Scattering Effects in Terahertz Wave Spectroscopy of Granulated Solids

Abstract

Applications of terahertz spectroscopy methods to granulated solids are substantially constrained by the effects of scattering of the incident terahertz waves by spatial heterogeneities of a sample. The experimental results obtained by means of terahertz time-domain spectroscopy are reported for the samples of granulated 2, 4-dinitrotoluene and alpha-lactose without any special filling. We show that in samples of the same substance, but prepared in a different manner, the overall increase of the background extinction with frequency and appearance of additional extinction maxima can obscure the true absorption features of the material. In samples with large grain size, of order of 400 m, a complicated irregular spectral structure is observed instead of a fingerprint maxima at 1.37 THz for alpha-lactose and at 1.08 THz for 2,4-dinitrotoluene. At higher frequencies no true spectral fingerprints are clearly seen in the raw experimental spectra of all the samples with the grain sizes of 130 m and more. The influence of grain size, air-filling factor and the dispersion properties of the materials is studied. Theoretical analysis is made with the use of Mie theory in simplifying approximations and taking into account interference between the waves transmitted through a porous sample. It has been shown that the scattering-induced masking effects can be properly simulated even for the air-filled objects with initially unknown spatial and dispersion parameters. The procedure of eliminating the influence of scattering effects from the raw spectral data is proposed. Detection of true high-frequency spectral fingerprints is demonstrated using this procedure for various grain sizes up to 400 m.Index Terms-Electromagnetic interference, electromagnetic propagation in absorbing media, submillimeter-wave measurements, submillimeter-wave propagation.

Nonlinear Plasmonic Metasurface Terahertz Emitters for Compact Terahertz Spectroscopy Systems

“…Measuring the two additional lines may either require systems with higher signal-to-noise ratio, or use of stricter sample preparation protocols, 2 as the spectroscopic response of granulated solids was shown to depend strongly on sample preparation. 34 …”

“…Measuring the two additional lines may either require systems with higher signal-to-noise ratio, or use of stricter sample preparation protocols, 2 as the spectroscopic response of granulated solids was shown to depend strongly on sample preparation. 34 To conclude, we have studied the possibility to drive THz PMAs by nanojoule femtosecond lasers, which are commonly used in compact THz spectroscopy and imaging systems. The 40 nm thick PMA generated linearly polarized single-cycle THz pulses with a bandwidth of ∼1.5 THz and noise level of under −30 dB, with NIR to THz conversion efficiency as high as that of a 0.1 mm thick ZnTe crystal.…”

Terahertz Scattering and Spectroscopic Characteristics of Polymethacryl Imide Microstructures

“…The above simulation process was carried out using MATLAB programming [7], [14], [15]. As can be seen that the application of Mie scattering theory to explain the extinction phenomenon of THz wave propagating through PMI foams is quite reasonable, as demonstrated by the consistency between the theoretical predictions and the experimental results, affirming our original hypothesis that the THz extinction results mainly from scattering rather than absorption [18]- [23].…”

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

“…To overcome these issues, a granular form of the material is often diluted into a low absorbing material, such as polyethylene powder, and the powder mixture then compressed into a thick pellet [5], [6]. Previous studies have investigated the influence of the particle size of granular materials on the spectral signature (absorbance, transmittance, absorption or attenuation coefficient spectra) of the material in the terahertz region, observing changes in the monotonic increase of the baseline, anomalous spectral features and distortions of vibrational absorption bands [7]- [12]. A study has recently suggested a curve-fitting equation to distinguish the absorption behavior of the material from scattering-induced effects in heterogeneous and scattering specimens [12].…”

“…Previous studies have investigated the influence of the particle size of granular materials on the spectral signature (absorbance, transmittance, absorption or attenuation coefficient spectra) of the material in the terahertz region, observing changes in the monotonic increase of the baseline, anomalous spectral features and distortions of vibrational absorption bands [7]- [12]. A study has recently suggested a curve-fitting equation to distinguish the absorption behavior of the material from scattering-induced effects in heterogeneous and scattering specimens [12]. Yet, no study has been made of the influence of particle size on refractive index values extracted using terahertz TDS.…”