Elimination of scattering effects in spectral measurement of granulated
materials using terahertz pulsed spectroscopy
Y. C. Shen,
1,a
P. F. Taday,
2
and M. Pepper
2
1
Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool L69 3GJ,
United Kingdom
2
TeraView Limited, Platinum Building, St John’s Innovation Park, Cambridge CB4 0WS, United Kingdom
Received 12 December 2007; accepted 15 January 2008; published online 5 February 2008
Spectral distortions are commonly observed in terahertz spectra of granulated materials. These
spurious structures in the spectroscopy are caused by scattering due to the refractive index mismatch
between the particles and their surrounding medium. We find that the scattering contribution is
random across sample positions and could be eliminated by summing and averaging multiple
measurements over a sample area. We present experimental results of both absorbing and
nonabsorbing particles in the size range 50–250 m and also give an empirical expres-
sion to describe the effect of grain size on the scattering-induced extinction as a function of
frequency. © 2008 American Institute of Physics. DOI: 10.1063/1.2840719
The terahertz region of the electromagnetic spectrum
spans the frequency range between the midinfrared and the
millimeter/microwave. The relatively unexplored central part
of the region 0.3–3 THz or 10–100 cm
-1
comprises fre-
quencies lower than those corresponding to most internal
vibrations of isolated small molecules. Instead, spectra con-
tain information on motions associated with coherent,
delocalized movements of large numbers of atoms and
molecules.
1
The newly developed technique of terahertz
pulsed spectroscopy TPS has been demonstrated to be a
powerful tool for studying these low-frequency vibrational
modes.
2–4
A major advantage of TPS is that the transient
electric field, not simply the intensity of the terahertz radia-
tion, is measured. This coherent detection scheme not only
yields terahertz spectra with excellent signal-to-noise ratio
and high dynamic range but also allows both the absorption
coefficient and refractive index to be obtained without the
need for Kramers–Kronig dispersion relationship.
2
Owing to
these advantages, TPS is being widely used in studying low-
frequency vibrational modes for a wide variety of samples
including chemical, biological, pharmaceutical, and security-
related materials.
5–10
In most of the work reported so far,
TPS measurements were performed on solid crystalline
sample powder in the form of pellet. Therefore, it is essen-
tial to investigate the scattering of terahertz radiation by par-
ticles presented in the sample and, in particular, the practical
ways to minimize the scattering effect for getting reliable
terahertz spectra of granulated materials.
The propagation of terahertz pulses in random media
comprising many 800-m-diameter Teflon spheres has been
studied previously,
11
and the scattering of terahertz pulses by
individual spheres or cylinders has also been measured.
12
These novel studies utilized the unique capabilities of TPS
for characterizing the scattered electric field with
unprecedented spatial and temporal resolution, leading to
further insight into the scattering phenomena.
13–15
In a recent
Food and Drug Administration FDA study, Wu et al.
16
measured terahertz spectra of a number of granulated phar-
maceutical materials with grain size comparable to terahertz
wavelength and found that the particle scattering becomes
significant at higher terahertz frequencies. Very recently,
Zurk et al.
17
measured terahertz scattering of granular mate-
rials polyethylene particles with two different grain sizes
and explained the observed scattering response using the
dense medium theory. In this paper, we study the effect of
particle scattering on TPS spectrum in the frequency range
0.3–3 THz 10–100 cm
-1
for both absorbing and nonab-
sorbing particles in the size range 50– 250 m. We find that
the scattering contribution could be eliminated by either a
novel procedure of summing and averaging over a mapped
area or, where this is possible, a traditional procedure of
proper sample preparation.
The nonabsorbing material used is the high-density poly-
ethylene PE powder Inducos 13 / 1 and 13 / 3, with particle
sizes 80 and 50–200 m, respectivelyInduchem AG,
Switzerland. The PE particles in these samples were further
size fractionated using test sieves Endecotts Ltd, UK into
six size categories: 53 m, 50–75, 75–80, 75–106, 106–
150, and 150–200 m. PE is nearly transparent with a
frequency-independent index of refraction of 1.53 in tera-
hertz region.
9
Sucrose was used as an absorbing material.
Granulated sucrose the coarse grain size up to 800 m was
first crushed to a coarse powder using pestle and mortar, and
then milled using a Specamill Specac Ltd, UK, and finally
size fractionated into five size categories: 52, 53–75, 75–
106, 106–150, and 150–250 m. For TPS measurements,
the particles were contained in a specially designed powder
cell with a volume of 0.53 cm
3
composed of two 3-mm-thick
PE windows separated by a 3-mm-thick spacer. In all TPS
measurements, the cell was filled with either 300 mg PE
powder or 60 mg sucrose diluted in 260 mg PE particle size
53 m. The filling factor, which is defined by the ratio of
the sample volume to the cell volume, was estimated to be
0.6 0.04 for above powder samples.
The experimental setup and operating principle for TPS
have been well documented.
2–4
In our experiments, we used
a TPS spectra 1000 transmission spectrometer TeraView
Limited, UK. Measurements were performed in rapid scan
mode at a rate of 30 scans / second, providing a useful spec-
tral range 0.2–3.0 THz 6–100 cm
-1
with a spectral reso-
lution of 40 GHz 1.3 cm
-1
. For each sample or reference
empty powder cell measurement, the terahertz waveform
a
Electronic mail: y.c.shen@liv.ac.uk.
APPLIED PHYSICS LETTERS 92, 051103 2008
0003-6951/2008/925/051103/3/$23.00 © 2008 American Institute of Physics 92, 051103-1
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp