Characterization of particulates using ultra-short laser pulses

Abstract

Here we propose a novel time-domain method for the real-time characterization and analysis of scatterings samples, which separates out different sample properties, such as the particle size distribution and packing fraction (the density of scatterers), and absorption. The method is based on measuring the temporal shape of ultrashort light pulses scattered through the sample. By using a temporal random walk approach based on the characteristic size distribution and packing fraction of the sample, we predict the temporal dispersion of the input pulses, i.e. their stretching into particular shapes and the generation of echoes. So far our results are obtained under the coarse approximation that the mean free path is equal to the particle size in the peak of their distribution, however, our results are in qualitative agreement with results published previously. For example, assuming Gaussian input pulses with 50 fs (FWHM) duration, our analysis shows that the temporal shape of the first scattered pulse replicates and thereby reveals the particle size distribution. Furthermore, the delay between the initial peak and subsequent peaks depends on the packing fraction and average particle size. We also show that these results are independent of absorption features. This approach appears promising for the characterization of mixtures of pharmaceutical particulates with time resolved scattering of fs-pulses.