Experiment and theory of the complex refractive index of dense colloidal media

The complex refractive index is analyzed by measuring its scattering attenuation µ_s, group index n_g, and group velocity dispersion (GVD) for 100 nm diameter silica nanoparticles dispersed in water. The experiments were performed for wavelengths between 410 nm and 930 nm. The experimental results were compared with different mixing models for the complex refractive index of colloidal suspensions. The group index linearly scaled with the volume fraction both in experiment and for all tested models. It was found that the GVD has a nonlinear dependence on volume fraction in agreement with the coupled dipole model of Parola et al. [J. Chem. Phys. 141, 124902 (2014)] The scattering attenuation is in good agreement with both the coupled dipole model and the low frequency quasi-crystalline approximation [J. Electromagn. Waves Appl. 2, 757 (1988)] that take particle correlations into account. With an iterative fitting procedure of all the data based on both the coupled dipole model and the quasi-crystalline approximation, the refractive index, porosity, and size of the nanoparticles were determined. We determined that the coupled dipole model is in best agreement with the data.