1st EOS Topical Meetingon Micro- and Nano-Optoelectronic Systems

Sensitive measurement of water content in dry material using
low-frequency terahertz time-domain spectroscopy systemequipped with micro-structured photoconductive antennas
Takeshi Yasui1,2 and Tsutomu Araki2
12Univ. Tokushima, Inst. Tech. and Sci., Tokushima, 770-8506, Japan
Osaka Univ., Grad. Sch. Engg. Sci., Toyonaka, 560-8531, Japane-mail: yasui@me.tokushima-u.ac.jp

Summary
We proposed sensitive measurement of water content in dry material using
low-frequency THz time-domain spectroscopy. Simultaneous use of micro-structured
bowtie-type photoconductive antennas for generation and detection of THz pulse
enables to achieve water content measurement in dry materials.
Introduction
Terahertz time-domain spectroscopy (THz-TDS) has emerged as a new mode forsensing, non-destructive inspection, and material characterization. One of mostimportant applications is a non-contact, remote determination of water content in a drymaterial because THz pulse possesses unique charactertics, such as free-space
propagation, excellent transmittance to nonmetal material, low scattering, lowinvasion, and most importantly, strong absorption due to rotational transition of polarwater molecules [1, 2]. For example, ultimate sensitivity to liquid water was effectively
applied to monitor moisture level of a leaf from a common houseplant [3]. However,
too strong absorption in a higher frequency in THz region (≈ 230 cm-1 at 1THz) [2]often limits samples to thin and/or considerably dry materials.
In this paper, we developed a low-frequency THz-TDS system equipped withmicro-ctructured bowtie-type photoconductive antenna (PCA) to reduce restrictions
on the sample in sensitive measurement of water content. The developed system isapplied for sensitive measurement of water content in coffee powder in a glass bottle.
Experimental setup
Figure 1 shows experimental setup for low-frequency THz-TDS system based on atransmission configuration. A mode-locked Ti:Sapphire laser (pulse duration = 80 fs,average power = 300 mW, center wavelength = 808 nm) is used for generation anddetection of THz pulse. The bowtie-type PCAs were fabricated on low-temperature
GaAs with bowtie length of 1 mm and a gap of 5 µm. The laser beam is split into twoways with a beam splitter (BS) for a pump beam and a probe beam. After passing
through an optical chopper, the pump beam is focused onto a PCA with a lens. The
radiated THz beam from the PCA emitter is collimated with a diameter of 25 mm by ahemispherical silicon lens (Si-L) and an off-axis parabolic mirror (OAP-M), and then isincident to a sample. The THz beam passing through a sample is focused onto a PCA
detector by another Si-L and another OAP-M. On the other hand, the probe beam via
an optical delay line is incident to the PCA detector and induces free photocarriers ata gap between two electrodes in PCA detector. When the THz pulse and the probe
pulse are incident to the PCA detector simultaneously, the electric field of the THz
pulse accelerates the photocarriers and hence causes transient photocurrent
between in the PCA detector. The resulting current signal is converted into a voltage
signal by a current preamplifier, and then the voltage signal is measured with a lock-inamplifier referring to the chopper. The temporal waveform of the THz electric field isobtained by sampling measurement using the optical delay line. Finally, FourierISBN 978-3-00-033711-6

Bremen, Germany
7 - 9 December 2011

1st EOS Topical Meetingon Micro- and Nano-Optoelectronic Systems

transform of the measured temporal waveform gives frequency spectra of amplitude
and phase in the THz radiation, directly.
ResultWe performed a non-contact, remote measurement of water content in instantcoffee powder in a glass bottle. Figure 2(a) shows comparison of the THz
transmittance spectra without sample, with an empty glass bottle, and with a glass
bottle including instant coffee powder. In general, usual glass strongly absorbs theTHz pulse because it contains the water. However, the low-frequency THz pulse canpass through a thick glass bottle even though higher frequency components are
considerably absorbed as shown in Fig. 2(a). On the other hand, the coffee powder
hardly attenuates the THz pulse because of low scattering characteristic. Hence, itenables us to monitor the water content in the coffee powder in the glass bottle using
lower frequency THz region. Figure 2(b) shows comparison of THz transmittancespectra before and 3.5 hours after forced humidifying (> 90 %RH). From absorption ofwater at 0.07 THz, increase of water content caused by the 3.5hours forced
humidifying is estimated to be 7.5 mg/cm3.
Conclusion
We demonstrated sensitive measurement of water content in dry material using
low-frequency THz-TDS method. The proposed method will be a powerful tool for
quality control of water content in dry materials.
References[1] L. Thrane et al., Chem. Phys. Lett. 240, 330-333 (1996).
[2] J. T. Kindt et al., J. Phys. Chem. 100, 10373-10379 (1996).
[3] D. M. Mittleman et al., IEEE JST-QE 2, 679-692 (1996).

PCA

PCA

Fig. 1. Experimental setup. BS: beam splitter, CRs: cornerreflectors, PCA: photoconductive antenna, Si-L: silicon lens,
OAP-M: off-axis parabolic mirror.

ISBN 978-3-00-033711-6

Fig. 2. (a) Comparison of THz
transmission
spectrawithout
sample, with an empty glass
bottle, and with a glass bottle
including coffee powder, (b)change of THz transmissionspectra by 3.5hours forced
humidifying.

Bremen, Germany
7 - 9 December 2011