Characterization of inhomogeneous samples by spectroscopic Mueller polarimetry

Light depolarization occurs whenever different polarization responses add up incoherently, as it may be the case with inhomogeneous samples. The most convenient technique to characterize such samples is Mueller matrix polarimetry, as it is the only one providing all the relevant information in presence of depolarization. We studied the case of small grating boxes surrounded by bare silicon, in conditions where both the gratings and the substrate were illuminated by the Mueller polarimeter beam. The grating optical response is modeled by using rigorous coupled-waves analysis, and added incoherently to that of the substrate by merely summing the corresponding Mueller matrices. The line width and the depth of the grating as well as the percentage of substrate in irradiated spot area were obtained by fitting the experimental data taken with controlled displacement of the light spot in the boundary region between grating and substrate. Accurate grating parameters could be obtained with the fraction of the spot area within the grating box was larger than 30%. Moreover, these parameters remained relatively constant when this fraction was further decreased to 5%.