Murphy, K N; Naftaly, M; Nordon, A; Markl, D (2023) Effect of particle size and concentration on low-frequency terahertz scattering in granular compacts. Optical Materials Express, 13 (8). pp. 2251-2263.

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Abstract

Fundamental knowledge of scattering in granular compacts is essential to ensure accuracy of spectroscopic measurements and determine material characteristics such as size and shape of scattering objects. Terahertz time-domain spectroscopy (THz-TDS) was employed to investigate the effect of particle size and concentration on scattering in specially fabricated compacts consisting of borosilicate microspheres in a polytetrafluoroethylene (PTFE) matrix. As expected, increasing particle size leads to an increase in overall scattering contribution. Scattering increases linearly at low concentrations, saturates at higher concentrations with a maximum level depending on particle size, and that the onset of saturation is independent of particle size. The effective refractive index becomes sublinear at high particle concentrations and exceeds the linear model at maximum density, which can cause errors in calculations based on it, such as porosity. The observed phenomena are attributed to the change in the fraction of photons propagating ballistically versus being scattered. At low concentrations, photons travel predominately ballistically through the PTFE matrix. At high concentrations, the photons again propagate ballistically through adjacent glass microspheres. In the intermediate regime, photons are predominately scattered.

Item Type:	Article
Subjects:	Optical Radiation and Photonics > Materials and Appearance
Divisions:	Electromagnetic & Electrochemical Technologies
Identification number/DOI:	10.1364/OME.494825
Last Modified:	14 Nov 2023 15:52
URI:	https://eprintspublications.npl.co.uk/id/eprint/9862