with respect to that of isolated grains. We propose that the indirect heating of the micelles via the ferrofluid grains (hyperthermia) could account for this nonlinear increase of the linear-optical-absorption coefficient as a function of the grains concentration.

Behavior of the nonlinear refractive indices and birefringence in the neighborhood of first- and second-order phase transitions in lyotropic liquid crystals.

We investigated the behavior of the nonlinear refractive indices (n(2)) and birefringence (Delta n(2)) in the vicinity of the nematic-to-isotropic (N-I) and nematic uniaxial-to-nematic biaxial (N-N) phase transitions in a lyotropic liquid crystal. The single-beam Z-scan technique is used to measure n(2) in different relative configurations of the electric field of the laser beam and the symmetry axes of the phases. In the N-I transition, the nonlinear optical birefringence shows a discontinuity at the transition temperature (T(c)), as observed in the linear birefringence. On the other hand, in the N-N transition, Delta n(2) was shown to be proportional to /T-T(c)/(-beta), with beta approximately 0.5, in both uniaxial and biaxial nematic domains. No discontinuity was observed on Delta n(2) in the N-N transition. The symmetric invariants of the order parameter were shown to be linear functions of the temperature in the uniaxial nematic domain, in good agreement with the mean-field prediction.