Abstract
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.
Highlights
T ERAHERTZ metrology is rapidly developing with the increased access to terahertz technologies
For some materials absorbing in the terahertz region, extraction of optical constants from transmission through a solid slab or sheet may prove to be difficult due to excessive losses in the signal transmitted through a thick slab, and/or due to the etalon effect arising from multiple internal reflections of terahertz pulses on Manuscript received September 28, 2015; revised January 21, 2016; accepted February 27, 2016
While the average value and standard deviation of the refractive index is lower for particles with sizes between 75 and 106 m, both are seen to increase with larger particle sizes, until reaching a standard deviation of 0.1 for particles larger than 250 m
Summary
T ERAHERTZ metrology is rapidly developing with the increased access to terahertz technologies. The study of optical properties of pigments in the terahertz region has enabled us to observe the influence of particle size as well as of the mixing procedure of the granular material on the homogeneity of spectroscopic pellets and on the overall value and precision of the optical constants. These observations are reported and discussed in the present letter, together with best practice of pellet preparation
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