Accurate determination of optical coefficients from chemical samples using terahertz time-domain spectroscopy and effective medium theory.
A technique for calculating the optical constants of powdered materials that have been pressed into pellets along with a diluent such as polyethylene, as is commonly used in the spectroscopy community, is introduced. The simple Beer-Lambert law typically used to calculate the optical constants has the inherent weakness that it treats the absorbing medium as a single nonporous solid, as opposed to a dielectric material embedded within a medium. This leads to a systematic underestimation of both the absorption coefficient and the refractive index, especially at low filling factors. Effective medium theory provides a way to calculate more accurate optical constants and produces similar optical constants across a wide range of filling factors, including crucially at the low filling factors commonly used experimentally. The technique can also be extended to estimate the porosity of the samples and provide a true, preparation-independent value for the optical properties of the sample material.
Parrott
Edward P
EP
Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street,Cambridge CB2 3RA, UK.
Zeitler
J Axel
JA
Gladden
Lynn F
LF
D000042:Absorption; D007785:Lactose / Q000737:chemistry; D055620:Optical Phenomena; D020959:Polyethylene / Q000737:chemistry; D016062:Porosity; D013057:Spectrum Analysis; D013997:Time Factors
D020959:Polyethylene; D007785:Lactose