Influence of Particle Size on Optical Constants From Pellets Measured With Terahertz Pulsed Spectroscopy

Particle size is shown to influence both the overall value and the measurement precision of the refractive index and absorption coefficient of a sample diluted with high density polyethylene (HDPE) powder and compressed in a pellet. However, excessive grinding of a sample before it is mixed with HDPE powder can result in the formation of large particle clusters. These aggregates can induce internal scattering of the terahertz pulse within the pellet as well as lead to different mechanical properties of the pellet, which may change the distribution of air pores. These phenomena result in extracted optical constants with low accuracy and precision. The present work introduces a simple experimental procedure to improve the precision and accuracy of optical constants from spectroscopic pellets. This procedure consists of moderately grinding the sample powder in fine HDPE powder with a pestle and mortar. The standard deviation of the refractive index of Prussian blue has been observed to reduce from 0.14 to 0.02 following this procedure.